+++ /dev/null
-Introdution
-===========
-
-Executive summary
------------------
-
-The rest of this section covers the scope of the kernel development process
-and the kinds of frustrations that developers and their employers can
-encounter there. There are a great many reasons why kernel code should be
-merged into the official ("mainline") kernel, including automatic
-availability to users, community support in many forms, and the ability to
-influence the direction of kernel development. Code contributed to the
-Linux kernel must be made available under a GPL-compatible license.
-
-:ref:`development_process` introduces the development process, the kernel
-release cycle, and the mechanics of the merge window. The various phases in
-the patch development, review, and merging cycle are covered. There is some
-discussion of tools and mailing lists. Developers wanting to get started
-with kernel development are encouraged to track down and fix bugs as an
-initial exercise.
-
-:ref:`development_early_stage` covers early-stage project planning, with an
-emphasis on involving the development community as soon as possible.
-
-:ref:`development_coding` is about the coding process; several pitfalls which
-have been encountered by other developers are discussed. Some requirements for
-patches are covered, and there is an introduction to some of the tools
-which can help to ensure that kernel patches are correct.
-
-:ref:`development_posting` talks about the process of posting patches for
-review. To be taken seriously by the development community, patches must be
-properly formatted and described, and they must be sent to the right place.
-Following the advice in this section should help to ensure the best
-possible reception for your work.
-
-:ref:`development_followthrough` covers what happens after posting patches; the
-job is far from done at that point. Working with reviewers is a crucial part
-of the development process; this section offers a number of tips on how to
-avoid problems at this important stage. Developers are cautioned against
-assuming that the job is done when a patch is merged into the mainline.
-
-:ref:`development_advancedtopics` introduces a couple of "advanced" topics:
-managing patches with git and reviewing patches posted by others.
-
-:ref:`development_conclusion` concludes the document with pointers to sources
-for more information on kernel development.
-
-What this document is about
----------------------------
-
-The Linux kernel, at over 8 million lines of code and well over 1000
-contributors to each release, is one of the largest and most active free
-software projects in existence. Since its humble beginning in 1991, this
-kernel has evolved into a best-of-breed operating system component which
-runs on pocket-sized digital music players, desktop PCs, the largest
-supercomputers in existence, and all types of systems in between. It is a
-robust, efficient, and scalable solution for almost any situation.
-
-With the growth of Linux has come an increase in the number of developers
-(and companies) wishing to participate in its development. Hardware
-vendors want to ensure that Linux supports their products well, making
-those products attractive to Linux users. Embedded systems vendors, who
-use Linux as a component in an integrated product, want Linux to be as
-capable and well-suited to the task at hand as possible. Distributors and
-other software vendors who base their products on Linux have a clear
-interest in the capabilities, performance, and reliability of the Linux
-kernel. And end users, too, will often wish to change Linux to make it
-better suit their needs.
-
-One of the most compelling features of Linux is that it is accessible to
-these developers; anybody with the requisite skills can improve Linux and
-influence the direction of its development. Proprietary products cannot
-offer this kind of openness, which is a characteristic of the free software
-process. But, if anything, the kernel is even more open than most other
-free software projects. A typical three-month kernel development cycle can
-involve over 1000 developers working for more than 100 different companies
-(or for no company at all).
-
-Working with the kernel development community is not especially hard. But,
-that notwithstanding, many potential contributors have experienced
-difficulties when trying to do kernel work. The kernel community has
-evolved its own distinct ways of operating which allow it to function
-smoothly (and produce a high-quality product) in an environment where
-thousands of lines of code are being changed every day. So it is not
-surprising that Linux kernel development process differs greatly from
-proprietary development methods.
-
-The kernel's development process may come across as strange and
-intimidating to new developers, but there are good reasons and solid
-experience behind it. A developer who does not understand the kernel
-community's ways (or, worse, who tries to flout or circumvent them) will
-have a frustrating experience in store. The development community, while
-being helpful to those who are trying to learn, has little time for those
-who will not listen or who do not care about the development process.
-
-It is hoped that those who read this document will be able to avoid that
-frustrating experience. There is a lot of material here, but the effort
-involved in reading it will be repaid in short order. The development
-community is always in need of developers who will help to make the kernel
-better; the following text should help you - or those who work for you -
-join our community.
-
-Credits
--------
-
-This document was written by Jonathan Corbet, corbet@lwn.net. It has been
-improved by comments from Johannes Berg, James Berry, Alex Chiang, Roland
-Dreier, Randy Dunlap, Jake Edge, Jiri Kosina, Matt Mackall, Arthur Marsh,
-Amanda McPherson, Andrew Morton, Andrew Price, Tsugikazu Shibata, and
-Jochen Voß.
-
-This work was supported by the Linux Foundation; thanks especially to
-Amanda McPherson, who saw the value of this effort and made it all happen.
-
-The importance of getting code into the mainline
-------------------------------------------------
-
-Some companies and developers occasionally wonder why they should bother
-learning how to work with the kernel community and get their code into the
-mainline kernel (the "mainline" being the kernel maintained by Linus
-Torvalds and used as a base by Linux distributors). In the short term,
-contributing code can look like an avoidable expense; it seems easier to
-just keep the code separate and support users directly. The truth of the
-matter is that keeping code separate ("out of tree") is a false economy.
-
-As a way of illustrating the costs of out-of-tree code, here are a few
-relevant aspects of the kernel development process; most of these will be
-discussed in greater detail later in this document. Consider:
-
-- Code which has been merged into the mainline kernel is available to all
- Linux users. It will automatically be present on all distributions which
- enable it. There is no need for driver disks, downloads, or the hassles
- of supporting multiple versions of multiple distributions; it all just
- works, for the developer and for the user. Incorporation into the
- mainline solves a large number of distribution and support problems.
-
-- While kernel developers strive to maintain a stable interface to user
- space, the internal kernel API is in constant flux. The lack of a stable
- internal interface is a deliberate design decision; it allows fundamental
- improvements to be made at any time and results in higher-quality code.
- But one result of that policy is that any out-of-tree code requires
- constant upkeep if it is to work with new kernels. Maintaining
- out-of-tree code requires significant amounts of work just to keep that
- code working.
-
- Code which is in the mainline, instead, does not require this work as the
- result of a simple rule requiring any developer who makes an API change
- to also fix any code that breaks as the result of that change. So code
- which has been merged into the mainline has significantly lower
- maintenance costs.
-
-- Beyond that, code which is in the kernel will often be improved by other
- developers. Surprising results can come from empowering your user
- community and customers to improve your product.
-
-- Kernel code is subjected to review, both before and after merging into
- the mainline. No matter how strong the original developer's skills are,
- this review process invariably finds ways in which the code can be
- improved. Often review finds severe bugs and security problems. This is
- especially true for code which has been developed in a closed
- environment; such code benefits strongly from review by outside
- developers. Out-of-tree code is lower-quality code.
-
-- Participation in the development process is your way to influence the
- direction of kernel development. Users who complain from the sidelines
- are heard, but active developers have a stronger voice - and the ability
- to implement changes which make the kernel work better for their needs.
-
-- When code is maintained separately, the possibility that a third party
- will contribute a different implementation of a similar feature always
- exists. Should that happen, getting your code merged will become much
- harder - to the point of impossibility. Then you will be faced with the
- unpleasant alternatives of either (1) maintaining a nonstandard feature
- out of tree indefinitely, or (2) abandoning your code and migrating your
- users over to the in-tree version.
-
-- Contribution of code is the fundamental action which makes the whole
- process work. By contributing your code you can add new functionality to
- the kernel and provide capabilities and examples which are of use to
- other kernel developers. If you have developed code for Linux (or are
- thinking about doing so), you clearly have an interest in the continued
- success of this platform; contributing code is one of the best ways to
- help ensure that success.
-
-All of the reasoning above applies to any out-of-tree kernel code,
-including code which is distributed in proprietary, binary-only form.
-There are, however, additional factors which should be taken into account
-before considering any sort of binary-only kernel code distribution. These
-include:
-
-- The legal issues around the distribution of proprietary kernel modules
- are cloudy at best; quite a few kernel copyright holders believe that
- most binary-only modules are derived products of the kernel and that, as
- a result, their distribution is a violation of the GNU General Public
- license (about which more will be said below). Your author is not a
- lawyer, and nothing in this document can possibly be considered to be
- legal advice. The true legal status of closed-source modules can only be
- determined by the courts. But the uncertainty which haunts those modules
- is there regardless.
-
-- Binary modules greatly increase the difficulty of debugging kernel
- problems, to the point that most kernel developers will not even try. So
- the distribution of binary-only modules will make it harder for your
- users to get support from the community.
-
-- Support is also harder for distributors of binary-only modules, who must
- provide a version of the module for every distribution and every kernel
- version they wish to support. Dozens of builds of a single module can
- be required to provide reasonably comprehensive coverage, and your users
- will have to upgrade your module separately every time they upgrade their
- kernel.
-
-- Everything that was said above about code review applies doubly to
- closed-source code. Since this code is not available at all, it cannot
- have been reviewed by the community and will, beyond doubt, have serious
- problems.
-
-Makers of embedded systems, in particular, may be tempted to disregard much
-of what has been said in this section in the belief that they are shipping
-a self-contained product which uses a frozen kernel version and requires no
-more development after its release. This argument misses the value of
-widespread code review and the value of allowing your users to add
-capabilities to your product. But these products, too, have a limited
-commercial life, after which a new version must be released. At that
-point, vendors whose code is in the mainline and well maintained will be
-much better positioned to get the new product ready for market quickly.
-
-Licensing
----------
-
-Code is contributed to the Linux kernel under a number of licenses, but all
-code must be compatible with version 2 of the GNU General Public License
-(GPLv2), which is the license covering the kernel distribution as a whole.
-In practice, that means that all code contributions are covered either by
-GPLv2 (with, optionally, language allowing distribution under later
-versions of the GPL) or the three-clause BSD license. Any contributions
-which are not covered by a compatible license will not be accepted into the
-kernel.
-
-Copyright assignments are not required (or requested) for code contributed
-to the kernel. All code merged into the mainline kernel retains its
-original ownership; as a result, the kernel now has thousands of owners.
-
-One implication of this ownership structure is that any attempt to change
-the licensing of the kernel is doomed to almost certain failure. There are
-few practical scenarios where the agreement of all copyright holders could
-be obtained (or their code removed from the kernel). So, in particular,
-there is no prospect of a migration to version 3 of the GPL in the
-foreseeable future.
-
-It is imperative that all code contributed to the kernel be legitimately
-free software. For that reason, code from anonymous (or pseudonymous)
-contributors will not be accepted. All contributors are required to "sign
-off" on their code, stating that the code can be distributed with the
-kernel under the GPL. Code which has not been licensed as free software by
-its owner, or which risks creating copyright-related problems for the
-kernel (such as code which derives from reverse-engineering efforts lacking
-proper safeguards) cannot be contributed.
-
-Questions about copyright-related issues are common on Linux development
-mailing lists. Such questions will normally receive no shortage of
-answers, but one should bear in mind that the people answering those
-questions are not lawyers and cannot provide legal advice. If you have
-legal questions relating to Linux source code, there is no substitute for
-talking with a lawyer who understands this field. Relying on answers
-obtained on technical mailing lists is a risky affair.
--- /dev/null
+Introdution
+===========
+
+Executive summary
+-----------------
+
+The rest of this section covers the scope of the kernel development process
+and the kinds of frustrations that developers and their employers can
+encounter there. There are a great many reasons why kernel code should be
+merged into the official ("mainline") kernel, including automatic
+availability to users, community support in many forms, and the ability to
+influence the direction of kernel development. Code contributed to the
+Linux kernel must be made available under a GPL-compatible license.
+
+:ref:`development_process` introduces the development process, the kernel
+release cycle, and the mechanics of the merge window. The various phases in
+the patch development, review, and merging cycle are covered. There is some
+discussion of tools and mailing lists. Developers wanting to get started
+with kernel development are encouraged to track down and fix bugs as an
+initial exercise.
+
+:ref:`development_early_stage` covers early-stage project planning, with an
+emphasis on involving the development community as soon as possible.
+
+:ref:`development_coding` is about the coding process; several pitfalls which
+have been encountered by other developers are discussed. Some requirements for
+patches are covered, and there is an introduction to some of the tools
+which can help to ensure that kernel patches are correct.
+
+:ref:`development_posting` talks about the process of posting patches for
+review. To be taken seriously by the development community, patches must be
+properly formatted and described, and they must be sent to the right place.
+Following the advice in this section should help to ensure the best
+possible reception for your work.
+
+:ref:`development_followthrough` covers what happens after posting patches; the
+job is far from done at that point. Working with reviewers is a crucial part
+of the development process; this section offers a number of tips on how to
+avoid problems at this important stage. Developers are cautioned against
+assuming that the job is done when a patch is merged into the mainline.
+
+:ref:`development_advancedtopics` introduces a couple of "advanced" topics:
+managing patches with git and reviewing patches posted by others.
+
+:ref:`development_conclusion` concludes the document with pointers to sources
+for more information on kernel development.
+
+What this document is about
+---------------------------
+
+The Linux kernel, at over 8 million lines of code and well over 1000
+contributors to each release, is one of the largest and most active free
+software projects in existence. Since its humble beginning in 1991, this
+kernel has evolved into a best-of-breed operating system component which
+runs on pocket-sized digital music players, desktop PCs, the largest
+supercomputers in existence, and all types of systems in between. It is a
+robust, efficient, and scalable solution for almost any situation.
+
+With the growth of Linux has come an increase in the number of developers
+(and companies) wishing to participate in its development. Hardware
+vendors want to ensure that Linux supports their products well, making
+those products attractive to Linux users. Embedded systems vendors, who
+use Linux as a component in an integrated product, want Linux to be as
+capable and well-suited to the task at hand as possible. Distributors and
+other software vendors who base their products on Linux have a clear
+interest in the capabilities, performance, and reliability of the Linux
+kernel. And end users, too, will often wish to change Linux to make it
+better suit their needs.
+
+One of the most compelling features of Linux is that it is accessible to
+these developers; anybody with the requisite skills can improve Linux and
+influence the direction of its development. Proprietary products cannot
+offer this kind of openness, which is a characteristic of the free software
+process. But, if anything, the kernel is even more open than most other
+free software projects. A typical three-month kernel development cycle can
+involve over 1000 developers working for more than 100 different companies
+(or for no company at all).
+
+Working with the kernel development community is not especially hard. But,
+that notwithstanding, many potential contributors have experienced
+difficulties when trying to do kernel work. The kernel community has
+evolved its own distinct ways of operating which allow it to function
+smoothly (and produce a high-quality product) in an environment where
+thousands of lines of code are being changed every day. So it is not
+surprising that Linux kernel development process differs greatly from
+proprietary development methods.
+
+The kernel's development process may come across as strange and
+intimidating to new developers, but there are good reasons and solid
+experience behind it. A developer who does not understand the kernel
+community's ways (or, worse, who tries to flout or circumvent them) will
+have a frustrating experience in store. The development community, while
+being helpful to those who are trying to learn, has little time for those
+who will not listen or who do not care about the development process.
+
+It is hoped that those who read this document will be able to avoid that
+frustrating experience. There is a lot of material here, but the effort
+involved in reading it will be repaid in short order. The development
+community is always in need of developers who will help to make the kernel
+better; the following text should help you - or those who work for you -
+join our community.
+
+Credits
+-------
+
+This document was written by Jonathan Corbet, corbet@lwn.net. It has been
+improved by comments from Johannes Berg, James Berry, Alex Chiang, Roland
+Dreier, Randy Dunlap, Jake Edge, Jiri Kosina, Matt Mackall, Arthur Marsh,
+Amanda McPherson, Andrew Morton, Andrew Price, Tsugikazu Shibata, and
+Jochen Voß.
+
+This work was supported by the Linux Foundation; thanks especially to
+Amanda McPherson, who saw the value of this effort and made it all happen.
+
+The importance of getting code into the mainline
+------------------------------------------------
+
+Some companies and developers occasionally wonder why they should bother
+learning how to work with the kernel community and get their code into the
+mainline kernel (the "mainline" being the kernel maintained by Linus
+Torvalds and used as a base by Linux distributors). In the short term,
+contributing code can look like an avoidable expense; it seems easier to
+just keep the code separate and support users directly. The truth of the
+matter is that keeping code separate ("out of tree") is a false economy.
+
+As a way of illustrating the costs of out-of-tree code, here are a few
+relevant aspects of the kernel development process; most of these will be
+discussed in greater detail later in this document. Consider:
+
+- Code which has been merged into the mainline kernel is available to all
+ Linux users. It will automatically be present on all distributions which
+ enable it. There is no need for driver disks, downloads, or the hassles
+ of supporting multiple versions of multiple distributions; it all just
+ works, for the developer and for the user. Incorporation into the
+ mainline solves a large number of distribution and support problems.
+
+- While kernel developers strive to maintain a stable interface to user
+ space, the internal kernel API is in constant flux. The lack of a stable
+ internal interface is a deliberate design decision; it allows fundamental
+ improvements to be made at any time and results in higher-quality code.
+ But one result of that policy is that any out-of-tree code requires
+ constant upkeep if it is to work with new kernels. Maintaining
+ out-of-tree code requires significant amounts of work just to keep that
+ code working.
+
+ Code which is in the mainline, instead, does not require this work as the
+ result of a simple rule requiring any developer who makes an API change
+ to also fix any code that breaks as the result of that change. So code
+ which has been merged into the mainline has significantly lower
+ maintenance costs.
+
+- Beyond that, code which is in the kernel will often be improved by other
+ developers. Surprising results can come from empowering your user
+ community and customers to improve your product.
+
+- Kernel code is subjected to review, both before and after merging into
+ the mainline. No matter how strong the original developer's skills are,
+ this review process invariably finds ways in which the code can be
+ improved. Often review finds severe bugs and security problems. This is
+ especially true for code which has been developed in a closed
+ environment; such code benefits strongly from review by outside
+ developers. Out-of-tree code is lower-quality code.
+
+- Participation in the development process is your way to influence the
+ direction of kernel development. Users who complain from the sidelines
+ are heard, but active developers have a stronger voice - and the ability
+ to implement changes which make the kernel work better for their needs.
+
+- When code is maintained separately, the possibility that a third party
+ will contribute a different implementation of a similar feature always
+ exists. Should that happen, getting your code merged will become much
+ harder - to the point of impossibility. Then you will be faced with the
+ unpleasant alternatives of either (1) maintaining a nonstandard feature
+ out of tree indefinitely, or (2) abandoning your code and migrating your
+ users over to the in-tree version.
+
+- Contribution of code is the fundamental action which makes the whole
+ process work. By contributing your code you can add new functionality to
+ the kernel and provide capabilities and examples which are of use to
+ other kernel developers. If you have developed code for Linux (or are
+ thinking about doing so), you clearly have an interest in the continued
+ success of this platform; contributing code is one of the best ways to
+ help ensure that success.
+
+All of the reasoning above applies to any out-of-tree kernel code,
+including code which is distributed in proprietary, binary-only form.
+There are, however, additional factors which should be taken into account
+before considering any sort of binary-only kernel code distribution. These
+include:
+
+- The legal issues around the distribution of proprietary kernel modules
+ are cloudy at best; quite a few kernel copyright holders believe that
+ most binary-only modules are derived products of the kernel and that, as
+ a result, their distribution is a violation of the GNU General Public
+ license (about which more will be said below). Your author is not a
+ lawyer, and nothing in this document can possibly be considered to be
+ legal advice. The true legal status of closed-source modules can only be
+ determined by the courts. But the uncertainty which haunts those modules
+ is there regardless.
+
+- Binary modules greatly increase the difficulty of debugging kernel
+ problems, to the point that most kernel developers will not even try. So
+ the distribution of binary-only modules will make it harder for your
+ users to get support from the community.
+
+- Support is also harder for distributors of binary-only modules, who must
+ provide a version of the module for every distribution and every kernel
+ version they wish to support. Dozens of builds of a single module can
+ be required to provide reasonably comprehensive coverage, and your users
+ will have to upgrade your module separately every time they upgrade their
+ kernel.
+
+- Everything that was said above about code review applies doubly to
+ closed-source code. Since this code is not available at all, it cannot
+ have been reviewed by the community and will, beyond doubt, have serious
+ problems.
+
+Makers of embedded systems, in particular, may be tempted to disregard much
+of what has been said in this section in the belief that they are shipping
+a self-contained product which uses a frozen kernel version and requires no
+more development after its release. This argument misses the value of
+widespread code review and the value of allowing your users to add
+capabilities to your product. But these products, too, have a limited
+commercial life, after which a new version must be released. At that
+point, vendors whose code is in the mainline and well maintained will be
+much better positioned to get the new product ready for market quickly.
+
+Licensing
+---------
+
+Code is contributed to the Linux kernel under a number of licenses, but all
+code must be compatible with version 2 of the GNU General Public License
+(GPLv2), which is the license covering the kernel distribution as a whole.
+In practice, that means that all code contributions are covered either by
+GPLv2 (with, optionally, language allowing distribution under later
+versions of the GPL) or the three-clause BSD license. Any contributions
+which are not covered by a compatible license will not be accepted into the
+kernel.
+
+Copyright assignments are not required (or requested) for code contributed
+to the kernel. All code merged into the mainline kernel retains its
+original ownership; as a result, the kernel now has thousands of owners.
+
+One implication of this ownership structure is that any attempt to change
+the licensing of the kernel is doomed to almost certain failure. There are
+few practical scenarios where the agreement of all copyright holders could
+be obtained (or their code removed from the kernel). So, in particular,
+there is no prospect of a migration to version 3 of the GPL in the
+foreseeable future.
+
+It is imperative that all code contributed to the kernel be legitimately
+free software. For that reason, code from anonymous (or pseudonymous)
+contributors will not be accepted. All contributors are required to "sign
+off" on their code, stating that the code can be distributed with the
+kernel under the GPL. Code which has not been licensed as free software by
+its owner, or which risks creating copyright-related problems for the
+kernel (such as code which derives from reverse-engineering efforts lacking
+proper safeguards) cannot be contributed.
+
+Questions about copyright-related issues are common on Linux development
+mailing lists. Such questions will normally receive no shortage of
+answers, but one should bear in mind that the people answering those
+questions are not lawyers and cannot provide legal advice. If you have
+legal questions relating to Linux source code, there is no substitute for
+talking with a lawyer who understands this field. Relying on answers
+obtained on technical mailing lists is a risky affair.
+++ /dev/null
-.. _development_process:
-
-How the development process works
-=================================
-
-Linux kernel development in the early 1990's was a pretty loose affair,
-with relatively small numbers of users and developers involved. With a
-user base in the millions and with some 2,000 developers involved over the
-course of one year, the kernel has since had to evolve a number of
-processes to keep development happening smoothly. A solid understanding of
-how the process works is required in order to be an effective part of it.
-
-The big picture
----------------
-
-The kernel developers use a loosely time-based release process, with a new
-major kernel release happening every two or three months. The recent
-release history looks like this:
-
- ====== =================
- 2.6.38 March 14, 2011
- 2.6.37 January 4, 2011
- 2.6.36 October 20, 2010
- 2.6.35 August 1, 2010
- 2.6.34 May 15, 2010
- 2.6.33 February 24, 2010
- ====== =================
-
-Every 2.6.x release is a major kernel release with new features, internal
-API changes, and more. A typical 2.6 release can contain nearly 10,000
-changesets with changes to several hundred thousand lines of code. 2.6 is
-thus the leading edge of Linux kernel development; the kernel uses a
-rolling development model which is continually integrating major changes.
-
-A relatively straightforward discipline is followed with regard to the
-merging of patches for each release. At the beginning of each development
-cycle, the "merge window" is said to be open. At that time, code which is
-deemed to be sufficiently stable (and which is accepted by the development
-community) is merged into the mainline kernel. The bulk of changes for a
-new development cycle (and all of the major changes) will be merged during
-this time, at a rate approaching 1,000 changes ("patches," or "changesets")
-per day.
-
-(As an aside, it is worth noting that the changes integrated during the
-merge window do not come out of thin air; they have been collected, tested,
-and staged ahead of time. How that process works will be described in
-detail later on).
-
-The merge window lasts for approximately two weeks. At the end of this
-time, Linus Torvalds will declare that the window is closed and release the
-first of the "rc" kernels. For the kernel which is destined to be 2.6.40,
-for example, the release which happens at the end of the merge window will
-be called 2.6.40-rc1. The -rc1 release is the signal that the time to
-merge new features has passed, and that the time to stabilize the next
-kernel has begun.
-
-Over the next six to ten weeks, only patches which fix problems should be
-submitted to the mainline. On occasion a more significant change will be
-allowed, but such occasions are rare; developers who try to merge new
-features outside of the merge window tend to get an unfriendly reception.
-As a general rule, if you miss the merge window for a given feature, the
-best thing to do is to wait for the next development cycle. (An occasional
-exception is made for drivers for previously-unsupported hardware; if they
-touch no in-tree code, they cannot cause regressions and should be safe to
-add at any time).
-
-As fixes make their way into the mainline, the patch rate will slow over
-time. Linus releases new -rc kernels about once a week; a normal series
-will get up to somewhere between -rc6 and -rc9 before the kernel is
-considered to be sufficiently stable and the final 2.6.x release is made.
-At that point the whole process starts over again.
-
-As an example, here is how the 2.6.38 development cycle went (all dates in
-2011):
-
- ============== ===============================
- January 4 2.6.37 stable release
- January 18 2.6.38-rc1, merge window closes
- January 21 2.6.38-rc2
- February 1 2.6.38-rc3
- February 7 2.6.38-rc4
- February 15 2.6.38-rc5
- February 21 2.6.38-rc6
- March 1 2.6.38-rc7
- March 7 2.6.38-rc8
- March 14 2.6.38 stable release
- ============== ===============================
-
-How do the developers decide when to close the development cycle and create
-the stable release? The most significant metric used is the list of
-regressions from previous releases. No bugs are welcome, but those which
-break systems which worked in the past are considered to be especially
-serious. For this reason, patches which cause regressions are looked upon
-unfavorably and are quite likely to be reverted during the stabilization
-period.
-
-The developers' goal is to fix all known regressions before the stable
-release is made. In the real world, this kind of perfection is hard to
-achieve; there are just too many variables in a project of this size.
-There comes a point where delaying the final release just makes the problem
-worse; the pile of changes waiting for the next merge window will grow
-larger, creating even more regressions the next time around. So most 2.6.x
-kernels go out with a handful of known regressions though, hopefully, none
-of them are serious.
-
-Once a stable release is made, its ongoing maintenance is passed off to the
-"stable team," currently consisting of Greg Kroah-Hartman. The stable team
-will release occasional updates to the stable release using the 2.6.x.y
-numbering scheme. To be considered for an update release, a patch must (1)
-fix a significant bug, and (2) already be merged into the mainline for the
-next development kernel. Kernels will typically receive stable updates for
-a little more than one development cycle past their initial release. So,
-for example, the 2.6.36 kernel's history looked like:
-
- ============== ===============================
- October 10 2.6.36 stable release
- November 22 2.6.36.1
- December 9 2.6.36.2
- January 7 2.6.36.3
- February 17 2.6.36.4
- ============== ===============================
-
-2.6.36.4 was the final stable update for the 2.6.36 release.
-
-Some kernels are designated "long term" kernels; they will receive support
-for a longer period. As of this writing, the current long term kernels
-and their maintainers are:
-
- ====== ====================== ===========================
- 2.6.27 Willy Tarreau (Deep-frozen stable kernel)
- 2.6.32 Greg Kroah-Hartman
- 2.6.35 Andi Kleen (Embedded flag kernel)
- ====== ====================== ===========================
-
-The selection of a kernel for long-term support is purely a matter of a
-maintainer having the need and the time to maintain that release. There
-are no known plans for long-term support for any specific upcoming
-release.
-
-
-The lifecycle of a patch
-------------------------
-
-Patches do not go directly from the developer's keyboard into the mainline
-kernel. There is, instead, a somewhat involved (if somewhat informal)
-process designed to ensure that each patch is reviewed for quality and that
-each patch implements a change which is desirable to have in the mainline.
-This process can happen quickly for minor fixes, or, in the case of large
-and controversial changes, go on for years. Much developer frustration
-comes from a lack of understanding of this process or from attempts to
-circumvent it.
-
-In the hopes of reducing that frustration, this document will describe how
-a patch gets into the kernel. What follows below is an introduction which
-describes the process in a somewhat idealized way. A much more detailed
-treatment will come in later sections.
-
-The stages that a patch goes through are, generally:
-
- - Design. This is where the real requirements for the patch - and the way
- those requirements will be met - are laid out. Design work is often
- done without involving the community, but it is better to do this work
- in the open if at all possible; it can save a lot of time redesigning
- things later.
-
- - Early review. Patches are posted to the relevant mailing list, and
- developers on that list reply with any comments they may have. This
- process should turn up any major problems with a patch if all goes
- well.
-
- - Wider review. When the patch is getting close to ready for mainline
- inclusion, it should be accepted by a relevant subsystem maintainer -
- though this acceptance is not a guarantee that the patch will make it
- all the way to the mainline. The patch will show up in the maintainer's
- subsystem tree and into the -next trees (described below). When the
- process works, this step leads to more extensive review of the patch and
- the discovery of any problems resulting from the integration of this
- patch with work being done by others.
-
-- Please note that most maintainers also have day jobs, so merging
- your patch may not be their highest priority. If your patch is
- getting feedback about changes that are needed, you should either
- make those changes or justify why they should not be made. If your
- patch has no review complaints but is not being merged by its
- appropriate subsystem or driver maintainer, you should be persistent
- in updating the patch to the current kernel so that it applies cleanly
- and keep sending it for review and merging.
-
- - Merging into the mainline. Eventually, a successful patch will be
- merged into the mainline repository managed by Linus Torvalds. More
- comments and/or problems may surface at this time; it is important that
- the developer be responsive to these and fix any issues which arise.
-
- - Stable release. The number of users potentially affected by the patch
- is now large, so, once again, new problems may arise.
-
- - Long-term maintenance. While it is certainly possible for a developer
- to forget about code after merging it, that sort of behavior tends to
- leave a poor impression in the development community. Merging code
- eliminates some of the maintenance burden, in that others will fix
- problems caused by API changes. But the original developer should
- continue to take responsibility for the code if it is to remain useful
- in the longer term.
-
-One of the largest mistakes made by kernel developers (or their employers)
-is to try to cut the process down to a single "merging into the mainline"
-step. This approach invariably leads to frustration for everybody
-involved.
-
-How patches get into the Kernel
--------------------------------
-
-There is exactly one person who can merge patches into the mainline kernel
-repository: Linus Torvalds. But, of the over 9,500 patches which went
-into the 2.6.38 kernel, only 112 (around 1.3%) were directly chosen by Linus
-himself. The kernel project has long since grown to a size where no single
-developer could possibly inspect and select every patch unassisted. The
-way the kernel developers have addressed this growth is through the use of
-a lieutenant system built around a chain of trust.
-
-The kernel code base is logically broken down into a set of subsystems:
-networking, specific architecture support, memory management, video
-devices, etc. Most subsystems have a designated maintainer, a developer
-who has overall responsibility for the code within that subsystem. These
-subsystem maintainers are the gatekeepers (in a loose way) for the portion
-of the kernel they manage; they are the ones who will (usually) accept a
-patch for inclusion into the mainline kernel.
-
-Subsystem maintainers each manage their own version of the kernel source
-tree, usually (but certainly not always) using the git source management
-tool. Tools like git (and related tools like quilt or mercurial) allow
-maintainers to track a list of patches, including authorship information
-and other metadata. At any given time, the maintainer can identify which
-patches in his or her repository are not found in the mainline.
-
-When the merge window opens, top-level maintainers will ask Linus to "pull"
-the patches they have selected for merging from their repositories. If
-Linus agrees, the stream of patches will flow up into his repository,
-becoming part of the mainline kernel. The amount of attention that Linus
-pays to specific patches received in a pull operation varies. It is clear
-that, sometimes, he looks quite closely. But, as a general rule, Linus
-trusts the subsystem maintainers to not send bad patches upstream.
-
-Subsystem maintainers, in turn, can pull patches from other maintainers.
-For example, the networking tree is built from patches which accumulated
-first in trees dedicated to network device drivers, wireless networking,
-etc. This chain of repositories can be arbitrarily long, though it rarely
-exceeds two or three links. Since each maintainer in the chain trusts
-those managing lower-level trees, this process is known as the "chain of
-trust."
-
-Clearly, in a system like this, getting patches into the kernel depends on
-finding the right maintainer. Sending patches directly to Linus is not
-normally the right way to go.
-
-
-Next trees
-----------
-
-The chain of subsystem trees guides the flow of patches into the kernel,
-but it also raises an interesting question: what if somebody wants to look
-at all of the patches which are being prepared for the next merge window?
-Developers will be interested in what other changes are pending to see
-whether there are any conflicts to worry about; a patch which changes a
-core kernel function prototype, for example, will conflict with any other
-patches which use the older form of that function. Reviewers and testers
-want access to the changes in their integrated form before all of those
-changes land in the mainline kernel. One could pull changes from all of
-the interesting subsystem trees, but that would be a big and error-prone
-job.
-
-The answer comes in the form of -next trees, where subsystem trees are
-collected for testing and review. The older of these trees, maintained by
-Andrew Morton, is called "-mm" (for memory management, which is how it got
-started). The -mm tree integrates patches from a long list of subsystem
-trees; it also has some patches aimed at helping with debugging.
-
-Beyond that, -mm contains a significant collection of patches which have
-been selected by Andrew directly. These patches may have been posted on a
-mailing list, or they may apply to a part of the kernel for which there is
-no designated subsystem tree. As a result, -mm operates as a sort of
-subsystem tree of last resort; if there is no other obvious path for a
-patch into the mainline, it is likely to end up in -mm. Miscellaneous
-patches which accumulate in -mm will eventually either be forwarded on to
-an appropriate subsystem tree or be sent directly to Linus. In a typical
-development cycle, approximately 5-10% of the patches going into the
-mainline get there via -mm.
-
-The current -mm patch is available in the "mmotm" (-mm of the moment)
-directory at:
-
- http://www.ozlabs.org/~akpm/mmotm/
-
-Use of the MMOTM tree is likely to be a frustrating experience, though;
-there is a definite chance that it will not even compile.
-
-The primary tree for next-cycle patch merging is linux-next, maintained by
-Stephen Rothwell. The linux-next tree is, by design, a snapshot of what
-the mainline is expected to look like after the next merge window closes.
-Linux-next trees are announced on the linux-kernel and linux-next mailing
-lists when they are assembled; they can be downloaded from:
-
- http://www.kernel.org/pub/linux/kernel/next/
-
-Linux-next has become an integral part of the kernel development process;
-all patches merged during a given merge window should really have found
-their way into linux-next some time before the merge window opens.
-
-
-Staging trees
--------------
-
-The kernel source tree contains the drivers/staging/ directory, where
-many sub-directories for drivers or filesystems that are on their way to
-being added to the kernel tree live. They remain in drivers/staging while
-they still need more work; once complete, they can be moved into the
-kernel proper. This is a way to keep track of drivers that aren't
-up to Linux kernel coding or quality standards, but people may want to use
-them and track development.
-
-Greg Kroah-Hartman currently maintains the staging tree. Drivers that
-still need work are sent to him, with each driver having its own
-subdirectory in drivers/staging/. Along with the driver source files, a
-TODO file should be present in the directory as well. The TODO file lists
-the pending work that the driver needs for acceptance into the kernel
-proper, as well as a list of people that should be Cc'd for any patches to
-the driver. Current rules require that drivers contributed to staging
-must, at a minimum, compile properly.
-
-Staging can be a relatively easy way to get new drivers into the mainline
-where, with luck, they will come to the attention of other developers and
-improve quickly. Entry into staging is not the end of the story, though;
-code in staging which is not seeing regular progress will eventually be
-removed. Distributors also tend to be relatively reluctant to enable
-staging drivers. So staging is, at best, a stop on the way toward becoming
-a proper mainline driver.
-
-
-Tools
------
-
-As can be seen from the above text, the kernel development process depends
-heavily on the ability to herd collections of patches in various
-directions. The whole thing would not work anywhere near as well as it
-does without suitably powerful tools. Tutorials on how to use these tools
-are well beyond the scope of this document, but there is space for a few
-pointers.
-
-By far the dominant source code management system used by the kernel
-community is git. Git is one of a number of distributed version control
-systems being developed in the free software community. It is well tuned
-for kernel development, in that it performs quite well when dealing with
-large repositories and large numbers of patches. It also has a reputation
-for being difficult to learn and use, though it has gotten better over
-time. Some sort of familiarity with git is almost a requirement for kernel
-developers; even if they do not use it for their own work, they'll need git
-to keep up with what other developers (and the mainline) are doing.
-
-Git is now packaged by almost all Linux distributions. There is a home
-page at:
-
- http://git-scm.com/
-
-That page has pointers to documentation and tutorials.
-
-Among the kernel developers who do not use git, the most popular choice is
-almost certainly Mercurial:
-
- http://www.selenic.com/mercurial/
-
-Mercurial shares many features with git, but it provides an interface which
-many find easier to use.
-
-The other tool worth knowing about is Quilt:
-
- http://savannah.nongnu.org/projects/quilt/
-
-Quilt is a patch management system, rather than a source code management
-system. It does not track history over time; it is, instead, oriented
-toward tracking a specific set of changes against an evolving code base.
-Some major subsystem maintainers use quilt to manage patches intended to go
-upstream. For the management of certain kinds of trees (-mm, for example),
-quilt is the best tool for the job.
-
-
-Mailing lists
--------------
-
-A great deal of Linux kernel development work is done by way of mailing
-lists. It is hard to be a fully-functioning member of the community
-without joining at least one list somewhere. But Linux mailing lists also
-represent a potential hazard to developers, who risk getting buried under a
-load of electronic mail, running afoul of the conventions used on the Linux
-lists, or both.
-
-Most kernel mailing lists are run on vger.kernel.org; the master list can
-be found at:
-
- http://vger.kernel.org/vger-lists.html
-
-There are lists hosted elsewhere, though; a number of them are at
-lists.redhat.com.
-
-The core mailing list for kernel development is, of course, linux-kernel.
-This list is an intimidating place to be; volume can reach 500 messages per
-day, the amount of noise is high, the conversation can be severely
-technical, and participants are not always concerned with showing a high
-degree of politeness. But there is no other place where the kernel
-development community comes together as a whole; developers who avoid this
-list will miss important information.
-
-There are a few hints which can help with linux-kernel survival:
-
-- Have the list delivered to a separate folder, rather than your main
- mailbox. One must be able to ignore the stream for sustained periods of
- time.
-
-- Do not try to follow every conversation - nobody else does. It is
- important to filter on both the topic of interest (though note that
- long-running conversations can drift away from the original subject
- without changing the email subject line) and the people who are
- participating.
-
-- Do not feed the trolls. If somebody is trying to stir up an angry
- response, ignore them.
-
-- When responding to linux-kernel email (or that on other lists) preserve
- the Cc: header for all involved. In the absence of a strong reason (such
- as an explicit request), you should never remove recipients. Always make
- sure that the person you are responding to is in the Cc: list. This
- convention also makes it unnecessary to explicitly ask to be copied on
- replies to your postings.
-
-- Search the list archives (and the net as a whole) before asking
- questions. Some developers can get impatient with people who clearly
- have not done their homework.
-
-- Avoid top-posting (the practice of putting your answer above the quoted
- text you are responding to). It makes your response harder to read and
- makes a poor impression.
-
-- Ask on the correct mailing list. Linux-kernel may be the general meeting
- point, but it is not the best place to find developers from all
- subsystems.
-
-The last point - finding the correct mailing list - is a common place for
-beginning developers to go wrong. Somebody who asks a networking-related
-question on linux-kernel will almost certainly receive a polite suggestion
-to ask on the netdev list instead, as that is the list frequented by most
-networking developers. Other lists exist for the SCSI, video4linux, IDE,
-filesystem, etc. subsystems. The best place to look for mailing lists is
-in the MAINTAINERS file packaged with the kernel source.
-
-
-Getting started with Kernel development
----------------------------------------
-
-Questions about how to get started with the kernel development process are
-common - from both individuals and companies. Equally common are missteps
-which make the beginning of the relationship harder than it has to be.
-
-Companies often look to hire well-known developers to get a development
-group started. This can, in fact, be an effective technique. But it also
-tends to be expensive and does not do much to grow the pool of experienced
-kernel developers. It is possible to bring in-house developers up to speed
-on Linux kernel development, given the investment of a bit of time. Taking
-this time can endow an employer with a group of developers who understand
-the kernel and the company both, and who can help to train others as well.
-Over the medium term, this is often the more profitable approach.
-
-Individual developers are often, understandably, at a loss for a place to
-start. Beginning with a large project can be intimidating; one often wants
-to test the waters with something smaller first. This is the point where
-some developers jump into the creation of patches fixing spelling errors or
-minor coding style issues. Unfortunately, such patches create a level of
-noise which is distracting for the development community as a whole, so,
-increasingly, they are looked down upon. New developers wishing to
-introduce themselves to the community will not get the sort of reception
-they wish for by these means.
-
-Andrew Morton gives this advice for aspiring kernel developers
-
-::
-
- The #1 project for all kernel beginners should surely be "make sure
- that the kernel runs perfectly at all times on all machines which
- you can lay your hands on". Usually the way to do this is to work
- with others on getting things fixed up (this can require
- persistence!) but that's fine - it's a part of kernel development.
-
-(http://lwn.net/Articles/283982/).
-
-In the absence of obvious problems to fix, developers are advised to look
-at the current lists of regressions and open bugs in general. There is
-never any shortage of issues in need of fixing; by addressing these issues,
-developers will gain experience with the process while, at the same time,
-building respect with the rest of the development community.
--- /dev/null
+.. _development_process:
+
+How the development process works
+=================================
+
+Linux kernel development in the early 1990's was a pretty loose affair,
+with relatively small numbers of users and developers involved. With a
+user base in the millions and with some 2,000 developers involved over the
+course of one year, the kernel has since had to evolve a number of
+processes to keep development happening smoothly. A solid understanding of
+how the process works is required in order to be an effective part of it.
+
+The big picture
+---------------
+
+The kernel developers use a loosely time-based release process, with a new
+major kernel release happening every two or three months. The recent
+release history looks like this:
+
+ ====== =================
+ 2.6.38 March 14, 2011
+ 2.6.37 January 4, 2011
+ 2.6.36 October 20, 2010
+ 2.6.35 August 1, 2010
+ 2.6.34 May 15, 2010
+ 2.6.33 February 24, 2010
+ ====== =================
+
+Every 2.6.x release is a major kernel release with new features, internal
+API changes, and more. A typical 2.6 release can contain nearly 10,000
+changesets with changes to several hundred thousand lines of code. 2.6 is
+thus the leading edge of Linux kernel development; the kernel uses a
+rolling development model which is continually integrating major changes.
+
+A relatively straightforward discipline is followed with regard to the
+merging of patches for each release. At the beginning of each development
+cycle, the "merge window" is said to be open. At that time, code which is
+deemed to be sufficiently stable (and which is accepted by the development
+community) is merged into the mainline kernel. The bulk of changes for a
+new development cycle (and all of the major changes) will be merged during
+this time, at a rate approaching 1,000 changes ("patches," or "changesets")
+per day.
+
+(As an aside, it is worth noting that the changes integrated during the
+merge window do not come out of thin air; they have been collected, tested,
+and staged ahead of time. How that process works will be described in
+detail later on).
+
+The merge window lasts for approximately two weeks. At the end of this
+time, Linus Torvalds will declare that the window is closed and release the
+first of the "rc" kernels. For the kernel which is destined to be 2.6.40,
+for example, the release which happens at the end of the merge window will
+be called 2.6.40-rc1. The -rc1 release is the signal that the time to
+merge new features has passed, and that the time to stabilize the next
+kernel has begun.
+
+Over the next six to ten weeks, only patches which fix problems should be
+submitted to the mainline. On occasion a more significant change will be
+allowed, but such occasions are rare; developers who try to merge new
+features outside of the merge window tend to get an unfriendly reception.
+As a general rule, if you miss the merge window for a given feature, the
+best thing to do is to wait for the next development cycle. (An occasional
+exception is made for drivers for previously-unsupported hardware; if they
+touch no in-tree code, they cannot cause regressions and should be safe to
+add at any time).
+
+As fixes make their way into the mainline, the patch rate will slow over
+time. Linus releases new -rc kernels about once a week; a normal series
+will get up to somewhere between -rc6 and -rc9 before the kernel is
+considered to be sufficiently stable and the final 2.6.x release is made.
+At that point the whole process starts over again.
+
+As an example, here is how the 2.6.38 development cycle went (all dates in
+2011):
+
+ ============== ===============================
+ January 4 2.6.37 stable release
+ January 18 2.6.38-rc1, merge window closes
+ January 21 2.6.38-rc2
+ February 1 2.6.38-rc3
+ February 7 2.6.38-rc4
+ February 15 2.6.38-rc5
+ February 21 2.6.38-rc6
+ March 1 2.6.38-rc7
+ March 7 2.6.38-rc8
+ March 14 2.6.38 stable release
+ ============== ===============================
+
+How do the developers decide when to close the development cycle and create
+the stable release? The most significant metric used is the list of
+regressions from previous releases. No bugs are welcome, but those which
+break systems which worked in the past are considered to be especially
+serious. For this reason, patches which cause regressions are looked upon
+unfavorably and are quite likely to be reverted during the stabilization
+period.
+
+The developers' goal is to fix all known regressions before the stable
+release is made. In the real world, this kind of perfection is hard to
+achieve; there are just too many variables in a project of this size.
+There comes a point where delaying the final release just makes the problem
+worse; the pile of changes waiting for the next merge window will grow
+larger, creating even more regressions the next time around. So most 2.6.x
+kernels go out with a handful of known regressions though, hopefully, none
+of them are serious.
+
+Once a stable release is made, its ongoing maintenance is passed off to the
+"stable team," currently consisting of Greg Kroah-Hartman. The stable team
+will release occasional updates to the stable release using the 2.6.x.y
+numbering scheme. To be considered for an update release, a patch must (1)
+fix a significant bug, and (2) already be merged into the mainline for the
+next development kernel. Kernels will typically receive stable updates for
+a little more than one development cycle past their initial release. So,
+for example, the 2.6.36 kernel's history looked like:
+
+ ============== ===============================
+ October 10 2.6.36 stable release
+ November 22 2.6.36.1
+ December 9 2.6.36.2
+ January 7 2.6.36.3
+ February 17 2.6.36.4
+ ============== ===============================
+
+2.6.36.4 was the final stable update for the 2.6.36 release.
+
+Some kernels are designated "long term" kernels; they will receive support
+for a longer period. As of this writing, the current long term kernels
+and their maintainers are:
+
+ ====== ====================== ===========================
+ 2.6.27 Willy Tarreau (Deep-frozen stable kernel)
+ 2.6.32 Greg Kroah-Hartman
+ 2.6.35 Andi Kleen (Embedded flag kernel)
+ ====== ====================== ===========================
+
+The selection of a kernel for long-term support is purely a matter of a
+maintainer having the need and the time to maintain that release. There
+are no known plans for long-term support for any specific upcoming
+release.
+
+
+The lifecycle of a patch
+------------------------
+
+Patches do not go directly from the developer's keyboard into the mainline
+kernel. There is, instead, a somewhat involved (if somewhat informal)
+process designed to ensure that each patch is reviewed for quality and that
+each patch implements a change which is desirable to have in the mainline.
+This process can happen quickly for minor fixes, or, in the case of large
+and controversial changes, go on for years. Much developer frustration
+comes from a lack of understanding of this process or from attempts to
+circumvent it.
+
+In the hopes of reducing that frustration, this document will describe how
+a patch gets into the kernel. What follows below is an introduction which
+describes the process in a somewhat idealized way. A much more detailed
+treatment will come in later sections.
+
+The stages that a patch goes through are, generally:
+
+ - Design. This is where the real requirements for the patch - and the way
+ those requirements will be met - are laid out. Design work is often
+ done without involving the community, but it is better to do this work
+ in the open if at all possible; it can save a lot of time redesigning
+ things later.
+
+ - Early review. Patches are posted to the relevant mailing list, and
+ developers on that list reply with any comments they may have. This
+ process should turn up any major problems with a patch if all goes
+ well.
+
+ - Wider review. When the patch is getting close to ready for mainline
+ inclusion, it should be accepted by a relevant subsystem maintainer -
+ though this acceptance is not a guarantee that the patch will make it
+ all the way to the mainline. The patch will show up in the maintainer's
+ subsystem tree and into the -next trees (described below). When the
+ process works, this step leads to more extensive review of the patch and
+ the discovery of any problems resulting from the integration of this
+ patch with work being done by others.
+
+- Please note that most maintainers also have day jobs, so merging
+ your patch may not be their highest priority. If your patch is
+ getting feedback about changes that are needed, you should either
+ make those changes or justify why they should not be made. If your
+ patch has no review complaints but is not being merged by its
+ appropriate subsystem or driver maintainer, you should be persistent
+ in updating the patch to the current kernel so that it applies cleanly
+ and keep sending it for review and merging.
+
+ - Merging into the mainline. Eventually, a successful patch will be
+ merged into the mainline repository managed by Linus Torvalds. More
+ comments and/or problems may surface at this time; it is important that
+ the developer be responsive to these and fix any issues which arise.
+
+ - Stable release. The number of users potentially affected by the patch
+ is now large, so, once again, new problems may arise.
+
+ - Long-term maintenance. While it is certainly possible for a developer
+ to forget about code after merging it, that sort of behavior tends to
+ leave a poor impression in the development community. Merging code
+ eliminates some of the maintenance burden, in that others will fix
+ problems caused by API changes. But the original developer should
+ continue to take responsibility for the code if it is to remain useful
+ in the longer term.
+
+One of the largest mistakes made by kernel developers (or their employers)
+is to try to cut the process down to a single "merging into the mainline"
+step. This approach invariably leads to frustration for everybody
+involved.
+
+How patches get into the Kernel
+-------------------------------
+
+There is exactly one person who can merge patches into the mainline kernel
+repository: Linus Torvalds. But, of the over 9,500 patches which went
+into the 2.6.38 kernel, only 112 (around 1.3%) were directly chosen by Linus
+himself. The kernel project has long since grown to a size where no single
+developer could possibly inspect and select every patch unassisted. The
+way the kernel developers have addressed this growth is through the use of
+a lieutenant system built around a chain of trust.
+
+The kernel code base is logically broken down into a set of subsystems:
+networking, specific architecture support, memory management, video
+devices, etc. Most subsystems have a designated maintainer, a developer
+who has overall responsibility for the code within that subsystem. These
+subsystem maintainers are the gatekeepers (in a loose way) for the portion
+of the kernel they manage; they are the ones who will (usually) accept a
+patch for inclusion into the mainline kernel.
+
+Subsystem maintainers each manage their own version of the kernel source
+tree, usually (but certainly not always) using the git source management
+tool. Tools like git (and related tools like quilt or mercurial) allow
+maintainers to track a list of patches, including authorship information
+and other metadata. At any given time, the maintainer can identify which
+patches in his or her repository are not found in the mainline.
+
+When the merge window opens, top-level maintainers will ask Linus to "pull"
+the patches they have selected for merging from their repositories. If
+Linus agrees, the stream of patches will flow up into his repository,
+becoming part of the mainline kernel. The amount of attention that Linus
+pays to specific patches received in a pull operation varies. It is clear
+that, sometimes, he looks quite closely. But, as a general rule, Linus
+trusts the subsystem maintainers to not send bad patches upstream.
+
+Subsystem maintainers, in turn, can pull patches from other maintainers.
+For example, the networking tree is built from patches which accumulated
+first in trees dedicated to network device drivers, wireless networking,
+etc. This chain of repositories can be arbitrarily long, though it rarely
+exceeds two or three links. Since each maintainer in the chain trusts
+those managing lower-level trees, this process is known as the "chain of
+trust."
+
+Clearly, in a system like this, getting patches into the kernel depends on
+finding the right maintainer. Sending patches directly to Linus is not
+normally the right way to go.
+
+
+Next trees
+----------
+
+The chain of subsystem trees guides the flow of patches into the kernel,
+but it also raises an interesting question: what if somebody wants to look
+at all of the patches which are being prepared for the next merge window?
+Developers will be interested in what other changes are pending to see
+whether there are any conflicts to worry about; a patch which changes a
+core kernel function prototype, for example, will conflict with any other
+patches which use the older form of that function. Reviewers and testers
+want access to the changes in their integrated form before all of those
+changes land in the mainline kernel. One could pull changes from all of
+the interesting subsystem trees, but that would be a big and error-prone
+job.
+
+The answer comes in the form of -next trees, where subsystem trees are
+collected for testing and review. The older of these trees, maintained by
+Andrew Morton, is called "-mm" (for memory management, which is how it got
+started). The -mm tree integrates patches from a long list of subsystem
+trees; it also has some patches aimed at helping with debugging.
+
+Beyond that, -mm contains a significant collection of patches which have
+been selected by Andrew directly. These patches may have been posted on a
+mailing list, or they may apply to a part of the kernel for which there is
+no designated subsystem tree. As a result, -mm operates as a sort of
+subsystem tree of last resort; if there is no other obvious path for a
+patch into the mainline, it is likely to end up in -mm. Miscellaneous
+patches which accumulate in -mm will eventually either be forwarded on to
+an appropriate subsystem tree or be sent directly to Linus. In a typical
+development cycle, approximately 5-10% of the patches going into the
+mainline get there via -mm.
+
+The current -mm patch is available in the "mmotm" (-mm of the moment)
+directory at:
+
+ http://www.ozlabs.org/~akpm/mmotm/
+
+Use of the MMOTM tree is likely to be a frustrating experience, though;
+there is a definite chance that it will not even compile.
+
+The primary tree for next-cycle patch merging is linux-next, maintained by
+Stephen Rothwell. The linux-next tree is, by design, a snapshot of what
+the mainline is expected to look like after the next merge window closes.
+Linux-next trees are announced on the linux-kernel and linux-next mailing
+lists when they are assembled; they can be downloaded from:
+
+ http://www.kernel.org/pub/linux/kernel/next/
+
+Linux-next has become an integral part of the kernel development process;
+all patches merged during a given merge window should really have found
+their way into linux-next some time before the merge window opens.
+
+
+Staging trees
+-------------
+
+The kernel source tree contains the drivers/staging/ directory, where
+many sub-directories for drivers or filesystems that are on their way to
+being added to the kernel tree live. They remain in drivers/staging while
+they still need more work; once complete, they can be moved into the
+kernel proper. This is a way to keep track of drivers that aren't
+up to Linux kernel coding or quality standards, but people may want to use
+them and track development.
+
+Greg Kroah-Hartman currently maintains the staging tree. Drivers that
+still need work are sent to him, with each driver having its own
+subdirectory in drivers/staging/. Along with the driver source files, a
+TODO file should be present in the directory as well. The TODO file lists
+the pending work that the driver needs for acceptance into the kernel
+proper, as well as a list of people that should be Cc'd for any patches to
+the driver. Current rules require that drivers contributed to staging
+must, at a minimum, compile properly.
+
+Staging can be a relatively easy way to get new drivers into the mainline
+where, with luck, they will come to the attention of other developers and
+improve quickly. Entry into staging is not the end of the story, though;
+code in staging which is not seeing regular progress will eventually be
+removed. Distributors also tend to be relatively reluctant to enable
+staging drivers. So staging is, at best, a stop on the way toward becoming
+a proper mainline driver.
+
+
+Tools
+-----
+
+As can be seen from the above text, the kernel development process depends
+heavily on the ability to herd collections of patches in various
+directions. The whole thing would not work anywhere near as well as it
+does without suitably powerful tools. Tutorials on how to use these tools
+are well beyond the scope of this document, but there is space for a few
+pointers.
+
+By far the dominant source code management system used by the kernel
+community is git. Git is one of a number of distributed version control
+systems being developed in the free software community. It is well tuned
+for kernel development, in that it performs quite well when dealing with
+large repositories and large numbers of patches. It also has a reputation
+for being difficult to learn and use, though it has gotten better over
+time. Some sort of familiarity with git is almost a requirement for kernel
+developers; even if they do not use it for their own work, they'll need git
+to keep up with what other developers (and the mainline) are doing.
+
+Git is now packaged by almost all Linux distributions. There is a home
+page at:
+
+ http://git-scm.com/
+
+That page has pointers to documentation and tutorials.
+
+Among the kernel developers who do not use git, the most popular choice is
+almost certainly Mercurial:
+
+ http://www.selenic.com/mercurial/
+
+Mercurial shares many features with git, but it provides an interface which
+many find easier to use.
+
+The other tool worth knowing about is Quilt:
+
+ http://savannah.nongnu.org/projects/quilt/
+
+Quilt is a patch management system, rather than a source code management
+system. It does not track history over time; it is, instead, oriented
+toward tracking a specific set of changes against an evolving code base.
+Some major subsystem maintainers use quilt to manage patches intended to go
+upstream. For the management of certain kinds of trees (-mm, for example),
+quilt is the best tool for the job.
+
+
+Mailing lists
+-------------
+
+A great deal of Linux kernel development work is done by way of mailing
+lists. It is hard to be a fully-functioning member of the community
+without joining at least one list somewhere. But Linux mailing lists also
+represent a potential hazard to developers, who risk getting buried under a
+load of electronic mail, running afoul of the conventions used on the Linux
+lists, or both.
+
+Most kernel mailing lists are run on vger.kernel.org; the master list can
+be found at:
+
+ http://vger.kernel.org/vger-lists.html
+
+There are lists hosted elsewhere, though; a number of them are at
+lists.redhat.com.
+
+The core mailing list for kernel development is, of course, linux-kernel.
+This list is an intimidating place to be; volume can reach 500 messages per
+day, the amount of noise is high, the conversation can be severely
+technical, and participants are not always concerned with showing a high
+degree of politeness. But there is no other place where the kernel
+development community comes together as a whole; developers who avoid this
+list will miss important information.
+
+There are a few hints which can help with linux-kernel survival:
+
+- Have the list delivered to a separate folder, rather than your main
+ mailbox. One must be able to ignore the stream for sustained periods of
+ time.
+
+- Do not try to follow every conversation - nobody else does. It is
+ important to filter on both the topic of interest (though note that
+ long-running conversations can drift away from the original subject
+ without changing the email subject line) and the people who are
+ participating.
+
+- Do not feed the trolls. If somebody is trying to stir up an angry
+ response, ignore them.
+
+- When responding to linux-kernel email (or that on other lists) preserve
+ the Cc: header for all involved. In the absence of a strong reason (such
+ as an explicit request), you should never remove recipients. Always make
+ sure that the person you are responding to is in the Cc: list. This
+ convention also makes it unnecessary to explicitly ask to be copied on
+ replies to your postings.
+
+- Search the list archives (and the net as a whole) before asking
+ questions. Some developers can get impatient with people who clearly
+ have not done their homework.
+
+- Avoid top-posting (the practice of putting your answer above the quoted
+ text you are responding to). It makes your response harder to read and
+ makes a poor impression.
+
+- Ask on the correct mailing list. Linux-kernel may be the general meeting
+ point, but it is not the best place to find developers from all
+ subsystems.
+
+The last point - finding the correct mailing list - is a common place for
+beginning developers to go wrong. Somebody who asks a networking-related
+question on linux-kernel will almost certainly receive a polite suggestion
+to ask on the netdev list instead, as that is the list frequented by most
+networking developers. Other lists exist for the SCSI, video4linux, IDE,
+filesystem, etc. subsystems. The best place to look for mailing lists is
+in the MAINTAINERS file packaged with the kernel source.
+
+
+Getting started with Kernel development
+---------------------------------------
+
+Questions about how to get started with the kernel development process are
+common - from both individuals and companies. Equally common are missteps
+which make the beginning of the relationship harder than it has to be.
+
+Companies often look to hire well-known developers to get a development
+group started. This can, in fact, be an effective technique. But it also
+tends to be expensive and does not do much to grow the pool of experienced
+kernel developers. It is possible to bring in-house developers up to speed
+on Linux kernel development, given the investment of a bit of time. Taking
+this time can endow an employer with a group of developers who understand
+the kernel and the company both, and who can help to train others as well.
+Over the medium term, this is often the more profitable approach.
+
+Individual developers are often, understandably, at a loss for a place to
+start. Beginning with a large project can be intimidating; one often wants
+to test the waters with something smaller first. This is the point where
+some developers jump into the creation of patches fixing spelling errors or
+minor coding style issues. Unfortunately, such patches create a level of
+noise which is distracting for the development community as a whole, so,
+increasingly, they are looked down upon. New developers wishing to
+introduce themselves to the community will not get the sort of reception
+they wish for by these means.
+
+Andrew Morton gives this advice for aspiring kernel developers
+
+::
+
+ The #1 project for all kernel beginners should surely be "make sure
+ that the kernel runs perfectly at all times on all machines which
+ you can lay your hands on". Usually the way to do this is to work
+ with others on getting things fixed up (this can require
+ persistence!) but that's fine - it's a part of kernel development.
+
+(http://lwn.net/Articles/283982/).
+
+In the absence of obvious problems to fix, developers are advised to look
+at the current lists of regressions and open bugs in general. There is
+never any shortage of issues in need of fixing; by addressing these issues,
+developers will gain experience with the process while, at the same time,
+building respect with the rest of the development community.
+++ /dev/null
-.. _development_early_stage:
-
-Early-stage planning
-====================
-
-When contemplating a Linux kernel development project, it can be tempting
-to jump right in and start coding. As with any significant project,
-though, much of the groundwork for success is best laid before the first
-line of code is written. Some time spent in early planning and
-communication can save far more time later on.
-
-
-Specifying the problem
-----------------------
-
-Like any engineering project, a successful kernel enhancement starts with a
-clear description of the problem to be solved. In some cases, this step is
-easy: when a driver is needed for a specific piece of hardware, for
-example. In others, though, it is tempting to confuse the real problem
-with the proposed solution, and that can lead to difficulties.
-
-Consider an example: some years ago, developers working with Linux audio
-sought a way to run applications without dropouts or other artifacts caused
-by excessive latency in the system. The solution they arrived at was a
-kernel module intended to hook into the Linux Security Module (LSM)
-framework; this module could be configured to give specific applications
-access to the realtime scheduler. This module was implemented and sent to
-the linux-kernel mailing list, where it immediately ran into problems.
-
-To the audio developers, this security module was sufficient to solve their
-immediate problem. To the wider kernel community, though, it was seen as a
-misuse of the LSM framework (which is not intended to confer privileges
-onto processes which they would not otherwise have) and a risk to system
-stability. Their preferred solutions involved realtime scheduling access
-via the rlimit mechanism for the short term, and ongoing latency reduction
-work in the long term.
-
-The audio community, however, could not see past the particular solution
-they had implemented; they were unwilling to accept alternatives. The
-resulting disagreement left those developers feeling disillusioned with the
-entire kernel development process; one of them went back to an audio list
-and posted this:
-
- There are a number of very good Linux kernel developers, but they
- tend to get outshouted by a large crowd of arrogant fools. Trying
- to communicate user requirements to these people is a waste of
- time. They are much too "intelligent" to listen to lesser mortals.
-
-(http://lwn.net/Articles/131776/).
-
-The reality of the situation was different; the kernel developers were far
-more concerned about system stability, long-term maintenance, and finding
-the right solution to the problem than they were with a specific module.
-The moral of the story is to focus on the problem - not a specific solution
-- and to discuss it with the development community before investing in the
-creation of a body of code.
-
-So, when contemplating a kernel development project, one should obtain
-answers to a short set of questions:
-
- - What, exactly, is the problem which needs to be solved?
-
- - Who are the users affected by this problem? Which use cases should the
- solution address?
-
- - How does the kernel fall short in addressing that problem now?
-
-Only then does it make sense to start considering possible solutions.
-
-
-Early discussion
-----------------
-
-When planning a kernel development project, it makes great sense to hold
-discussions with the community before launching into implementation. Early
-communication can save time and trouble in a number of ways:
-
- - It may well be that the problem is addressed by the kernel in ways which
- you have not understood. The Linux kernel is large and has a number of
- features and capabilities which are not immediately obvious. Not all
- kernel capabilities are documented as well as one might like, and it is
- easy to miss things. Your author has seen the posting of a complete
- driver which duplicated an existing driver that the new author had been
- unaware of. Code which reinvents existing wheels is not only wasteful;
- it will also not be accepted into the mainline kernel.
-
- - There may be elements of the proposed solution which will not be
- acceptable for mainline merging. It is better to find out about
- problems like this before writing the code.
-
- - It's entirely possible that other developers have thought about the
- problem; they may have ideas for a better solution, and may be willing
- to help in the creation of that solution.
-
-Years of experience with the kernel development community have taught a
-clear lesson: kernel code which is designed and developed behind closed
-doors invariably has problems which are only revealed when the code is
-released into the community. Sometimes these problems are severe,
-requiring months or years of effort before the code can be brought up to
-the kernel community's standards. Some examples include:
-
- - The Devicescape network stack was designed and implemented for
- single-processor systems. It could not be merged into the mainline
- until it was made suitable for multiprocessor systems. Retrofitting
- locking and such into code is a difficult task; as a result, the merging
- of this code (now called mac80211) was delayed for over a year.
-
- - The Reiser4 filesystem included a number of capabilities which, in the
- core kernel developers' opinion, should have been implemented in the
- virtual filesystem layer instead. It also included features which could
- not easily be implemented without exposing the system to user-caused
- deadlocks. The late revelation of these problems - and refusal to
- address some of them - has caused Reiser4 to stay out of the mainline
- kernel.
-
- - The AppArmor security module made use of internal virtual filesystem
- data structures in ways which were considered to be unsafe and
- unreliable. This concern (among others) kept AppArmor out of the
- mainline for years.
-
-In each of these cases, a great deal of pain and extra work could have been
-avoided with some early discussion with the kernel developers.
-
-
-Who do you talk to?
--------------------
-
-When developers decide to take their plans public, the next question will
-be: where do we start? The answer is to find the right mailing list(s) and
-the right maintainer. For mailing lists, the best approach is to look in
-the MAINTAINERS file for a relevant place to post. If there is a suitable
-subsystem list, posting there is often preferable to posting on
-linux-kernel; you are more likely to reach developers with expertise in the
-relevant subsystem and the environment may be more supportive.
-
-Finding maintainers can be a bit harder. Again, the MAINTAINERS file is
-the place to start. That file tends to not always be up to date, though,
-and not all subsystems are represented there. The person listed in the
-MAINTAINERS file may, in fact, not be the person who is actually acting in
-that role currently. So, when there is doubt about who to contact, a
-useful trick is to use git (and "git log" in particular) to see who is
-currently active within the subsystem of interest. Look at who is writing
-patches, and who, if anybody, is attaching Signed-off-by lines to those
-patches. Those are the people who will be best placed to help with a new
-development project.
-
-The task of finding the right maintainer is sometimes challenging enough
-that the kernel developers have added a script to ease the process:
-
-::
-
- .../scripts/get_maintainer.pl
-
-This script will return the current maintainer(s) for a given file or
-directory when given the "-f" option. If passed a patch on the
-command line, it will list the maintainers who should probably receive
-copies of the patch. There are a number of options regulating how hard
-get_maintainer.pl will search for maintainers; please be careful about
-using the more aggressive options as you may end up including developers
-who have no real interest in the code you are modifying.
-
-If all else fails, talking to Andrew Morton can be an effective way to
-track down a maintainer for a specific piece of code.
-
-
-When to post?
--------------
-
-If possible, posting your plans during the early stages can only be
-helpful. Describe the problem being solved and any plans that have been
-made on how the implementation will be done. Any information you can
-provide can help the development community provide useful input on the
-project.
-
-One discouraging thing which can happen at this stage is not a hostile
-reaction, but, instead, little or no reaction at all. The sad truth of the
-matter is (1) kernel developers tend to be busy, (2) there is no shortage
-of people with grand plans and little code (or even prospect of code) to
-back them up, and (3) nobody is obligated to review or comment on ideas
-posted by others. Beyond that, high-level designs often hide problems
-which are only reviewed when somebody actually tries to implement those
-designs; for that reason, kernel developers would rather see the code.
-
-If a request-for-comments posting yields little in the way of comments, do
-not assume that it means there is no interest in the project.
-Unfortunately, you also cannot assume that there are no problems with your
-idea. The best thing to do in this situation is to proceed, keeping the
-community informed as you go.
-
-
-Getting official buy-in
------------------------
-
-If your work is being done in a corporate environment - as most Linux
-kernel work is - you must, obviously, have permission from suitably
-empowered managers before you can post your company's plans or code to a
-public mailing list. The posting of code which has not been cleared for
-release under a GPL-compatible license can be especially problematic; the
-sooner that a company's management and legal staff can agree on the posting
-of a kernel development project, the better off everybody involved will be.
-
-Some readers may be thinking at this point that their kernel work is
-intended to support a product which does not yet have an officially
-acknowledged existence. Revealing their employer's plans on a public
-mailing list may not be a viable option. In cases like this, it is worth
-considering whether the secrecy is really necessary; there is often no real
-need to keep development plans behind closed doors.
-
-That said, there are also cases where a company legitimately cannot
-disclose its plans early in the development process. Companies with
-experienced kernel developers may choose to proceed in an open-loop manner
-on the assumption that they will be able to avoid serious integration
-problems later. For companies without that sort of in-house expertise, the
-best option is often to hire an outside developer to review the plans under
-a non-disclosure agreement. The Linux Foundation operates an NDA program
-designed to help with this sort of situation; more information can be found
-at:
-
- http://www.linuxfoundation.org/en/NDA_program
-
-This kind of review is often enough to avoid serious problems later on
-without requiring public disclosure of the project.
--- /dev/null
+.. _development_early_stage:
+
+Early-stage planning
+====================
+
+When contemplating a Linux kernel development project, it can be tempting
+to jump right in and start coding. As with any significant project,
+though, much of the groundwork for success is best laid before the first
+line of code is written. Some time spent in early planning and
+communication can save far more time later on.
+
+
+Specifying the problem
+----------------------
+
+Like any engineering project, a successful kernel enhancement starts with a
+clear description of the problem to be solved. In some cases, this step is
+easy: when a driver is needed for a specific piece of hardware, for
+example. In others, though, it is tempting to confuse the real problem
+with the proposed solution, and that can lead to difficulties.
+
+Consider an example: some years ago, developers working with Linux audio
+sought a way to run applications without dropouts or other artifacts caused
+by excessive latency in the system. The solution they arrived at was a
+kernel module intended to hook into the Linux Security Module (LSM)
+framework; this module could be configured to give specific applications
+access to the realtime scheduler. This module was implemented and sent to
+the linux-kernel mailing list, where it immediately ran into problems.
+
+To the audio developers, this security module was sufficient to solve their
+immediate problem. To the wider kernel community, though, it was seen as a
+misuse of the LSM framework (which is not intended to confer privileges
+onto processes which they would not otherwise have) and a risk to system
+stability. Their preferred solutions involved realtime scheduling access
+via the rlimit mechanism for the short term, and ongoing latency reduction
+work in the long term.
+
+The audio community, however, could not see past the particular solution
+they had implemented; they were unwilling to accept alternatives. The
+resulting disagreement left those developers feeling disillusioned with the
+entire kernel development process; one of them went back to an audio list
+and posted this:
+
+ There are a number of very good Linux kernel developers, but they
+ tend to get outshouted by a large crowd of arrogant fools. Trying
+ to communicate user requirements to these people is a waste of
+ time. They are much too "intelligent" to listen to lesser mortals.
+
+(http://lwn.net/Articles/131776/).
+
+The reality of the situation was different; the kernel developers were far
+more concerned about system stability, long-term maintenance, and finding
+the right solution to the problem than they were with a specific module.
+The moral of the story is to focus on the problem - not a specific solution
+- and to discuss it with the development community before investing in the
+creation of a body of code.
+
+So, when contemplating a kernel development project, one should obtain
+answers to a short set of questions:
+
+ - What, exactly, is the problem which needs to be solved?
+
+ - Who are the users affected by this problem? Which use cases should the
+ solution address?
+
+ - How does the kernel fall short in addressing that problem now?
+
+Only then does it make sense to start considering possible solutions.
+
+
+Early discussion
+----------------
+
+When planning a kernel development project, it makes great sense to hold
+discussions with the community before launching into implementation. Early
+communication can save time and trouble in a number of ways:
+
+ - It may well be that the problem is addressed by the kernel in ways which
+ you have not understood. The Linux kernel is large and has a number of
+ features and capabilities which are not immediately obvious. Not all
+ kernel capabilities are documented as well as one might like, and it is
+ easy to miss things. Your author has seen the posting of a complete
+ driver which duplicated an existing driver that the new author had been
+ unaware of. Code which reinvents existing wheels is not only wasteful;
+ it will also not be accepted into the mainline kernel.
+
+ - There may be elements of the proposed solution which will not be
+ acceptable for mainline merging. It is better to find out about
+ problems like this before writing the code.
+
+ - It's entirely possible that other developers have thought about the
+ problem; they may have ideas for a better solution, and may be willing
+ to help in the creation of that solution.
+
+Years of experience with the kernel development community have taught a
+clear lesson: kernel code which is designed and developed behind closed
+doors invariably has problems which are only revealed when the code is
+released into the community. Sometimes these problems are severe,
+requiring months or years of effort before the code can be brought up to
+the kernel community's standards. Some examples include:
+
+ - The Devicescape network stack was designed and implemented for
+ single-processor systems. It could not be merged into the mainline
+ until it was made suitable for multiprocessor systems. Retrofitting
+ locking and such into code is a difficult task; as a result, the merging
+ of this code (now called mac80211) was delayed for over a year.
+
+ - The Reiser4 filesystem included a number of capabilities which, in the
+ core kernel developers' opinion, should have been implemented in the
+ virtual filesystem layer instead. It also included features which could
+ not easily be implemented without exposing the system to user-caused
+ deadlocks. The late revelation of these problems - and refusal to
+ address some of them - has caused Reiser4 to stay out of the mainline
+ kernel.
+
+ - The AppArmor security module made use of internal virtual filesystem
+ data structures in ways which were considered to be unsafe and
+ unreliable. This concern (among others) kept AppArmor out of the
+ mainline for years.
+
+In each of these cases, a great deal of pain and extra work could have been
+avoided with some early discussion with the kernel developers.
+
+
+Who do you talk to?
+-------------------
+
+When developers decide to take their plans public, the next question will
+be: where do we start? The answer is to find the right mailing list(s) and
+the right maintainer. For mailing lists, the best approach is to look in
+the MAINTAINERS file for a relevant place to post. If there is a suitable
+subsystem list, posting there is often preferable to posting on
+linux-kernel; you are more likely to reach developers with expertise in the
+relevant subsystem and the environment may be more supportive.
+
+Finding maintainers can be a bit harder. Again, the MAINTAINERS file is
+the place to start. That file tends to not always be up to date, though,
+and not all subsystems are represented there. The person listed in the
+MAINTAINERS file may, in fact, not be the person who is actually acting in
+that role currently. So, when there is doubt about who to contact, a
+useful trick is to use git (and "git log" in particular) to see who is
+currently active within the subsystem of interest. Look at who is writing
+patches, and who, if anybody, is attaching Signed-off-by lines to those
+patches. Those are the people who will be best placed to help with a new
+development project.
+
+The task of finding the right maintainer is sometimes challenging enough
+that the kernel developers have added a script to ease the process:
+
+::
+
+ .../scripts/get_maintainer.pl
+
+This script will return the current maintainer(s) for a given file or
+directory when given the "-f" option. If passed a patch on the
+command line, it will list the maintainers who should probably receive
+copies of the patch. There are a number of options regulating how hard
+get_maintainer.pl will search for maintainers; please be careful about
+using the more aggressive options as you may end up including developers
+who have no real interest in the code you are modifying.
+
+If all else fails, talking to Andrew Morton can be an effective way to
+track down a maintainer for a specific piece of code.
+
+
+When to post?
+-------------
+
+If possible, posting your plans during the early stages can only be
+helpful. Describe the problem being solved and any plans that have been
+made on how the implementation will be done. Any information you can
+provide can help the development community provide useful input on the
+project.
+
+One discouraging thing which can happen at this stage is not a hostile
+reaction, but, instead, little or no reaction at all. The sad truth of the
+matter is (1) kernel developers tend to be busy, (2) there is no shortage
+of people with grand plans and little code (or even prospect of code) to
+back them up, and (3) nobody is obligated to review or comment on ideas
+posted by others. Beyond that, high-level designs often hide problems
+which are only reviewed when somebody actually tries to implement those
+designs; for that reason, kernel developers would rather see the code.
+
+If a request-for-comments posting yields little in the way of comments, do
+not assume that it means there is no interest in the project.
+Unfortunately, you also cannot assume that there are no problems with your
+idea. The best thing to do in this situation is to proceed, keeping the
+community informed as you go.
+
+
+Getting official buy-in
+-----------------------
+
+If your work is being done in a corporate environment - as most Linux
+kernel work is - you must, obviously, have permission from suitably
+empowered managers before you can post your company's plans or code to a
+public mailing list. The posting of code which has not been cleared for
+release under a GPL-compatible license can be especially problematic; the
+sooner that a company's management and legal staff can agree on the posting
+of a kernel development project, the better off everybody involved will be.
+
+Some readers may be thinking at this point that their kernel work is
+intended to support a product which does not yet have an officially
+acknowledged existence. Revealing their employer's plans on a public
+mailing list may not be a viable option. In cases like this, it is worth
+considering whether the secrecy is really necessary; there is often no real
+need to keep development plans behind closed doors.
+
+That said, there are also cases where a company legitimately cannot
+disclose its plans early in the development process. Companies with
+experienced kernel developers may choose to proceed in an open-loop manner
+on the assumption that they will be able to avoid serious integration
+problems later. For companies without that sort of in-house expertise, the
+best option is often to hire an outside developer to review the plans under
+a non-disclosure agreement. The Linux Foundation operates an NDA program
+designed to help with this sort of situation; more information can be found
+at:
+
+ http://www.linuxfoundation.org/en/NDA_program
+
+This kind of review is often enough to avoid serious problems later on
+without requiring public disclosure of the project.
+++ /dev/null
-.. _development_coding:
-
-Getting the code right
-======================
-
-While there is much to be said for a solid and community-oriented design
-process, the proof of any kernel development project is in the resulting
-code. It is the code which will be examined by other developers and merged
-(or not) into the mainline tree. So it is the quality of this code which
-will determine the ultimate success of the project.
-
-This section will examine the coding process. We'll start with a look at a
-number of ways in which kernel developers can go wrong. Then the focus
-will shift toward doing things right and the tools which can help in that
-quest.
-
-
-Pitfalls
----------
-
-Coding style
-************
-
-The kernel has long had a standard coding style, described in
-Documentation/CodingStyle. For much of that time, the policies described
-in that file were taken as being, at most, advisory. As a result, there is
-a substantial amount of code in the kernel which does not meet the coding
-style guidelines. The presence of that code leads to two independent
-hazards for kernel developers.
-
-The first of these is to believe that the kernel coding standards do not
-matter and are not enforced. The truth of the matter is that adding new
-code to the kernel is very difficult if that code is not coded according to
-the standard; many developers will request that the code be reformatted
-before they will even review it. A code base as large as the kernel
-requires some uniformity of code to make it possible for developers to
-quickly understand any part of it. So there is no longer room for
-strangely-formatted code.
-
-Occasionally, the kernel's coding style will run into conflict with an
-employer's mandated style. In such cases, the kernel's style will have to
-win before the code can be merged. Putting code into the kernel means
-giving up a degree of control in a number of ways - including control over
-how the code is formatted.
-
-The other trap is to assume that code which is already in the kernel is
-urgently in need of coding style fixes. Developers may start to generate
-reformatting patches as a way of gaining familiarity with the process, or
-as a way of getting their name into the kernel changelogs - or both. But
-pure coding style fixes are seen as noise by the development community;
-they tend to get a chilly reception. So this type of patch is best
-avoided. It is natural to fix the style of a piece of code while working
-on it for other reasons, but coding style changes should not be made for
-their own sake.
-
-The coding style document also should not be read as an absolute law which
-can never be transgressed. If there is a good reason to go against the
-style (a line which becomes far less readable if split to fit within the
-80-column limit, for example), just do it.
-
-
-Abstraction layers
-******************
-
-Computer Science professors teach students to make extensive use of
-abstraction layers in the name of flexibility and information hiding.
-Certainly the kernel makes extensive use of abstraction; no project
-involving several million lines of code could do otherwise and survive.
-But experience has shown that excessive or premature abstraction can be
-just as harmful as premature optimization. Abstraction should be used to
-the level required and no further.
-
-At a simple level, consider a function which has an argument which is
-always passed as zero by all callers. One could retain that argument just
-in case somebody eventually needs to use the extra flexibility that it
-provides. By that time, though, chances are good that the code which
-implements this extra argument has been broken in some subtle way which was
-never noticed - because it has never been used. Or, when the need for
-extra flexibility arises, it does not do so in a way which matches the
-programmer's early expectation. Kernel developers will routinely submit
-patches to remove unused arguments; they should, in general, not be added
-in the first place.
-
-Abstraction layers which hide access to hardware - often to allow the bulk
-of a driver to be used with multiple operating systems - are especially
-frowned upon. Such layers obscure the code and may impose a performance
-penalty; they do not belong in the Linux kernel.
-
-On the other hand, if you find yourself copying significant amounts of code
-from another kernel subsystem, it is time to ask whether it would, in fact,
-make sense to pull out some of that code into a separate library or to
-implement that functionality at a higher level. There is no value in
-replicating the same code throughout the kernel.
-
-
-#ifdef and preprocessor use in general
-**************************************
-
-The C preprocessor seems to present a powerful temptation to some C
-programmers, who see it as a way to efficiently encode a great deal of
-flexibility into a source file. But the preprocessor is not C, and heavy
-use of it results in code which is much harder for others to read and
-harder for the compiler to check for correctness. Heavy preprocessor use
-is almost always a sign of code which needs some cleanup work.
-
-Conditional compilation with #ifdef is, indeed, a powerful feature, and it
-is used within the kernel. But there is little desire to see code which is
-sprinkled liberally with #ifdef blocks. As a general rule, #ifdef use
-should be confined to header files whenever possible.
-Conditionally-compiled code can be confined to functions which, if the code
-is not to be present, simply become empty. The compiler will then quietly
-optimize out the call to the empty function. The result is far cleaner
-code which is easier to follow.
-
-C preprocessor macros present a number of hazards, including possible
-multiple evaluation of expressions with side effects and no type safety.
-If you are tempted to define a macro, consider creating an inline function
-instead. The code which results will be the same, but inline functions are
-easier to read, do not evaluate their arguments multiple times, and allow
-the compiler to perform type checking on the arguments and return value.
-
-
-Inline functions
-****************
-
-Inline functions present a hazard of their own, though. Programmers can
-become enamored of the perceived efficiency inherent in avoiding a function
-call and fill a source file with inline functions. Those functions,
-however, can actually reduce performance. Since their code is replicated
-at each call site, they end up bloating the size of the compiled kernel.
-That, in turn, creates pressure on the processor's memory caches, which can
-slow execution dramatically. Inline functions, as a rule, should be quite
-small and relatively rare. The cost of a function call, after all, is not
-that high; the creation of large numbers of inline functions is a classic
-example of premature optimization.
-
-In general, kernel programmers ignore cache effects at their peril. The
-classic time/space tradeoff taught in beginning data structures classes
-often does not apply to contemporary hardware. Space *is* time, in that a
-larger program will run slower than one which is more compact.
-
-More recent compilers take an increasingly active role in deciding whether
-a given function should actually be inlined or not. So the liberal
-placement of "inline" keywords may not just be excessive; it could also be
-irrelevant.
-
-
-Locking
-*******
-
-In May, 2006, the "Devicescape" networking stack was, with great
-fanfare, released under the GPL and made available for inclusion in the
-mainline kernel. This donation was welcome news; support for wireless
-networking in Linux was considered substandard at best, and the Devicescape
-stack offered the promise of fixing that situation. Yet, this code did not
-actually make it into the mainline until June, 2007 (2.6.22). What
-happened?
-
-This code showed a number of signs of having been developed behind
-corporate doors. But one large problem in particular was that it was not
-designed to work on multiprocessor systems. Before this networking stack
-(now called mac80211) could be merged, a locking scheme needed to be
-retrofitted onto it.
-
-Once upon a time, Linux kernel code could be developed without thinking
-about the concurrency issues presented by multiprocessor systems. Now,
-however, this document is being written on a dual-core laptop. Even on
-single-processor systems, work being done to improve responsiveness will
-raise the level of concurrency within the kernel. The days when kernel
-code could be written without thinking about locking are long past.
-
-Any resource (data structures, hardware registers, etc.) which could be
-accessed concurrently by more than one thread must be protected by a lock.
-New code should be written with this requirement in mind; retrofitting
-locking after the fact is a rather more difficult task. Kernel developers
-should take the time to understand the available locking primitives well
-enough to pick the right tool for the job. Code which shows a lack of
-attention to concurrency will have a difficult path into the mainline.
-
-
-Regressions
-***********
-
-One final hazard worth mentioning is this: it can be tempting to make a
-change (which may bring big improvements) which causes something to break
-for existing users. This kind of change is called a "regression," and
-regressions have become most unwelcome in the mainline kernel. With few
-exceptions, changes which cause regressions will be backed out if the
-regression cannot be fixed in a timely manner. Far better to avoid the
-regression in the first place.
-
-It is often argued that a regression can be justified if it causes things
-to work for more people than it creates problems for. Why not make a
-change if it brings new functionality to ten systems for each one it
-breaks? The best answer to this question was expressed by Linus in July,
-2007:
-
-::
-
- So we don't fix bugs by introducing new problems. That way lies
- madness, and nobody ever knows if you actually make any real
- progress at all. Is it two steps forwards, one step back, or one
- step forward and two steps back?
-
-(http://lwn.net/Articles/243460/).
-
-An especially unwelcome type of regression is any sort of change to the
-user-space ABI. Once an interface has been exported to user space, it must
-be supported indefinitely. This fact makes the creation of user-space
-interfaces particularly challenging: since they cannot be changed in
-incompatible ways, they must be done right the first time. For this
-reason, a great deal of thought, clear documentation, and wide review for
-user-space interfaces is always required.
-
-
-Code checking tools
--------------------
-
-For now, at least, the writing of error-free code remains an ideal that few
-of us can reach. What we can hope to do, though, is to catch and fix as
-many of those errors as possible before our code goes into the mainline
-kernel. To that end, the kernel developers have put together an impressive
-array of tools which can catch a wide variety of obscure problems in an
-automated way. Any problem caught by the computer is a problem which will
-not afflict a user later on, so it stands to reason that the automated
-tools should be used whenever possible.
-
-The first step is simply to heed the warnings produced by the compiler.
-Contemporary versions of gcc can detect (and warn about) a large number of
-potential errors. Quite often, these warnings point to real problems.
-Code submitted for review should, as a rule, not produce any compiler
-warnings. When silencing warnings, take care to understand the real cause
-and try to avoid "fixes" which make the warning go away without addressing
-its cause.
-
-Note that not all compiler warnings are enabled by default. Build the
-kernel with "make EXTRA_CFLAGS=-W" to get the full set.
-
-The kernel provides several configuration options which turn on debugging
-features; most of these are found in the "kernel hacking" submenu. Several
-of these options should be turned on for any kernel used for development or
-testing purposes. In particular, you should turn on:
-
- - ENABLE_WARN_DEPRECATED, ENABLE_MUST_CHECK, and FRAME_WARN to get an
- extra set of warnings for problems like the use of deprecated interfaces
- or ignoring an important return value from a function. The output
- generated by these warnings can be verbose, but one need not worry about
- warnings from other parts of the kernel.
-
- - DEBUG_OBJECTS will add code to track the lifetime of various objects
- created by the kernel and warn when things are done out of order. If
- you are adding a subsystem which creates (and exports) complex objects
- of its own, consider adding support for the object debugging
- infrastructure.
-
- - DEBUG_SLAB can find a variety of memory allocation and use errors; it
- should be used on most development kernels.
-
- - DEBUG_SPINLOCK, DEBUG_ATOMIC_SLEEP, and DEBUG_MUTEXES will find a
- number of common locking errors.
-
-There are quite a few other debugging options, some of which will be
-discussed below. Some of them have a significant performance impact and
-should not be used all of the time. But some time spent learning the
-available options will likely be paid back many times over in short order.
-
-One of the heavier debugging tools is the locking checker, or "lockdep."
-This tool will track the acquisition and release of every lock (spinlock or
-mutex) in the system, the order in which locks are acquired relative to
-each other, the current interrupt environment, and more. It can then
-ensure that locks are always acquired in the same order, that the same
-interrupt assumptions apply in all situations, and so on. In other words,
-lockdep can find a number of scenarios in which the system could, on rare
-occasion, deadlock. This kind of problem can be painful (for both
-developers and users) in a deployed system; lockdep allows them to be found
-in an automated manner ahead of time. Code with any sort of non-trivial
-locking should be run with lockdep enabled before being submitted for
-inclusion.
-
-As a diligent kernel programmer, you will, beyond doubt, check the return
-status of any operation (such as a memory allocation) which can fail. The
-fact of the matter, though, is that the resulting failure recovery paths
-are, probably, completely untested. Untested code tends to be broken code;
-you could be much more confident of your code if all those error-handling
-paths had been exercised a few times.
-
-The kernel provides a fault injection framework which can do exactly that,
-especially where memory allocations are involved. With fault injection
-enabled, a configurable percentage of memory allocations will be made to
-fail; these failures can be restricted to a specific range of code.
-Running with fault injection enabled allows the programmer to see how the
-code responds when things go badly. See
-Documentation/fault-injection/fault-injection.txt for more information on
-how to use this facility.
-
-Other kinds of errors can be found with the "sparse" static analysis tool.
-With sparse, the programmer can be warned about confusion between
-user-space and kernel-space addresses, mixture of big-endian and
-small-endian quantities, the passing of integer values where a set of bit
-flags is expected, and so on. Sparse must be installed separately (it can
-be found at https://sparse.wiki.kernel.org/index.php/Main_Page if your
-distributor does not package it); it can then be run on the code by adding
-"C=1" to your make command.
-
-The "Coccinelle" tool (http://coccinelle.lip6.fr/) is able to find a wide
-variety of potential coding problems; it can also propose fixes for those
-problems. Quite a few "semantic patches" for the kernel have been packaged
-under the scripts/coccinelle directory; running "make coccicheck" will run
-through those semantic patches and report on any problems found. See
-Documentation/coccinelle.txt for more information.
-
-Other kinds of portability errors are best found by compiling your code for
-other architectures. If you do not happen to have an S/390 system or a
-Blackfin development board handy, you can still perform the compilation
-step. A large set of cross compilers for x86 systems can be found at
-
- http://www.kernel.org/pub/tools/crosstool/
-
-Some time spent installing and using these compilers will help avoid
-embarrassment later.
-
-
-Documentation
--------------
-
-Documentation has often been more the exception than the rule with kernel
-development. Even so, adequate documentation will help to ease the merging
-of new code into the kernel, make life easier for other developers, and
-will be helpful for your users. In many cases, the addition of
-documentation has become essentially mandatory.
-
-The first piece of documentation for any patch is its associated
-changelog. Log entries should describe the problem being solved, the form
-of the solution, the people who worked on the patch, any relevant
-effects on performance, and anything else that might be needed to
-understand the patch. Be sure that the changelog says *why* the patch is
-worth applying; a surprising number of developers fail to provide that
-information.
-
-Any code which adds a new user-space interface - including new sysfs or
-/proc files - should include documentation of that interface which enables
-user-space developers to know what they are working with. See
-Documentation/ABI/README for a description of how this documentation should
-be formatted and what information needs to be provided.
-
-The file Documentation/kernel-parameters.txt describes all of the kernel's
-boot-time parameters. Any patch which adds new parameters should add the
-appropriate entries to this file.
-
-Any new configuration options must be accompanied by help text which
-clearly explains the options and when the user might want to select them.
-
-Internal API information for many subsystems is documented by way of
-specially-formatted comments; these comments can be extracted and formatted
-in a number of ways by the "kernel-doc" script. If you are working within
-a subsystem which has kerneldoc comments, you should maintain them and add
-them, as appropriate, for externally-available functions. Even in areas
-which have not been so documented, there is no harm in adding kerneldoc
-comments for the future; indeed, this can be a useful activity for
-beginning kernel developers. The format of these comments, along with some
-information on how to create kerneldoc templates can be found in the file
-Documentation/kernel-documentation.rst.
-
-Anybody who reads through a significant amount of existing kernel code will
-note that, often, comments are most notable by their absence. Once again,
-the expectations for new code are higher than they were in the past;
-merging uncommented code will be harder. That said, there is little desire
-for verbosely-commented code. The code should, itself, be readable, with
-comments explaining the more subtle aspects.
-
-Certain things should always be commented. Uses of memory barriers should
-be accompanied by a line explaining why the barrier is necessary. The
-locking rules for data structures generally need to be explained somewhere.
-Major data structures need comprehensive documentation in general.
-Non-obvious dependencies between separate bits of code should be pointed
-out. Anything which might tempt a code janitor to make an incorrect
-"cleanup" needs a comment saying why it is done the way it is. And so on.
-
-
-Internal API changes
---------------------
-
-The binary interface provided by the kernel to user space cannot be broken
-except under the most severe circumstances. The kernel's internal
-programming interfaces, instead, are highly fluid and can be changed when
-the need arises. If you find yourself having to work around a kernel API,
-or simply not using a specific functionality because it does not meet your
-needs, that may be a sign that the API needs to change. As a kernel
-developer, you are empowered to make such changes.
-
-There are, of course, some catches. API changes can be made, but they need
-to be well justified. So any patch making an internal API change should be
-accompanied by a description of what the change is and why it is
-necessary. This kind of change should also be broken out into a separate
-patch, rather than buried within a larger patch.
-
-The other catch is that a developer who changes an internal API is
-generally charged with the task of fixing any code within the kernel tree
-which is broken by the change. For a widely-used function, this duty can
-lead to literally hundreds or thousands of changes - many of which are
-likely to conflict with work being done by other developers. Needless to
-say, this can be a large job, so it is best to be sure that the
-justification is solid. Note that the Coccinelle tool can help with
-wide-ranging API changes.
-
-When making an incompatible API change, one should, whenever possible,
-ensure that code which has not been updated is caught by the compiler.
-This will help you to be sure that you have found all in-tree uses of that
-interface. It will also alert developers of out-of-tree code that there is
-a change that they need to respond to. Supporting out-of-tree code is not
-something that kernel developers need to be worried about, but we also do
-not have to make life harder for out-of-tree developers than it needs to
-be.
--- /dev/null
+.. _development_coding:
+
+Getting the code right
+======================
+
+While there is much to be said for a solid and community-oriented design
+process, the proof of any kernel development project is in the resulting
+code. It is the code which will be examined by other developers and merged
+(or not) into the mainline tree. So it is the quality of this code which
+will determine the ultimate success of the project.
+
+This section will examine the coding process. We'll start with a look at a
+number of ways in which kernel developers can go wrong. Then the focus
+will shift toward doing things right and the tools which can help in that
+quest.
+
+
+Pitfalls
+---------
+
+Coding style
+************
+
+The kernel has long had a standard coding style, described in
+Documentation/CodingStyle. For much of that time, the policies described
+in that file were taken as being, at most, advisory. As a result, there is
+a substantial amount of code in the kernel which does not meet the coding
+style guidelines. The presence of that code leads to two independent
+hazards for kernel developers.
+
+The first of these is to believe that the kernel coding standards do not
+matter and are not enforced. The truth of the matter is that adding new
+code to the kernel is very difficult if that code is not coded according to
+the standard; many developers will request that the code be reformatted
+before they will even review it. A code base as large as the kernel
+requires some uniformity of code to make it possible for developers to
+quickly understand any part of it. So there is no longer room for
+strangely-formatted code.
+
+Occasionally, the kernel's coding style will run into conflict with an
+employer's mandated style. In such cases, the kernel's style will have to
+win before the code can be merged. Putting code into the kernel means
+giving up a degree of control in a number of ways - including control over
+how the code is formatted.
+
+The other trap is to assume that code which is already in the kernel is
+urgently in need of coding style fixes. Developers may start to generate
+reformatting patches as a way of gaining familiarity with the process, or
+as a way of getting their name into the kernel changelogs - or both. But
+pure coding style fixes are seen as noise by the development community;
+they tend to get a chilly reception. So this type of patch is best
+avoided. It is natural to fix the style of a piece of code while working
+on it for other reasons, but coding style changes should not be made for
+their own sake.
+
+The coding style document also should not be read as an absolute law which
+can never be transgressed. If there is a good reason to go against the
+style (a line which becomes far less readable if split to fit within the
+80-column limit, for example), just do it.
+
+
+Abstraction layers
+******************
+
+Computer Science professors teach students to make extensive use of
+abstraction layers in the name of flexibility and information hiding.
+Certainly the kernel makes extensive use of abstraction; no project
+involving several million lines of code could do otherwise and survive.
+But experience has shown that excessive or premature abstraction can be
+just as harmful as premature optimization. Abstraction should be used to
+the level required and no further.
+
+At a simple level, consider a function which has an argument which is
+always passed as zero by all callers. One could retain that argument just
+in case somebody eventually needs to use the extra flexibility that it
+provides. By that time, though, chances are good that the code which
+implements this extra argument has been broken in some subtle way which was
+never noticed - because it has never been used. Or, when the need for
+extra flexibility arises, it does not do so in a way which matches the
+programmer's early expectation. Kernel developers will routinely submit
+patches to remove unused arguments; they should, in general, not be added
+in the first place.
+
+Abstraction layers which hide access to hardware - often to allow the bulk
+of a driver to be used with multiple operating systems - are especially
+frowned upon. Such layers obscure the code and may impose a performance
+penalty; they do not belong in the Linux kernel.
+
+On the other hand, if you find yourself copying significant amounts of code
+from another kernel subsystem, it is time to ask whether it would, in fact,
+make sense to pull out some of that code into a separate library or to
+implement that functionality at a higher level. There is no value in
+replicating the same code throughout the kernel.
+
+
+#ifdef and preprocessor use in general
+**************************************
+
+The C preprocessor seems to present a powerful temptation to some C
+programmers, who see it as a way to efficiently encode a great deal of
+flexibility into a source file. But the preprocessor is not C, and heavy
+use of it results in code which is much harder for others to read and
+harder for the compiler to check for correctness. Heavy preprocessor use
+is almost always a sign of code which needs some cleanup work.
+
+Conditional compilation with #ifdef is, indeed, a powerful feature, and it
+is used within the kernel. But there is little desire to see code which is
+sprinkled liberally with #ifdef blocks. As a general rule, #ifdef use
+should be confined to header files whenever possible.
+Conditionally-compiled code can be confined to functions which, if the code
+is not to be present, simply become empty. The compiler will then quietly
+optimize out the call to the empty function. The result is far cleaner
+code which is easier to follow.
+
+C preprocessor macros present a number of hazards, including possible
+multiple evaluation of expressions with side effects and no type safety.
+If you are tempted to define a macro, consider creating an inline function
+instead. The code which results will be the same, but inline functions are
+easier to read, do not evaluate their arguments multiple times, and allow
+the compiler to perform type checking on the arguments and return value.
+
+
+Inline functions
+****************
+
+Inline functions present a hazard of their own, though. Programmers can
+become enamored of the perceived efficiency inherent in avoiding a function
+call and fill a source file with inline functions. Those functions,
+however, can actually reduce performance. Since their code is replicated
+at each call site, they end up bloating the size of the compiled kernel.
+That, in turn, creates pressure on the processor's memory caches, which can
+slow execution dramatically. Inline functions, as a rule, should be quite
+small and relatively rare. The cost of a function call, after all, is not
+that high; the creation of large numbers of inline functions is a classic
+example of premature optimization.
+
+In general, kernel programmers ignore cache effects at their peril. The
+classic time/space tradeoff taught in beginning data structures classes
+often does not apply to contemporary hardware. Space *is* time, in that a
+larger program will run slower than one which is more compact.
+
+More recent compilers take an increasingly active role in deciding whether
+a given function should actually be inlined or not. So the liberal
+placement of "inline" keywords may not just be excessive; it could also be
+irrelevant.
+
+
+Locking
+*******
+
+In May, 2006, the "Devicescape" networking stack was, with great
+fanfare, released under the GPL and made available for inclusion in the
+mainline kernel. This donation was welcome news; support for wireless
+networking in Linux was considered substandard at best, and the Devicescape
+stack offered the promise of fixing that situation. Yet, this code did not
+actually make it into the mainline until June, 2007 (2.6.22). What
+happened?
+
+This code showed a number of signs of having been developed behind
+corporate doors. But one large problem in particular was that it was not
+designed to work on multiprocessor systems. Before this networking stack
+(now called mac80211) could be merged, a locking scheme needed to be
+retrofitted onto it.
+
+Once upon a time, Linux kernel code could be developed without thinking
+about the concurrency issues presented by multiprocessor systems. Now,
+however, this document is being written on a dual-core laptop. Even on
+single-processor systems, work being done to improve responsiveness will
+raise the level of concurrency within the kernel. The days when kernel
+code could be written without thinking about locking are long past.
+
+Any resource (data structures, hardware registers, etc.) which could be
+accessed concurrently by more than one thread must be protected by a lock.
+New code should be written with this requirement in mind; retrofitting
+locking after the fact is a rather more difficult task. Kernel developers
+should take the time to understand the available locking primitives well
+enough to pick the right tool for the job. Code which shows a lack of
+attention to concurrency will have a difficult path into the mainline.
+
+
+Regressions
+***********
+
+One final hazard worth mentioning is this: it can be tempting to make a
+change (which may bring big improvements) which causes something to break
+for existing users. This kind of change is called a "regression," and
+regressions have become most unwelcome in the mainline kernel. With few
+exceptions, changes which cause regressions will be backed out if the
+regression cannot be fixed in a timely manner. Far better to avoid the
+regression in the first place.
+
+It is often argued that a regression can be justified if it causes things
+to work for more people than it creates problems for. Why not make a
+change if it brings new functionality to ten systems for each one it
+breaks? The best answer to this question was expressed by Linus in July,
+2007:
+
+::
+
+ So we don't fix bugs by introducing new problems. That way lies
+ madness, and nobody ever knows if you actually make any real
+ progress at all. Is it two steps forwards, one step back, or one
+ step forward and two steps back?
+
+(http://lwn.net/Articles/243460/).
+
+An especially unwelcome type of regression is any sort of change to the
+user-space ABI. Once an interface has been exported to user space, it must
+be supported indefinitely. This fact makes the creation of user-space
+interfaces particularly challenging: since they cannot be changed in
+incompatible ways, they must be done right the first time. For this
+reason, a great deal of thought, clear documentation, and wide review for
+user-space interfaces is always required.
+
+
+Code checking tools
+-------------------
+
+For now, at least, the writing of error-free code remains an ideal that few
+of us can reach. What we can hope to do, though, is to catch and fix as
+many of those errors as possible before our code goes into the mainline
+kernel. To that end, the kernel developers have put together an impressive
+array of tools which can catch a wide variety of obscure problems in an
+automated way. Any problem caught by the computer is a problem which will
+not afflict a user later on, so it stands to reason that the automated
+tools should be used whenever possible.
+
+The first step is simply to heed the warnings produced by the compiler.
+Contemporary versions of gcc can detect (and warn about) a large number of
+potential errors. Quite often, these warnings point to real problems.
+Code submitted for review should, as a rule, not produce any compiler
+warnings. When silencing warnings, take care to understand the real cause
+and try to avoid "fixes" which make the warning go away without addressing
+its cause.
+
+Note that not all compiler warnings are enabled by default. Build the
+kernel with "make EXTRA_CFLAGS=-W" to get the full set.
+
+The kernel provides several configuration options which turn on debugging
+features; most of these are found in the "kernel hacking" submenu. Several
+of these options should be turned on for any kernel used for development or
+testing purposes. In particular, you should turn on:
+
+ - ENABLE_WARN_DEPRECATED, ENABLE_MUST_CHECK, and FRAME_WARN to get an
+ extra set of warnings for problems like the use of deprecated interfaces
+ or ignoring an important return value from a function. The output
+ generated by these warnings can be verbose, but one need not worry about
+ warnings from other parts of the kernel.
+
+ - DEBUG_OBJECTS will add code to track the lifetime of various objects
+ created by the kernel and warn when things are done out of order. If
+ you are adding a subsystem which creates (and exports) complex objects
+ of its own, consider adding support for the object debugging
+ infrastructure.
+
+ - DEBUG_SLAB can find a variety of memory allocation and use errors; it
+ should be used on most development kernels.
+
+ - DEBUG_SPINLOCK, DEBUG_ATOMIC_SLEEP, and DEBUG_MUTEXES will find a
+ number of common locking errors.
+
+There are quite a few other debugging options, some of which will be
+discussed below. Some of them have a significant performance impact and
+should not be used all of the time. But some time spent learning the
+available options will likely be paid back many times over in short order.
+
+One of the heavier debugging tools is the locking checker, or "lockdep."
+This tool will track the acquisition and release of every lock (spinlock or
+mutex) in the system, the order in which locks are acquired relative to
+each other, the current interrupt environment, and more. It can then
+ensure that locks are always acquired in the same order, that the same
+interrupt assumptions apply in all situations, and so on. In other words,
+lockdep can find a number of scenarios in which the system could, on rare
+occasion, deadlock. This kind of problem can be painful (for both
+developers and users) in a deployed system; lockdep allows them to be found
+in an automated manner ahead of time. Code with any sort of non-trivial
+locking should be run with lockdep enabled before being submitted for
+inclusion.
+
+As a diligent kernel programmer, you will, beyond doubt, check the return
+status of any operation (such as a memory allocation) which can fail. The
+fact of the matter, though, is that the resulting failure recovery paths
+are, probably, completely untested. Untested code tends to be broken code;
+you could be much more confident of your code if all those error-handling
+paths had been exercised a few times.
+
+The kernel provides a fault injection framework which can do exactly that,
+especially where memory allocations are involved. With fault injection
+enabled, a configurable percentage of memory allocations will be made to
+fail; these failures can be restricted to a specific range of code.
+Running with fault injection enabled allows the programmer to see how the
+code responds when things go badly. See
+Documentation/fault-injection/fault-injection.txt for more information on
+how to use this facility.
+
+Other kinds of errors can be found with the "sparse" static analysis tool.
+With sparse, the programmer can be warned about confusion between
+user-space and kernel-space addresses, mixture of big-endian and
+small-endian quantities, the passing of integer values where a set of bit
+flags is expected, and so on. Sparse must be installed separately (it can
+be found at https://sparse.wiki.kernel.org/index.php/Main_Page if your
+distributor does not package it); it can then be run on the code by adding
+"C=1" to your make command.
+
+The "Coccinelle" tool (http://coccinelle.lip6.fr/) is able to find a wide
+variety of potential coding problems; it can also propose fixes for those
+problems. Quite a few "semantic patches" for the kernel have been packaged
+under the scripts/coccinelle directory; running "make coccicheck" will run
+through those semantic patches and report on any problems found. See
+Documentation/coccinelle.txt for more information.
+
+Other kinds of portability errors are best found by compiling your code for
+other architectures. If you do not happen to have an S/390 system or a
+Blackfin development board handy, you can still perform the compilation
+step. A large set of cross compilers for x86 systems can be found at
+
+ http://www.kernel.org/pub/tools/crosstool/
+
+Some time spent installing and using these compilers will help avoid
+embarrassment later.
+
+
+Documentation
+-------------
+
+Documentation has often been more the exception than the rule with kernel
+development. Even so, adequate documentation will help to ease the merging
+of new code into the kernel, make life easier for other developers, and
+will be helpful for your users. In many cases, the addition of
+documentation has become essentially mandatory.
+
+The first piece of documentation for any patch is its associated
+changelog. Log entries should describe the problem being solved, the form
+of the solution, the people who worked on the patch, any relevant
+effects on performance, and anything else that might be needed to
+understand the patch. Be sure that the changelog says *why* the patch is
+worth applying; a surprising number of developers fail to provide that
+information.
+
+Any code which adds a new user-space interface - including new sysfs or
+/proc files - should include documentation of that interface which enables
+user-space developers to know what they are working with. See
+Documentation/ABI/README for a description of how this documentation should
+be formatted and what information needs to be provided.
+
+The file Documentation/kernel-parameters.txt describes all of the kernel's
+boot-time parameters. Any patch which adds new parameters should add the
+appropriate entries to this file.
+
+Any new configuration options must be accompanied by help text which
+clearly explains the options and when the user might want to select them.
+
+Internal API information for many subsystems is documented by way of
+specially-formatted comments; these comments can be extracted and formatted
+in a number of ways by the "kernel-doc" script. If you are working within
+a subsystem which has kerneldoc comments, you should maintain them and add
+them, as appropriate, for externally-available functions. Even in areas
+which have not been so documented, there is no harm in adding kerneldoc
+comments for the future; indeed, this can be a useful activity for
+beginning kernel developers. The format of these comments, along with some
+information on how to create kerneldoc templates can be found in the file
+Documentation/kernel-documentation.rst.
+
+Anybody who reads through a significant amount of existing kernel code will
+note that, often, comments are most notable by their absence. Once again,
+the expectations for new code are higher than they were in the past;
+merging uncommented code will be harder. That said, there is little desire
+for verbosely-commented code. The code should, itself, be readable, with
+comments explaining the more subtle aspects.
+
+Certain things should always be commented. Uses of memory barriers should
+be accompanied by a line explaining why the barrier is necessary. The
+locking rules for data structures generally need to be explained somewhere.
+Major data structures need comprehensive documentation in general.
+Non-obvious dependencies between separate bits of code should be pointed
+out. Anything which might tempt a code janitor to make an incorrect
+"cleanup" needs a comment saying why it is done the way it is. And so on.
+
+
+Internal API changes
+--------------------
+
+The binary interface provided by the kernel to user space cannot be broken
+except under the most severe circumstances. The kernel's internal
+programming interfaces, instead, are highly fluid and can be changed when
+the need arises. If you find yourself having to work around a kernel API,
+or simply not using a specific functionality because it does not meet your
+needs, that may be a sign that the API needs to change. As a kernel
+developer, you are empowered to make such changes.
+
+There are, of course, some catches. API changes can be made, but they need
+to be well justified. So any patch making an internal API change should be
+accompanied by a description of what the change is and why it is
+necessary. This kind of change should also be broken out into a separate
+patch, rather than buried within a larger patch.
+
+The other catch is that a developer who changes an internal API is
+generally charged with the task of fixing any code within the kernel tree
+which is broken by the change. For a widely-used function, this duty can
+lead to literally hundreds or thousands of changes - many of which are
+likely to conflict with work being done by other developers. Needless to
+say, this can be a large job, so it is best to be sure that the
+justification is solid. Note that the Coccinelle tool can help with
+wide-ranging API changes.
+
+When making an incompatible API change, one should, whenever possible,
+ensure that code which has not been updated is caught by the compiler.
+This will help you to be sure that you have found all in-tree uses of that
+interface. It will also alert developers of out-of-tree code that there is
+a change that they need to respond to. Supporting out-of-tree code is not
+something that kernel developers need to be worried about, but we also do
+not have to make life harder for out-of-tree developers than it needs to
+be.
+++ /dev/null
-.. _development_posting:
-
-Posting patches
-===============
-
-Sooner or later, the time comes when your work is ready to be presented to
-the community for review and, eventually, inclusion into the mainline
-kernel. Unsurprisingly, the kernel development community has evolved a set
-of conventions and procedures which are used in the posting of patches;
-following them will make life much easier for everybody involved. This
-document will attempt to cover these expectations in reasonable detail;
-more information can also be found in the files SubmittingPatches,
-SubmittingDrivers, and SubmitChecklist in the kernel documentation
-directory.
-
-
-When to post
-------------
-
-There is a constant temptation to avoid posting patches before they are
-completely "ready." For simple patches, that is not a problem. If the
-work being done is complex, though, there is a lot to be gained by getting
-feedback from the community before the work is complete. So you should
-consider posting in-progress work, or even making a git tree available so
-that interested developers can catch up with your work at any time.
-
-When posting code which is not yet considered ready for inclusion, it is a
-good idea to say so in the posting itself. Also mention any major work
-which remains to be done and any known problems. Fewer people will look at
-patches which are known to be half-baked, but those who do will come in
-with the idea that they can help you drive the work in the right direction.
-
-
-Before creating patches
------------------------
-
-There are a number of things which should be done before you consider
-sending patches to the development community. These include:
-
- - Test the code to the extent that you can. Make use of the kernel's
- debugging tools, ensure that the kernel will build with all reasonable
- combinations of configuration options, use cross-compilers to build for
- different architectures, etc.
-
- - Make sure your code is compliant with the kernel coding style
- guidelines.
-
- - Does your change have performance implications? If so, you should run
- benchmarks showing what the impact (or benefit) of your change is; a
- summary of the results should be included with the patch.
-
- - Be sure that you have the right to post the code. If this work was done
- for an employer, the employer likely has a right to the work and must be
- agreeable with its release under the GPL.
-
-As a general rule, putting in some extra thought before posting code almost
-always pays back the effort in short order.
-
-
-Patch preparation
------------------
-
-The preparation of patches for posting can be a surprising amount of work,
-but, once again, attempting to save time here is not generally advisable
-even in the short term.
-
-Patches must be prepared against a specific version of the kernel. As a
-general rule, a patch should be based on the current mainline as found in
-Linus's git tree. When basing on mainline, start with a well-known release
-point - a stable or -rc release - rather than branching off the mainline at
-an arbitrary spot.
-
-It may become necessary to make versions against -mm, linux-next, or a
-subsystem tree, though, to facilitate wider testing and review. Depending
-on the area of your patch and what is going on elsewhere, basing a patch
-against these other trees can require a significant amount of work
-resolving conflicts and dealing with API changes.
-
-Only the most simple changes should be formatted as a single patch;
-everything else should be made as a logical series of changes. Splitting
-up patches is a bit of an art; some developers spend a long time figuring
-out how to do it in the way that the community expects. There are a few
-rules of thumb, however, which can help considerably:
-
- - The patch series you post will almost certainly not be the series of
- changes found in your working revision control system. Instead, the
- changes you have made need to be considered in their final form, then
- split apart in ways which make sense. The developers are interested in
- discrete, self-contained changes, not the path you took to get to those
- changes.
-
- - Each logically independent change should be formatted as a separate
- patch. These changes can be small ("add a field to this structure") or
- large (adding a significant new driver, for example), but they should be
- conceptually small and amenable to a one-line description. Each patch
- should make a specific change which can be reviewed on its own and
- verified to do what it says it does.
-
- - As a way of restating the guideline above: do not mix different types of
- changes in the same patch. If a single patch fixes a critical security
- bug, rearranges a few structures, and reformats the code, there is a
- good chance that it will be passed over and the important fix will be
- lost.
-
- - Each patch should yield a kernel which builds and runs properly; if your
- patch series is interrupted in the middle, the result should still be a
- working kernel. Partial application of a patch series is a common
- scenario when the "git bisect" tool is used to find regressions; if the
- result is a broken kernel, you will make life harder for developers and
- users who are engaging in the noble work of tracking down problems.
-
- - Do not overdo it, though. One developer once posted a set of edits
- to a single file as 500 separate patches - an act which did not make him
- the most popular person on the kernel mailing list. A single patch can
- be reasonably large as long as it still contains a single *logical*
- change.
-
- - It can be tempting to add a whole new infrastructure with a series of
- patches, but to leave that infrastructure unused until the final patch
- in the series enables the whole thing. This temptation should be
- avoided if possible; if that series adds regressions, bisection will
- finger the last patch as the one which caused the problem, even though
- the real bug is elsewhere. Whenever possible, a patch which adds new
- code should make that code active immediately.
-
-Working to create the perfect patch series can be a frustrating process
-which takes quite a bit of time and thought after the "real work" has been
-done. When done properly, though, it is time well spent.
-
-
-Patch formatting and changelogs
--------------------------------
-
-So now you have a perfect series of patches for posting, but the work is
-not done quite yet. Each patch needs to be formatted into a message which
-quickly and clearly communicates its purpose to the rest of the world. To
-that end, each patch will be composed of the following:
-
- - An optional "From" line naming the author of the patch. This line is
- only necessary if you are passing on somebody else's patch via email,
- but it never hurts to add it when in doubt.
-
- - A one-line description of what the patch does. This message should be
- enough for a reader who sees it with no other context to figure out the
- scope of the patch; it is the line that will show up in the "short form"
- changelogs. This message is usually formatted with the relevant
- subsystem name first, followed by the purpose of the patch. For
- example:
-
- ::
-
- gpio: fix build on CONFIG_GPIO_SYSFS=n
-
- - A blank line followed by a detailed description of the contents of the
- patch. This description can be as long as is required; it should say
- what the patch does and why it should be applied to the kernel.
-
- - One or more tag lines, with, at a minimum, one Signed-off-by: line from
- the author of the patch. Tags will be described in more detail below.
-
-The items above, together, form the changelog for the patch. Writing good
-changelogs is a crucial but often-neglected art; it's worth spending
-another moment discussing this issue. When writing a changelog, you should
-bear in mind that a number of different people will be reading your words.
-These include subsystem maintainers and reviewers who need to decide
-whether the patch should be included, distributors and other maintainers
-trying to decide whether a patch should be backported to other kernels, bug
-hunters wondering whether the patch is responsible for a problem they are
-chasing, users who want to know how the kernel has changed, and more. A
-good changelog conveys the needed information to all of these people in the
-most direct and concise way possible.
-
-To that end, the summary line should describe the effects of and motivation
-for the change as well as possible given the one-line constraint. The
-detailed description can then amplify on those topics and provide any
-needed additional information. If the patch fixes a bug, cite the commit
-which introduced the bug if possible (and please provide both the commit ID
-and the title when citing commits). If a problem is associated with
-specific log or compiler output, include that output to help others
-searching for a solution to the same problem. If the change is meant to
-support other changes coming in later patch, say so. If internal APIs are
-changed, detail those changes and how other developers should respond. In
-general, the more you can put yourself into the shoes of everybody who will
-be reading your changelog, the better that changelog (and the kernel as a
-whole) will be.
-
-Needless to say, the changelog should be the text used when committing the
-change to a revision control system. It will be followed by:
-
- - The patch itself, in the unified ("-u") patch format. Using the "-p"
- option to diff will associate function names with changes, making the
- resulting patch easier for others to read.
-
-You should avoid including changes to irrelevant files (those generated by
-the build process, for example, or editor backup files) in the patch. The
-file "dontdiff" in the Documentation directory can help in this regard;
-pass it to diff with the "-X" option.
-
-The tags mentioned above are used to describe how various developers have
-been associated with the development of this patch. They are described in
-detail in the SubmittingPatches document; what follows here is a brief
-summary. Each of these lines has the format:
-
-::
-
- tag: Full Name <email address> optional-other-stuff
-
-The tags in common use are:
-
- - Signed-off-by: this is a developer's certification that he or she has
- the right to submit the patch for inclusion into the kernel. It is an
- agreement to the Developer's Certificate of Origin, the full text of
- which can be found in Documentation/SubmittingPatches. Code without a
- proper signoff cannot be merged into the mainline.
-
- - Acked-by: indicates an agreement by another developer (often a
- maintainer of the relevant code) that the patch is appropriate for
- inclusion into the kernel.
-
- - Tested-by: states that the named person has tested the patch and found
- it to work.
-
- - Reviewed-by: the named developer has reviewed the patch for correctness;
- see the reviewer's statement in Documentation/SubmittingPatches for more
- detail.
-
- - Reported-by: names a user who reported a problem which is fixed by this
- patch; this tag is used to give credit to the (often underappreciated)
- people who test our code and let us know when things do not work
- correctly.
-
- - Cc: the named person received a copy of the patch and had the
- opportunity to comment on it.
-
-Be careful in the addition of tags to your patches: only Cc: is appropriate
-for addition without the explicit permission of the person named.
-
-
-Sending the patch
------------------
-
-Before you mail your patches, there are a couple of other things you should
-take care of:
-
- - Are you sure that your mailer will not corrupt the patches? Patches
- which have had gratuitous white-space changes or line wrapping performed
- by the mail client will not apply at the other end, and often will not
- be examined in any detail. If there is any doubt at all, mail the patch
- to yourself and convince yourself that it shows up intact.
-
- Documentation/email-clients.txt has some helpful hints on making
- specific mail clients work for sending patches.
-
- - Are you sure your patch is free of silly mistakes? You should always
- run patches through scripts/checkpatch.pl and address the complaints it
- comes up with. Please bear in mind that checkpatch.pl, while being the
- embodiment of a fair amount of thought about what kernel patches should
- look like, is not smarter than you. If fixing a checkpatch.pl complaint
- would make the code worse, don't do it.
-
-Patches should always be sent as plain text. Please do not send them as
-attachments; that makes it much harder for reviewers to quote sections of
-the patch in their replies. Instead, just put the patch directly into your
-message.
-
-When mailing patches, it is important to send copies to anybody who might
-be interested in it. Unlike some other projects, the kernel encourages
-people to err on the side of sending too many copies; don't assume that the
-relevant people will see your posting on the mailing lists. In particular,
-copies should go to:
-
- - The maintainer(s) of the affected subsystem(s). As described earlier,
- the MAINTAINERS file is the first place to look for these people.
-
- - Other developers who have been working in the same area - especially
- those who might be working there now. Using git to see who else has
- modified the files you are working on can be helpful.
-
- - If you are responding to a bug report or a feature request, copy the
- original poster as well.
-
- - Send a copy to the relevant mailing list, or, if nothing else applies,
- the linux-kernel list.
-
- - If you are fixing a bug, think about whether the fix should go into the
- next stable update. If so, stable@vger.kernel.org should get a copy of
- the patch. Also add a "Cc: stable@vger.kernel.org" to the tags within
- the patch itself; that will cause the stable team to get a notification
- when your fix goes into the mainline.
-
-When selecting recipients for a patch, it is good to have an idea of who
-you think will eventually accept the patch and get it merged. While it
-is possible to send patches directly to Linus Torvalds and have him merge
-them, things are not normally done that way. Linus is busy, and there are
-subsystem maintainers who watch over specific parts of the kernel. Usually
-you will be wanting that maintainer to merge your patches. If there is no
-obvious maintainer, Andrew Morton is often the patch target of last resort.
-
-Patches need good subject lines. The canonical format for a patch line is
-something like:
-
-::
-
- [PATCH nn/mm] subsys: one-line description of the patch
-
-where "nn" is the ordinal number of the patch, "mm" is the total number of
-patches in the series, and "subsys" is the name of the affected subsystem.
-Clearly, nn/mm can be omitted for a single, standalone patch.
-
-If you have a significant series of patches, it is customary to send an
-introductory description as part zero. This convention is not universally
-followed though; if you use it, remember that information in the
-introduction does not make it into the kernel changelogs. So please ensure
-that the patches, themselves, have complete changelog information.
-
-In general, the second and following parts of a multi-part patch should be
-sent as a reply to the first part so that they all thread together at the
-receiving end. Tools like git and quilt have commands to mail out a set of
-patches with the proper threading. If you have a long series, though, and
-are using git, please stay away from the --chain-reply-to option to avoid
-creating exceptionally deep nesting.
--- /dev/null
+.. _development_posting:
+
+Posting patches
+===============
+
+Sooner or later, the time comes when your work is ready to be presented to
+the community for review and, eventually, inclusion into the mainline
+kernel. Unsurprisingly, the kernel development community has evolved a set
+of conventions and procedures which are used in the posting of patches;
+following them will make life much easier for everybody involved. This
+document will attempt to cover these expectations in reasonable detail;
+more information can also be found in the files SubmittingPatches,
+SubmittingDrivers, and SubmitChecklist in the kernel documentation
+directory.
+
+
+When to post
+------------
+
+There is a constant temptation to avoid posting patches before they are
+completely "ready." For simple patches, that is not a problem. If the
+work being done is complex, though, there is a lot to be gained by getting
+feedback from the community before the work is complete. So you should
+consider posting in-progress work, or even making a git tree available so
+that interested developers can catch up with your work at any time.
+
+When posting code which is not yet considered ready for inclusion, it is a
+good idea to say so in the posting itself. Also mention any major work
+which remains to be done and any known problems. Fewer people will look at
+patches which are known to be half-baked, but those who do will come in
+with the idea that they can help you drive the work in the right direction.
+
+
+Before creating patches
+-----------------------
+
+There are a number of things which should be done before you consider
+sending patches to the development community. These include:
+
+ - Test the code to the extent that you can. Make use of the kernel's
+ debugging tools, ensure that the kernel will build with all reasonable
+ combinations of configuration options, use cross-compilers to build for
+ different architectures, etc.
+
+ - Make sure your code is compliant with the kernel coding style
+ guidelines.
+
+ - Does your change have performance implications? If so, you should run
+ benchmarks showing what the impact (or benefit) of your change is; a
+ summary of the results should be included with the patch.
+
+ - Be sure that you have the right to post the code. If this work was done
+ for an employer, the employer likely has a right to the work and must be
+ agreeable with its release under the GPL.
+
+As a general rule, putting in some extra thought before posting code almost
+always pays back the effort in short order.
+
+
+Patch preparation
+-----------------
+
+The preparation of patches for posting can be a surprising amount of work,
+but, once again, attempting to save time here is not generally advisable
+even in the short term.
+
+Patches must be prepared against a specific version of the kernel. As a
+general rule, a patch should be based on the current mainline as found in
+Linus's git tree. When basing on mainline, start with a well-known release
+point - a stable or -rc release - rather than branching off the mainline at
+an arbitrary spot.
+
+It may become necessary to make versions against -mm, linux-next, or a
+subsystem tree, though, to facilitate wider testing and review. Depending
+on the area of your patch and what is going on elsewhere, basing a patch
+against these other trees can require a significant amount of work
+resolving conflicts and dealing with API changes.
+
+Only the most simple changes should be formatted as a single patch;
+everything else should be made as a logical series of changes. Splitting
+up patches is a bit of an art; some developers spend a long time figuring
+out how to do it in the way that the community expects. There are a few
+rules of thumb, however, which can help considerably:
+
+ - The patch series you post will almost certainly not be the series of
+ changes found in your working revision control system. Instead, the
+ changes you have made need to be considered in their final form, then
+ split apart in ways which make sense. The developers are interested in
+ discrete, self-contained changes, not the path you took to get to those
+ changes.
+
+ - Each logically independent change should be formatted as a separate
+ patch. These changes can be small ("add a field to this structure") or
+ large (adding a significant new driver, for example), but they should be
+ conceptually small and amenable to a one-line description. Each patch
+ should make a specific change which can be reviewed on its own and
+ verified to do what it says it does.
+
+ - As a way of restating the guideline above: do not mix different types of
+ changes in the same patch. If a single patch fixes a critical security
+ bug, rearranges a few structures, and reformats the code, there is a
+ good chance that it will be passed over and the important fix will be
+ lost.
+
+ - Each patch should yield a kernel which builds and runs properly; if your
+ patch series is interrupted in the middle, the result should still be a
+ working kernel. Partial application of a patch series is a common
+ scenario when the "git bisect" tool is used to find regressions; if the
+ result is a broken kernel, you will make life harder for developers and
+ users who are engaging in the noble work of tracking down problems.
+
+ - Do not overdo it, though. One developer once posted a set of edits
+ to a single file as 500 separate patches - an act which did not make him
+ the most popular person on the kernel mailing list. A single patch can
+ be reasonably large as long as it still contains a single *logical*
+ change.
+
+ - It can be tempting to add a whole new infrastructure with a series of
+ patches, but to leave that infrastructure unused until the final patch
+ in the series enables the whole thing. This temptation should be
+ avoided if possible; if that series adds regressions, bisection will
+ finger the last patch as the one which caused the problem, even though
+ the real bug is elsewhere. Whenever possible, a patch which adds new
+ code should make that code active immediately.
+
+Working to create the perfect patch series can be a frustrating process
+which takes quite a bit of time and thought after the "real work" has been
+done. When done properly, though, it is time well spent.
+
+
+Patch formatting and changelogs
+-------------------------------
+
+So now you have a perfect series of patches for posting, but the work is
+not done quite yet. Each patch needs to be formatted into a message which
+quickly and clearly communicates its purpose to the rest of the world. To
+that end, each patch will be composed of the following:
+
+ - An optional "From" line naming the author of the patch. This line is
+ only necessary if you are passing on somebody else's patch via email,
+ but it never hurts to add it when in doubt.
+
+ - A one-line description of what the patch does. This message should be
+ enough for a reader who sees it with no other context to figure out the
+ scope of the patch; it is the line that will show up in the "short form"
+ changelogs. This message is usually formatted with the relevant
+ subsystem name first, followed by the purpose of the patch. For
+ example:
+
+ ::
+
+ gpio: fix build on CONFIG_GPIO_SYSFS=n
+
+ - A blank line followed by a detailed description of the contents of the
+ patch. This description can be as long as is required; it should say
+ what the patch does and why it should be applied to the kernel.
+
+ - One or more tag lines, with, at a minimum, one Signed-off-by: line from
+ the author of the patch. Tags will be described in more detail below.
+
+The items above, together, form the changelog for the patch. Writing good
+changelogs is a crucial but often-neglected art; it's worth spending
+another moment discussing this issue. When writing a changelog, you should
+bear in mind that a number of different people will be reading your words.
+These include subsystem maintainers and reviewers who need to decide
+whether the patch should be included, distributors and other maintainers
+trying to decide whether a patch should be backported to other kernels, bug
+hunters wondering whether the patch is responsible for a problem they are
+chasing, users who want to know how the kernel has changed, and more. A
+good changelog conveys the needed information to all of these people in the
+most direct and concise way possible.
+
+To that end, the summary line should describe the effects of and motivation
+for the change as well as possible given the one-line constraint. The
+detailed description can then amplify on those topics and provide any
+needed additional information. If the patch fixes a bug, cite the commit
+which introduced the bug if possible (and please provide both the commit ID
+and the title when citing commits). If a problem is associated with
+specific log or compiler output, include that output to help others
+searching for a solution to the same problem. If the change is meant to
+support other changes coming in later patch, say so. If internal APIs are
+changed, detail those changes and how other developers should respond. In
+general, the more you can put yourself into the shoes of everybody who will
+be reading your changelog, the better that changelog (and the kernel as a
+whole) will be.
+
+Needless to say, the changelog should be the text used when committing the
+change to a revision control system. It will be followed by:
+
+ - The patch itself, in the unified ("-u") patch format. Using the "-p"
+ option to diff will associate function names with changes, making the
+ resulting patch easier for others to read.
+
+You should avoid including changes to irrelevant files (those generated by
+the build process, for example, or editor backup files) in the patch. The
+file "dontdiff" in the Documentation directory can help in this regard;
+pass it to diff with the "-X" option.
+
+The tags mentioned above are used to describe how various developers have
+been associated with the development of this patch. They are described in
+detail in the SubmittingPatches document; what follows here is a brief
+summary. Each of these lines has the format:
+
+::
+
+ tag: Full Name <email address> optional-other-stuff
+
+The tags in common use are:
+
+ - Signed-off-by: this is a developer's certification that he or she has
+ the right to submit the patch for inclusion into the kernel. It is an
+ agreement to the Developer's Certificate of Origin, the full text of
+ which can be found in Documentation/SubmittingPatches. Code without a
+ proper signoff cannot be merged into the mainline.
+
+ - Acked-by: indicates an agreement by another developer (often a
+ maintainer of the relevant code) that the patch is appropriate for
+ inclusion into the kernel.
+
+ - Tested-by: states that the named person has tested the patch and found
+ it to work.
+
+ - Reviewed-by: the named developer has reviewed the patch for correctness;
+ see the reviewer's statement in Documentation/SubmittingPatches for more
+ detail.
+
+ - Reported-by: names a user who reported a problem which is fixed by this
+ patch; this tag is used to give credit to the (often underappreciated)
+ people who test our code and let us know when things do not work
+ correctly.
+
+ - Cc: the named person received a copy of the patch and had the
+ opportunity to comment on it.
+
+Be careful in the addition of tags to your patches: only Cc: is appropriate
+for addition without the explicit permission of the person named.
+
+
+Sending the patch
+-----------------
+
+Before you mail your patches, there are a couple of other things you should
+take care of:
+
+ - Are you sure that your mailer will not corrupt the patches? Patches
+ which have had gratuitous white-space changes or line wrapping performed
+ by the mail client will not apply at the other end, and often will not
+ be examined in any detail. If there is any doubt at all, mail the patch
+ to yourself and convince yourself that it shows up intact.
+
+ Documentation/email-clients.txt has some helpful hints on making
+ specific mail clients work for sending patches.
+
+ - Are you sure your patch is free of silly mistakes? You should always
+ run patches through scripts/checkpatch.pl and address the complaints it
+ comes up with. Please bear in mind that checkpatch.pl, while being the
+ embodiment of a fair amount of thought about what kernel patches should
+ look like, is not smarter than you. If fixing a checkpatch.pl complaint
+ would make the code worse, don't do it.
+
+Patches should always be sent as plain text. Please do not send them as
+attachments; that makes it much harder for reviewers to quote sections of
+the patch in their replies. Instead, just put the patch directly into your
+message.
+
+When mailing patches, it is important to send copies to anybody who might
+be interested in it. Unlike some other projects, the kernel encourages
+people to err on the side of sending too many copies; don't assume that the
+relevant people will see your posting on the mailing lists. In particular,
+copies should go to:
+
+ - The maintainer(s) of the affected subsystem(s). As described earlier,
+ the MAINTAINERS file is the first place to look for these people.
+
+ - Other developers who have been working in the same area - especially
+ those who might be working there now. Using git to see who else has
+ modified the files you are working on can be helpful.
+
+ - If you are responding to a bug report or a feature request, copy the
+ original poster as well.
+
+ - Send a copy to the relevant mailing list, or, if nothing else applies,
+ the linux-kernel list.
+
+ - If you are fixing a bug, think about whether the fix should go into the
+ next stable update. If so, stable@vger.kernel.org should get a copy of
+ the patch. Also add a "Cc: stable@vger.kernel.org" to the tags within
+ the patch itself; that will cause the stable team to get a notification
+ when your fix goes into the mainline.
+
+When selecting recipients for a patch, it is good to have an idea of who
+you think will eventually accept the patch and get it merged. While it
+is possible to send patches directly to Linus Torvalds and have him merge
+them, things are not normally done that way. Linus is busy, and there are
+subsystem maintainers who watch over specific parts of the kernel. Usually
+you will be wanting that maintainer to merge your patches. If there is no
+obvious maintainer, Andrew Morton is often the patch target of last resort.
+
+Patches need good subject lines. The canonical format for a patch line is
+something like:
+
+::
+
+ [PATCH nn/mm] subsys: one-line description of the patch
+
+where "nn" is the ordinal number of the patch, "mm" is the total number of
+patches in the series, and "subsys" is the name of the affected subsystem.
+Clearly, nn/mm can be omitted for a single, standalone patch.
+
+If you have a significant series of patches, it is customary to send an
+introductory description as part zero. This convention is not universally
+followed though; if you use it, remember that information in the
+introduction does not make it into the kernel changelogs. So please ensure
+that the patches, themselves, have complete changelog information.
+
+In general, the second and following parts of a multi-part patch should be
+sent as a reply to the first part so that they all thread together at the
+receiving end. Tools like git and quilt have commands to mail out a set of
+patches with the proper threading. If you have a long series, though, and
+are using git, please stay away from the --chain-reply-to option to avoid
+creating exceptionally deep nesting.
+++ /dev/null
-.. _development_followthrough:
-
-Followthrough
-=============
-
-At this point, you have followed the guidelines given so far and, with the
-addition of your own engineering skills, have posted a perfect series of
-patches. One of the biggest mistakes that even experienced kernel
-developers can make is to conclude that their work is now done. In truth,
-posting patches indicates a transition into the next stage of the process,
-with, possibly, quite a bit of work yet to be done.
-
-It is a rare patch which is so good at its first posting that there is no
-room for improvement. The kernel development process recognizes this fact,
-and, as a result, is heavily oriented toward the improvement of posted
-code. You, as the author of that code, will be expected to work with the
-kernel community to ensure that your code is up to the kernel's quality
-standards. A failure to participate in this process is quite likely to
-prevent the inclusion of your patches into the mainline.
-
-
-Working with reviewers
-----------------------
-
-A patch of any significance will result in a number of comments from other
-developers as they review the code. Working with reviewers can be, for
-many developers, the most intimidating part of the kernel development
-process. Life can be made much easier, though, if you keep a few things in
-mind:
-
- - If you have explained your patch well, reviewers will understand its
- value and why you went to the trouble of writing it. But that value
- will not keep them from asking a fundamental question: what will it be
- like to maintain a kernel with this code in it five or ten years later?
- Many of the changes you may be asked to make - from coding style tweaks
- to substantial rewrites - come from the understanding that Linux will
- still be around and under development a decade from now.
-
- - Code review is hard work, and it is a relatively thankless occupation;
- people remember who wrote kernel code, but there is little lasting fame
- for those who reviewed it. So reviewers can get grumpy, especially when
- they see the same mistakes being made over and over again. If you get a
- review which seems angry, insulting, or outright offensive, resist the
- impulse to respond in kind. Code review is about the code, not about
- the people, and code reviewers are not attacking you personally.
-
- - Similarly, code reviewers are not trying to promote their employers'
- agendas at the expense of your own. Kernel developers often expect to
- be working on the kernel years from now, but they understand that their
- employer could change. They truly are, almost without exception,
- working toward the creation of the best kernel they can; they are not
- trying to create discomfort for their employers' competitors.
-
-What all of this comes down to is that, when reviewers send you comments,
-you need to pay attention to the technical observations that they are
-making. Do not let their form of expression or your own pride keep that
-from happening. When you get review comments on a patch, take the time to
-understand what the reviewer is trying to say. If possible, fix the things
-that the reviewer is asking you to fix. And respond back to the reviewer:
-thank them, and describe how you will answer their questions.
-
-Note that you do not have to agree with every change suggested by
-reviewers. If you believe that the reviewer has misunderstood your code,
-explain what is really going on. If you have a technical objection to a
-suggested change, describe it and justify your solution to the problem. If
-your explanations make sense, the reviewer will accept them. Should your
-explanation not prove persuasive, though, especially if others start to
-agree with the reviewer, take some time to think things over again. It can
-be easy to become blinded by your own solution to a problem to the point
-that you don't realize that something is fundamentally wrong or, perhaps,
-you're not even solving the right problem.
-
-Andrew Morton has suggested that every review comment which does not result
-in a code change should result in an additional code comment instead; that
-can help future reviewers avoid the questions which came up the first time
-around.
-
-One fatal mistake is to ignore review comments in the hope that they will
-go away. They will not go away. If you repost code without having
-responded to the comments you got the time before, you're likely to find
-that your patches go nowhere.
-
-Speaking of reposting code: please bear in mind that reviewers are not
-going to remember all the details of the code you posted the last time
-around. So it is always a good idea to remind reviewers of previously
-raised issues and how you dealt with them; the patch changelog is a good
-place for this kind of information. Reviewers should not have to search
-through list archives to familiarize themselves with what was said last
-time; if you help them get a running start, they will be in a better mood
-when they revisit your code.
-
-What if you've tried to do everything right and things still aren't going
-anywhere? Most technical disagreements can be resolved through discussion,
-but there are times when somebody simply has to make a decision. If you
-honestly believe that this decision is going against you wrongly, you can
-always try appealing to a higher power. As of this writing, that higher
-power tends to be Andrew Morton. Andrew has a great deal of respect in the
-kernel development community; he can often unjam a situation which seems to
-be hopelessly blocked. Appealing to Andrew should not be done lightly,
-though, and not before all other alternatives have been explored. And bear
-in mind, of course, that he may not agree with you either.
-
-
-What happens next
------------------
-
-If a patch is considered to be a good thing to add to the kernel, and once
-most of the review issues have been resolved, the next step is usually
-entry into a subsystem maintainer's tree. How that works varies from one
-subsystem to the next; each maintainer has his or her own way of doing
-things. In particular, there may be more than one tree - one, perhaps,
-dedicated to patches planned for the next merge window, and another for
-longer-term work.
-
-For patches applying to areas for which there is no obvious subsystem tree
-(memory management patches, for example), the default tree often ends up
-being -mm. Patches which affect multiple subsystems can also end up going
-through the -mm tree.
-
-Inclusion into a subsystem tree can bring a higher level of visibility to a
-patch. Now other developers working with that tree will get the patch by
-default. Subsystem trees typically feed linux-next as well, making their
-contents visible to the development community as a whole. At this point,
-there's a good chance that you will get more comments from a new set of
-reviewers; these comments need to be answered as in the previous round.
-
-What may also happen at this point, depending on the nature of your patch,
-is that conflicts with work being done by others turn up. In the worst
-case, heavy patch conflicts can result in some work being put on the back
-burner so that the remaining patches can be worked into shape and merged.
-Other times, conflict resolution will involve working with the other
-developers and, possibly, moving some patches between trees to ensure that
-everything applies cleanly. This work can be a pain, but count your
-blessings: before the advent of the linux-next tree, these conflicts often
-only turned up during the merge window and had to be addressed in a hurry.
-Now they can be resolved at leisure, before the merge window opens.
-
-Some day, if all goes well, you'll log on and see that your patch has been
-merged into the mainline kernel. Congratulations! Once the celebration is
-complete (and you have added yourself to the MAINTAINERS file), though, it
-is worth remembering an important little fact: the job still is not done.
-Merging into the mainline brings its own challenges.
-
-To begin with, the visibility of your patch has increased yet again. There
-may be a new round of comments from developers who had not been aware of
-the patch before. It may be tempting to ignore them, since there is no
-longer any question of your code being merged. Resist that temptation,
-though; you still need to be responsive to developers who have questions or
-suggestions.
-
-More importantly, though: inclusion into the mainline puts your code into
-the hands of a much larger group of testers. Even if you have contributed
-a driver for hardware which is not yet available, you will be surprised by
-how many people will build your code into their kernels. And, of course,
-where there are testers, there will be bug reports.
-
-The worst sort of bug reports are regressions. If your patch causes a
-regression, you'll find an uncomfortable number of eyes upon you;
-regressions need to be fixed as soon as possible. If you are unwilling or
-unable to fix the regression (and nobody else does it for you), your patch
-will almost certainly be removed during the stabilization period. Beyond
-negating all of the work you have done to get your patch into the mainline,
-having a patch pulled as the result of a failure to fix a regression could
-well make it harder for you to get work merged in the future.
-
-After any regressions have been dealt with, there may be other, ordinary
-bugs to deal with. The stabilization period is your best opportunity to
-fix these bugs and ensure that your code's debut in a mainline kernel
-release is as solid as possible. So, please, answer bug reports, and fix
-the problems if at all possible. That's what the stabilization period is
-for; you can start creating cool new patches once any problems with the old
-ones have been taken care of.
-
-And don't forget that there are other milestones which may also create bug
-reports: the next mainline stable release, when prominent distributors pick
-up a version of the kernel containing your patch, etc. Continuing to
-respond to these reports is a matter of basic pride in your work. If that
-is insufficient motivation, though, it's also worth considering that the
-development community remembers developers who lose interest in their code
-after it's merged. The next time you post a patch, they will be evaluating
-it with the assumption that you will not be around to maintain it
-afterward.
-
-
-Other things that can happen
------------------------------
-
-One day, you may open your mail client and see that somebody has mailed you
-a patch to your code. That is one of the advantages of having your code
-out there in the open, after all. If you agree with the patch, you can
-either forward it on to the subsystem maintainer (be sure to include a
-proper From: line so that the attribution is correct, and add a signoff of
-your own), or send an Acked-by: response back and let the original poster
-send it upward.
-
-If you disagree with the patch, send a polite response explaining why. If
-possible, tell the author what changes need to be made to make the patch
-acceptable to you. There is a certain resistance to merging patches which
-are opposed by the author and maintainer of the code, but it only goes so
-far. If you are seen as needlessly blocking good work, those patches will
-eventually flow around you and get into the mainline anyway. In the Linux
-kernel, nobody has absolute veto power over any code. Except maybe Linus.
-
-On very rare occasion, you may see something completely different: another
-developer posts a different solution to your problem. At that point,
-chances are that one of the two patches will not be merged, and "mine was
-here first" is not considered to be a compelling technical argument. If
-somebody else's patch displaces yours and gets into the mainline, there is
-really only one way to respond: be pleased that your problem got solved and
-get on with your work. Having one's work shoved aside in this manner can
-be hurtful and discouraging, but the community will remember your reaction
-long after they have forgotten whose patch actually got merged.
--- /dev/null
+.. _development_followthrough:
+
+Followthrough
+=============
+
+At this point, you have followed the guidelines given so far and, with the
+addition of your own engineering skills, have posted a perfect series of
+patches. One of the biggest mistakes that even experienced kernel
+developers can make is to conclude that their work is now done. In truth,
+posting patches indicates a transition into the next stage of the process,
+with, possibly, quite a bit of work yet to be done.
+
+It is a rare patch which is so good at its first posting that there is no
+room for improvement. The kernel development process recognizes this fact,
+and, as a result, is heavily oriented toward the improvement of posted
+code. You, as the author of that code, will be expected to work with the
+kernel community to ensure that your code is up to the kernel's quality
+standards. A failure to participate in this process is quite likely to
+prevent the inclusion of your patches into the mainline.
+
+
+Working with reviewers
+----------------------
+
+A patch of any significance will result in a number of comments from other
+developers as they review the code. Working with reviewers can be, for
+many developers, the most intimidating part of the kernel development
+process. Life can be made much easier, though, if you keep a few things in
+mind:
+
+ - If you have explained your patch well, reviewers will understand its
+ value and why you went to the trouble of writing it. But that value
+ will not keep them from asking a fundamental question: what will it be
+ like to maintain a kernel with this code in it five or ten years later?
+ Many of the changes you may be asked to make - from coding style tweaks
+ to substantial rewrites - come from the understanding that Linux will
+ still be around and under development a decade from now.
+
+ - Code review is hard work, and it is a relatively thankless occupation;
+ people remember who wrote kernel code, but there is little lasting fame
+ for those who reviewed it. So reviewers can get grumpy, especially when
+ they see the same mistakes being made over and over again. If you get a
+ review which seems angry, insulting, or outright offensive, resist the
+ impulse to respond in kind. Code review is about the code, not about
+ the people, and code reviewers are not attacking you personally.
+
+ - Similarly, code reviewers are not trying to promote their employers'
+ agendas at the expense of your own. Kernel developers often expect to
+ be working on the kernel years from now, but they understand that their
+ employer could change. They truly are, almost without exception,
+ working toward the creation of the best kernel they can; they are not
+ trying to create discomfort for their employers' competitors.
+
+What all of this comes down to is that, when reviewers send you comments,
+you need to pay attention to the technical observations that they are
+making. Do not let their form of expression or your own pride keep that
+from happening. When you get review comments on a patch, take the time to
+understand what the reviewer is trying to say. If possible, fix the things
+that the reviewer is asking you to fix. And respond back to the reviewer:
+thank them, and describe how you will answer their questions.
+
+Note that you do not have to agree with every change suggested by
+reviewers. If you believe that the reviewer has misunderstood your code,
+explain what is really going on. If you have a technical objection to a
+suggested change, describe it and justify your solution to the problem. If
+your explanations make sense, the reviewer will accept them. Should your
+explanation not prove persuasive, though, especially if others start to
+agree with the reviewer, take some time to think things over again. It can
+be easy to become blinded by your own solution to a problem to the point
+that you don't realize that something is fundamentally wrong or, perhaps,
+you're not even solving the right problem.
+
+Andrew Morton has suggested that every review comment which does not result
+in a code change should result in an additional code comment instead; that
+can help future reviewers avoid the questions which came up the first time
+around.
+
+One fatal mistake is to ignore review comments in the hope that they will
+go away. They will not go away. If you repost code without having
+responded to the comments you got the time before, you're likely to find
+that your patches go nowhere.
+
+Speaking of reposting code: please bear in mind that reviewers are not
+going to remember all the details of the code you posted the last time
+around. So it is always a good idea to remind reviewers of previously
+raised issues and how you dealt with them; the patch changelog is a good
+place for this kind of information. Reviewers should not have to search
+through list archives to familiarize themselves with what was said last
+time; if you help them get a running start, they will be in a better mood
+when they revisit your code.
+
+What if you've tried to do everything right and things still aren't going
+anywhere? Most technical disagreements can be resolved through discussion,
+but there are times when somebody simply has to make a decision. If you
+honestly believe that this decision is going against you wrongly, you can
+always try appealing to a higher power. As of this writing, that higher
+power tends to be Andrew Morton. Andrew has a great deal of respect in the
+kernel development community; he can often unjam a situation which seems to
+be hopelessly blocked. Appealing to Andrew should not be done lightly,
+though, and not before all other alternatives have been explored. And bear
+in mind, of course, that he may not agree with you either.
+
+
+What happens next
+-----------------
+
+If a patch is considered to be a good thing to add to the kernel, and once
+most of the review issues have been resolved, the next step is usually
+entry into a subsystem maintainer's tree. How that works varies from one
+subsystem to the next; each maintainer has his or her own way of doing
+things. In particular, there may be more than one tree - one, perhaps,
+dedicated to patches planned for the next merge window, and another for
+longer-term work.
+
+For patches applying to areas for which there is no obvious subsystem tree
+(memory management patches, for example), the default tree often ends up
+being -mm. Patches which affect multiple subsystems can also end up going
+through the -mm tree.
+
+Inclusion into a subsystem tree can bring a higher level of visibility to a
+patch. Now other developers working with that tree will get the patch by
+default. Subsystem trees typically feed linux-next as well, making their
+contents visible to the development community as a whole. At this point,
+there's a good chance that you will get more comments from a new set of
+reviewers; these comments need to be answered as in the previous round.
+
+What may also happen at this point, depending on the nature of your patch,
+is that conflicts with work being done by others turn up. In the worst
+case, heavy patch conflicts can result in some work being put on the back
+burner so that the remaining patches can be worked into shape and merged.
+Other times, conflict resolution will involve working with the other
+developers and, possibly, moving some patches between trees to ensure that
+everything applies cleanly. This work can be a pain, but count your
+blessings: before the advent of the linux-next tree, these conflicts often
+only turned up during the merge window and had to be addressed in a hurry.
+Now they can be resolved at leisure, before the merge window opens.
+
+Some day, if all goes well, you'll log on and see that your patch has been
+merged into the mainline kernel. Congratulations! Once the celebration is
+complete (and you have added yourself to the MAINTAINERS file), though, it
+is worth remembering an important little fact: the job still is not done.
+Merging into the mainline brings its own challenges.
+
+To begin with, the visibility of your patch has increased yet again. There
+may be a new round of comments from developers who had not been aware of
+the patch before. It may be tempting to ignore them, since there is no
+longer any question of your code being merged. Resist that temptation,
+though; you still need to be responsive to developers who have questions or
+suggestions.
+
+More importantly, though: inclusion into the mainline puts your code into
+the hands of a much larger group of testers. Even if you have contributed
+a driver for hardware which is not yet available, you will be surprised by
+how many people will build your code into their kernels. And, of course,
+where there are testers, there will be bug reports.
+
+The worst sort of bug reports are regressions. If your patch causes a
+regression, you'll find an uncomfortable number of eyes upon you;
+regressions need to be fixed as soon as possible. If you are unwilling or
+unable to fix the regression (and nobody else does it for you), your patch
+will almost certainly be removed during the stabilization period. Beyond
+negating all of the work you have done to get your patch into the mainline,
+having a patch pulled as the result of a failure to fix a regression could
+well make it harder for you to get work merged in the future.
+
+After any regressions have been dealt with, there may be other, ordinary
+bugs to deal with. The stabilization period is your best opportunity to
+fix these bugs and ensure that your code's debut in a mainline kernel
+release is as solid as possible. So, please, answer bug reports, and fix
+the problems if at all possible. That's what the stabilization period is
+for; you can start creating cool new patches once any problems with the old
+ones have been taken care of.
+
+And don't forget that there are other milestones which may also create bug
+reports: the next mainline stable release, when prominent distributors pick
+up a version of the kernel containing your patch, etc. Continuing to
+respond to these reports is a matter of basic pride in your work. If that
+is insufficient motivation, though, it's also worth considering that the
+development community remembers developers who lose interest in their code
+after it's merged. The next time you post a patch, they will be evaluating
+it with the assumption that you will not be around to maintain it
+afterward.
+
+
+Other things that can happen
+-----------------------------
+
+One day, you may open your mail client and see that somebody has mailed you
+a patch to your code. That is one of the advantages of having your code
+out there in the open, after all. If you agree with the patch, you can
+either forward it on to the subsystem maintainer (be sure to include a
+proper From: line so that the attribution is correct, and add a signoff of
+your own), or send an Acked-by: response back and let the original poster
+send it upward.
+
+If you disagree with the patch, send a polite response explaining why. If
+possible, tell the author what changes need to be made to make the patch
+acceptable to you. There is a certain resistance to merging patches which
+are opposed by the author and maintainer of the code, but it only goes so
+far. If you are seen as needlessly blocking good work, those patches will
+eventually flow around you and get into the mainline anyway. In the Linux
+kernel, nobody has absolute veto power over any code. Except maybe Linus.
+
+On very rare occasion, you may see something completely different: another
+developer posts a different solution to your problem. At that point,
+chances are that one of the two patches will not be merged, and "mine was
+here first" is not considered to be a compelling technical argument. If
+somebody else's patch displaces yours and gets into the mainline, there is
+really only one way to respond: be pleased that your problem got solved and
+get on with your work. Having one's work shoved aside in this manner can
+be hurtful and discouraging, but the community will remember your reaction
+long after they have forgotten whose patch actually got merged.
+++ /dev/null
-.. _development_advancedtopics:
-
-Advanced topics
-===============
-
-At this point, hopefully, you have a handle on how the development process
-works. There is still more to learn, however! This section will cover a
-number of topics which can be helpful for developers wanting to become a
-regular part of the Linux kernel development process.
-
-Managing patches with git
--------------------------
-
-The use of distributed version control for the kernel began in early 2002,
-when Linus first started playing with the proprietary BitKeeper
-application. While BitKeeper was controversial, the approach to software
-version management it embodied most certainly was not. Distributed version
-control enabled an immediate acceleration of the kernel development
-project. In current times, there are several free alternatives to
-BitKeeper. For better or for worse, the kernel project has settled on git
-as its tool of choice.
-
-Managing patches with git can make life much easier for the developer,
-especially as the volume of those patches grows. Git also has its rough
-edges and poses certain hazards; it is a young and powerful tool which is
-still being civilized by its developers. This document will not attempt to
-teach the reader how to use git; that would be sufficient material for a
-long document in its own right. Instead, the focus here will be on how git
-fits into the kernel development process in particular. Developers who
-wish to come up to speed with git will find more information at:
-
- http://git-scm.com/
-
- http://www.kernel.org/pub/software/scm/git/docs/user-manual.html
-
-and on various tutorials found on the web.
-
-The first order of business is to read the above sites and get a solid
-understanding of how git works before trying to use it to make patches
-available to others. A git-using developer should be able to obtain a copy
-of the mainline repository, explore the revision history, commit changes to
-the tree, use branches, etc. An understanding of git's tools for the
-rewriting of history (such as rebase) is also useful. Git comes with its
-own terminology and concepts; a new user of git should know about refs,
-remote branches, the index, fast-forward merges, pushes and pulls, detached
-heads, etc. It can all be a little intimidating at the outset, but the
-concepts are not that hard to grasp with a bit of study.
-
-Using git to generate patches for submission by email can be a good
-exercise while coming up to speed.
-
-When you are ready to start putting up git trees for others to look at, you
-will, of course, need a server that can be pulled from. Setting up such a
-server with git-daemon is relatively straightforward if you have a system
-which is accessible to the Internet. Otherwise, free, public hosting sites
-(Github, for example) are starting to appear on the net. Established
-developers can get an account on kernel.org, but those are not easy to come
-by; see http://kernel.org/faq/ for more information.
-
-The normal git workflow involves the use of a lot of branches. Each line
-of development can be separated into a separate "topic branch" and
-maintained independently. Branches in git are cheap, there is no reason to
-not make free use of them. And, in any case, you should not do your
-development in any branch which you intend to ask others to pull from.
-Publicly-available branches should be created with care; merge in patches
-from development branches when they are in complete form and ready to go -
-not before.
-
-Git provides some powerful tools which can allow you to rewrite your
-development history. An inconvenient patch (one which breaks bisection,
-say, or which has some other sort of obvious bug) can be fixed in place or
-made to disappear from the history entirely. A patch series can be
-rewritten as if it had been written on top of today's mainline, even though
-you have been working on it for months. Changes can be transparently
-shifted from one branch to another. And so on. Judicious use of git's
-ability to revise history can help in the creation of clean patch sets with
-fewer problems.
-
-Excessive use of this capability can lead to other problems, though, beyond
-a simple obsession for the creation of the perfect project history.
-Rewriting history will rewrite the changes contained in that history,
-turning a tested (hopefully) kernel tree into an untested one. But, beyond
-that, developers cannot easily collaborate if they do not have a shared
-view of the project history; if you rewrite history which other developers
-have pulled into their repositories, you will make life much more difficult
-for those developers. So a simple rule of thumb applies here: history
-which has been exported to others should generally be seen as immutable
-thereafter.
-
-So, once you push a set of changes to your publicly-available server, those
-changes should not be rewritten. Git will attempt to enforce this rule if
-you try to push changes which do not result in a fast-forward merge
-(i.e. changes which do not share the same history). It is possible to
-override this check, and there may be times when it is necessary to rewrite
-an exported tree. Moving changesets between trees to avoid conflicts in
-linux-next is one example. But such actions should be rare. This is one
-of the reasons why development should be done in private branches (which
-can be rewritten if necessary) and only moved into public branches when
-it's in a reasonably advanced state.
-
-As the mainline (or other tree upon which a set of changes is based)
-advances, it is tempting to merge with that tree to stay on the leading
-edge. For a private branch, rebasing can be an easy way to keep up with
-another tree, but rebasing is not an option once a tree is exported to the
-world. Once that happens, a full merge must be done. Merging occasionally
-makes good sense, but overly frequent merges can clutter the history
-needlessly. Suggested technique in this case is to merge infrequently, and
-generally only at specific release points (such as a mainline -rc
-release). If you are nervous about specific changes, you can always
-perform test merges in a private branch. The git "rerere" tool can be
-useful in such situations; it remembers how merge conflicts were resolved
-so that you don't have to do the same work twice.
-
-One of the biggest recurring complaints about tools like git is this: the
-mass movement of patches from one repository to another makes it easy to
-slip in ill-advised changes which go into the mainline below the review
-radar. Kernel developers tend to get unhappy when they see that kind of
-thing happening; putting up a git tree with unreviewed or off-topic patches
-can affect your ability to get trees pulled in the future. Quoting Linus:
-
-::
-
- You can send me patches, but for me to pull a git patch from you, I
- need to know that you know what you're doing, and I need to be able
- to trust things *without* then having to go and check every
- individual change by hand.
-
-(http://lwn.net/Articles/224135/).
-
-To avoid this kind of situation, ensure that all patches within a given
-branch stick closely to the associated topic; a "driver fixes" branch
-should not be making changes to the core memory management code. And, most
-importantly, do not use a git tree to bypass the review process. Post an
-occasional summary of the tree to the relevant list, and, when the time is
-right, request that the tree be included in linux-next.
-
-If and when others start to send patches for inclusion into your tree,
-don't forget to review them. Also ensure that you maintain the correct
-authorship information; the git "am" tool does its best in this regard, but
-you may have to add a "From:" line to the patch if it has been relayed to
-you via a third party.
-
-When requesting a pull, be sure to give all the relevant information: where
-your tree is, what branch to pull, and what changes will result from the
-pull. The git request-pull command can be helpful in this regard; it will
-format the request as other developers expect, and will also check to be
-sure that you have remembered to push those changes to the public server.
-
-
-Reviewing patches
------------------
-
-Some readers will certainly object to putting this section with "advanced
-topics" on the grounds that even beginning kernel developers should be
-reviewing patches. It is certainly true that there is no better way to
-learn how to program in the kernel environment than by looking at code
-posted by others. In addition, reviewers are forever in short supply; by
-looking at code you can make a significant contribution to the process as a
-whole.
-
-Reviewing code can be an intimidating prospect, especially for a new kernel
-developer who may well feel nervous about questioning code - in public -
-which has been posted by those with more experience. Even code written by
-the most experienced developers can be improved, though. Perhaps the best
-piece of advice for reviewers (all reviewers) is this: phrase review
-comments as questions rather than criticisms. Asking "how does the lock
-get released in this path?" will always work better than stating "the
-locking here is wrong."
-
-Different developers will review code from different points of view. Some
-are mostly concerned with coding style and whether code lines have trailing
-white space. Others will focus primarily on whether the change implemented
-by the patch as a whole is a good thing for the kernel or not. Yet others
-will check for problematic locking, excessive stack usage, possible
-security issues, duplication of code found elsewhere, adequate
-documentation, adverse effects on performance, user-space ABI changes, etc.
-All types of review, if they lead to better code going into the kernel, are
-welcome and worthwhile.
-
-
--- /dev/null
+.. _development_advancedtopics:
+
+Advanced topics
+===============
+
+At this point, hopefully, you have a handle on how the development process
+works. There is still more to learn, however! This section will cover a
+number of topics which can be helpful for developers wanting to become a
+regular part of the Linux kernel development process.
+
+Managing patches with git
+-------------------------
+
+The use of distributed version control for the kernel began in early 2002,
+when Linus first started playing with the proprietary BitKeeper
+application. While BitKeeper was controversial, the approach to software
+version management it embodied most certainly was not. Distributed version
+control enabled an immediate acceleration of the kernel development
+project. In current times, there are several free alternatives to
+BitKeeper. For better or for worse, the kernel project has settled on git
+as its tool of choice.
+
+Managing patches with git can make life much easier for the developer,
+especially as the volume of those patches grows. Git also has its rough
+edges and poses certain hazards; it is a young and powerful tool which is
+still being civilized by its developers. This document will not attempt to
+teach the reader how to use git; that would be sufficient material for a
+long document in its own right. Instead, the focus here will be on how git
+fits into the kernel development process in particular. Developers who
+wish to come up to speed with git will find more information at:
+
+ http://git-scm.com/
+
+ http://www.kernel.org/pub/software/scm/git/docs/user-manual.html
+
+and on various tutorials found on the web.
+
+The first order of business is to read the above sites and get a solid
+understanding of how git works before trying to use it to make patches
+available to others. A git-using developer should be able to obtain a copy
+of the mainline repository, explore the revision history, commit changes to
+the tree, use branches, etc. An understanding of git's tools for the
+rewriting of history (such as rebase) is also useful. Git comes with its
+own terminology and concepts; a new user of git should know about refs,
+remote branches, the index, fast-forward merges, pushes and pulls, detached
+heads, etc. It can all be a little intimidating at the outset, but the
+concepts are not that hard to grasp with a bit of study.
+
+Using git to generate patches for submission by email can be a good
+exercise while coming up to speed.
+
+When you are ready to start putting up git trees for others to look at, you
+will, of course, need a server that can be pulled from. Setting up such a
+server with git-daemon is relatively straightforward if you have a system
+which is accessible to the Internet. Otherwise, free, public hosting sites
+(Github, for example) are starting to appear on the net. Established
+developers can get an account on kernel.org, but those are not easy to come
+by; see http://kernel.org/faq/ for more information.
+
+The normal git workflow involves the use of a lot of branches. Each line
+of development can be separated into a separate "topic branch" and
+maintained independently. Branches in git are cheap, there is no reason to
+not make free use of them. And, in any case, you should not do your
+development in any branch which you intend to ask others to pull from.
+Publicly-available branches should be created with care; merge in patches
+from development branches when they are in complete form and ready to go -
+not before.
+
+Git provides some powerful tools which can allow you to rewrite your
+development history. An inconvenient patch (one which breaks bisection,
+say, or which has some other sort of obvious bug) can be fixed in place or
+made to disappear from the history entirely. A patch series can be
+rewritten as if it had been written on top of today's mainline, even though
+you have been working on it for months. Changes can be transparently
+shifted from one branch to another. And so on. Judicious use of git's
+ability to revise history can help in the creation of clean patch sets with
+fewer problems.
+
+Excessive use of this capability can lead to other problems, though, beyond
+a simple obsession for the creation of the perfect project history.
+Rewriting history will rewrite the changes contained in that history,
+turning a tested (hopefully) kernel tree into an untested one. But, beyond
+that, developers cannot easily collaborate if they do not have a shared
+view of the project history; if you rewrite history which other developers
+have pulled into their repositories, you will make life much more difficult
+for those developers. So a simple rule of thumb applies here: history
+which has been exported to others should generally be seen as immutable
+thereafter.
+
+So, once you push a set of changes to your publicly-available server, those
+changes should not be rewritten. Git will attempt to enforce this rule if
+you try to push changes which do not result in a fast-forward merge
+(i.e. changes which do not share the same history). It is possible to
+override this check, and there may be times when it is necessary to rewrite
+an exported tree. Moving changesets between trees to avoid conflicts in
+linux-next is one example. But such actions should be rare. This is one
+of the reasons why development should be done in private branches (which
+can be rewritten if necessary) and only moved into public branches when
+it's in a reasonably advanced state.
+
+As the mainline (or other tree upon which a set of changes is based)
+advances, it is tempting to merge with that tree to stay on the leading
+edge. For a private branch, rebasing can be an easy way to keep up with
+another tree, but rebasing is not an option once a tree is exported to the
+world. Once that happens, a full merge must be done. Merging occasionally
+makes good sense, but overly frequent merges can clutter the history
+needlessly. Suggested technique in this case is to merge infrequently, and
+generally only at specific release points (such as a mainline -rc
+release). If you are nervous about specific changes, you can always
+perform test merges in a private branch. The git "rerere" tool can be
+useful in such situations; it remembers how merge conflicts were resolved
+so that you don't have to do the same work twice.
+
+One of the biggest recurring complaints about tools like git is this: the
+mass movement of patches from one repository to another makes it easy to
+slip in ill-advised changes which go into the mainline below the review
+radar. Kernel developers tend to get unhappy when they see that kind of
+thing happening; putting up a git tree with unreviewed or off-topic patches
+can affect your ability to get trees pulled in the future. Quoting Linus:
+
+::
+
+ You can send me patches, but for me to pull a git patch from you, I
+ need to know that you know what you're doing, and I need to be able
+ to trust things *without* then having to go and check every
+ individual change by hand.
+
+(http://lwn.net/Articles/224135/).
+
+To avoid this kind of situation, ensure that all patches within a given
+branch stick closely to the associated topic; a "driver fixes" branch
+should not be making changes to the core memory management code. And, most
+importantly, do not use a git tree to bypass the review process. Post an
+occasional summary of the tree to the relevant list, and, when the time is
+right, request that the tree be included in linux-next.
+
+If and when others start to send patches for inclusion into your tree,
+don't forget to review them. Also ensure that you maintain the correct
+authorship information; the git "am" tool does its best in this regard, but
+you may have to add a "From:" line to the patch if it has been relayed to
+you via a third party.
+
+When requesting a pull, be sure to give all the relevant information: where
+your tree is, what branch to pull, and what changes will result from the
+pull. The git request-pull command can be helpful in this regard; it will
+format the request as other developers expect, and will also check to be
+sure that you have remembered to push those changes to the public server.
+
+
+Reviewing patches
+-----------------
+
+Some readers will certainly object to putting this section with "advanced
+topics" on the grounds that even beginning kernel developers should be
+reviewing patches. It is certainly true that there is no better way to
+learn how to program in the kernel environment than by looking at code
+posted by others. In addition, reviewers are forever in short supply; by
+looking at code you can make a significant contribution to the process as a
+whole.
+
+Reviewing code can be an intimidating prospect, especially for a new kernel
+developer who may well feel nervous about questioning code - in public -
+which has been posted by those with more experience. Even code written by
+the most experienced developers can be improved, though. Perhaps the best
+piece of advice for reviewers (all reviewers) is this: phrase review
+comments as questions rather than criticisms. Asking "how does the lock
+get released in this path?" will always work better than stating "the
+locking here is wrong."
+
+Different developers will review code from different points of view. Some
+are mostly concerned with coding style and whether code lines have trailing
+white space. Others will focus primarily on whether the change implemented
+by the patch as a whole is a good thing for the kernel or not. Yet others
+will check for problematic locking, excessive stack usage, possible
+security issues, duplication of code found elsewhere, adequate
+documentation, adverse effects on performance, user-space ABI changes, etc.
+All types of review, if they lead to better code going into the kernel, are
+welcome and worthwhile.
+
+
+++ /dev/null
-.. _development_conclusion:
-
-For more information
-====================
-
-There are numerous sources of information on Linux kernel development and
-related topics. First among those will always be the Documentation
-directory found in the kernel source distribution. The top-level HOWTO
-file is an important starting point; SubmittingPatches and
-SubmittingDrivers are also something which all kernel developers should
-read. Many internal kernel APIs are documented using the kerneldoc
-mechanism; "make htmldocs" or "make pdfdocs" can be used to generate those
-documents in HTML or PDF format (though the version of TeX shipped by some
-distributions runs into internal limits and fails to process the documents
-properly).
-
-Various web sites discuss kernel development at all levels of detail. Your
-author would like to humbly suggest http://lwn.net/ as a source;
-information on many specific kernel topics can be found via the LWN kernel
-index at:
-
- http://lwn.net/Kernel/Index/
-
-Beyond that, a valuable resource for kernel developers is:
-
- http://kernelnewbies.org/
-
-And, of course, one should not forget http://kernel.org/, the definitive
-location for kernel release information.
-
-There are a number of books on kernel development:
-
- Linux Device Drivers, 3rd Edition (Jonathan Corbet, Alessandro
- Rubini, and Greg Kroah-Hartman). Online at
- http://lwn.net/Kernel/LDD3/.
-
- Linux Kernel Development (Robert Love).
-
- Understanding the Linux Kernel (Daniel Bovet and Marco Cesati).
-
-All of these books suffer from a common fault, though: they tend to be
-somewhat obsolete by the time they hit the shelves, and they have been on
-the shelves for a while now. Still, there is quite a bit of good
-information to be found there.
-
-Documentation for git can be found at:
-
- http://www.kernel.org/pub/software/scm/git/docs/
-
- http://www.kernel.org/pub/software/scm/git/docs/user-manual.html
-
-
-Conclusion
-==========
-
-Congratulations to anybody who has made it through this long-winded
-document. Hopefully it has provided a helpful understanding of how the
-Linux kernel is developed and how you can participate in that process.
-
-In the end, it's the participation that matters. Any open source software
-project is no more than the sum of what its contributors put into it. The
-Linux kernel has progressed as quickly and as well as it has because it has
-been helped by an impressively large group of developers, all of whom are
-working to make it better. The kernel is a premier example of what can be
-done when thousands of people work together toward a common goal.
-
-The kernel can always benefit from a larger developer base, though. There
-is always more work to do. But, just as importantly, most other
-participants in the Linux ecosystem can benefit through contributing to the
-kernel. Getting code into the mainline is the key to higher code quality,
-lower maintenance and distribution costs, a higher level of influence over
-the direction of kernel development, and more. It is a situation where
-everybody involved wins. Fire up your editor and come join us; you will be
-more than welcome.
--- /dev/null
+.. _development_conclusion:
+
+For more information
+====================
+
+There are numerous sources of information on Linux kernel development and
+related topics. First among those will always be the Documentation
+directory found in the kernel source distribution. The top-level HOWTO
+file is an important starting point; SubmittingPatches and
+SubmittingDrivers are also something which all kernel developers should
+read. Many internal kernel APIs are documented using the kerneldoc
+mechanism; "make htmldocs" or "make pdfdocs" can be used to generate those
+documents in HTML or PDF format (though the version of TeX shipped by some
+distributions runs into internal limits and fails to process the documents
+properly).
+
+Various web sites discuss kernel development at all levels of detail. Your
+author would like to humbly suggest http://lwn.net/ as a source;
+information on many specific kernel topics can be found via the LWN kernel
+index at:
+
+ http://lwn.net/Kernel/Index/
+
+Beyond that, a valuable resource for kernel developers is:
+
+ http://kernelnewbies.org/
+
+And, of course, one should not forget http://kernel.org/, the definitive
+location for kernel release information.
+
+There are a number of books on kernel development:
+
+ Linux Device Drivers, 3rd Edition (Jonathan Corbet, Alessandro
+ Rubini, and Greg Kroah-Hartman). Online at
+ http://lwn.net/Kernel/LDD3/.
+
+ Linux Kernel Development (Robert Love).
+
+ Understanding the Linux Kernel (Daniel Bovet and Marco Cesati).
+
+All of these books suffer from a common fault, though: they tend to be
+somewhat obsolete by the time they hit the shelves, and they have been on
+the shelves for a while now. Still, there is quite a bit of good
+information to be found there.
+
+Documentation for git can be found at:
+
+ http://www.kernel.org/pub/software/scm/git/docs/
+
+ http://www.kernel.org/pub/software/scm/git/docs/user-manual.html
+
+
+Conclusion
+==========
+
+Congratulations to anybody who has made it through this long-winded
+document. Hopefully it has provided a helpful understanding of how the
+Linux kernel is developed and how you can participate in that process.
+
+In the end, it's the participation that matters. Any open source software
+project is no more than the sum of what its contributors put into it. The
+Linux kernel has progressed as quickly and as well as it has because it has
+been helped by an impressively large group of developers, all of whom are
+working to make it better. The kernel is a premier example of what can be
+done when thousands of people work together toward a common goal.
+
+The kernel can always benefit from a larger developer base, though. There
+is always more work to do. But, just as importantly, most other
+participants in the Linux ecosystem can benefit through contributing to the
+kernel. Getting code into the mainline is the key to higher code quality,
+lower maintenance and distribution costs, a higher level of influence over
+the direction of kernel development, and more. It is a situation where
+everybody involved wins. Fire up your editor and come join us; you will be
+more than welcome.
projects / cascardo / linux.git / commitdiff