--- /dev/null
+This document describes the generic device tree binding for IOMMUs and their
+master(s).
+
+
+IOMMU device node:
+==================
+
+An IOMMU can provide the following services:
+
+* Remap address space to allow devices to access physical memory ranges that
+ they otherwise wouldn't be capable of accessing.
+
+ Example: 32-bit DMA to 64-bit physical addresses
+
+* Implement scatter-gather at page level granularity so that the device does
+ not have to.
+
+* Provide system protection against "rogue" DMA by forcing all accesses to go
+ through the IOMMU and faulting when encountering accesses to unmapped
+ address regions.
+
+* Provide address space isolation between multiple contexts.
+
+ Example: Virtualization
+
+Device nodes compatible with this binding represent hardware with some of the
+above capabilities.
+
+IOMMUs can be single-master or multiple-master. Single-master IOMMU devices
+typically have a fixed association to the master device, whereas multiple-
+master IOMMU devices can translate accesses from more than one master.
+
+The device tree node of the IOMMU device's parent bus must contain a valid
+"dma-ranges" property that describes how the physical address space of the
+IOMMU maps to memory. An empty "dma-ranges" property means that there is a
+1:1 mapping from IOMMU to memory.
+
+Required properties:
+--------------------
+- #iommu-cells: The number of cells in an IOMMU specifier needed to encode an
+ address.
+
+The meaning of the IOMMU specifier is defined by the device tree binding of
+the specific IOMMU. Below are a few examples of typical use-cases:
+
+- #iommu-cells = <0>: Single master IOMMU devices are not configurable and
+ therefore no additional information needs to be encoded in the specifier.
+ This may also apply to multiple master IOMMU devices that do not allow the
+ association of masters to be configured. Note that an IOMMU can by design
+ be multi-master yet only expose a single master in a given configuration.
+ In such cases the number of cells will usually be 1 as in the next case.
+- #iommu-cells = <1>: Multiple master IOMMU devices may need to be configured
+ in order to enable translation for a given master. In such cases the single
+ address cell corresponds to the master device's ID. In some cases more than
+ one cell can be required to represent a single master ID.
+- #iommu-cells = <4>: Some IOMMU devices allow the DMA window for masters to
+ be configured. The first cell of the address in this may contain the master
+ device's ID for example, while the second cell could contain the start of
+ the DMA window for the given device. The length of the DMA window is given
+ by the third and fourth cells.
+
+Note that these are merely examples and real-world use-cases may use different
+definitions to represent their individual needs. Always refer to the specific
+IOMMU binding for the exact meaning of the cells that make up the specifier.
+
+
+IOMMU master node:
+==================
+
+Devices that access memory through an IOMMU are called masters. A device can
+have multiple master interfaces (to one or more IOMMU devices).
+
+Required properties:
+--------------------
+- iommus: A list of phandle and IOMMU specifier pairs that describe the IOMMU
+ master interfaces of the device. One entry in the list describes one master
+ interface of the device.
+
+When an "iommus" property is specified in a device tree node, the IOMMU will
+be used for address translation. If a "dma-ranges" property exists in the
+device's parent node it will be ignored. An exception to this rule is if the
+referenced IOMMU is disabled, in which case the "dma-ranges" property of the
+parent shall take effect. Note that merely disabling a device tree node does
+not guarantee that the IOMMU is really disabled since the hardware may not
+have a means to turn off translation. But it is invalid in such cases to
+disable the IOMMU's device tree node in the first place because it would
+prevent any driver from properly setting up the translations.
+
+
+Notes:
+======
+
+One possible extension to the above is to use an "iommus" property along with
+a "dma-ranges" property in a bus device node (such as PCI host bridges). This
+can be useful to describe how children on the bus relate to the IOMMU if they
+are not explicitly listed in the device tree (e.g. PCI devices). However, the
+requirements of that use-case haven't been fully determined yet. Implementing
+this is therefore not recommended without further discussion and extension of
+this binding.
+
+
+Examples:
+=========
+
+Single-master IOMMU:
+--------------------
+
+ iommu {
+ #iommu-cells = <0>;
+ };
+
+ master {
+ iommus = <&{/iommu}>;
+ };
+
+Multiple-master IOMMU with fixed associations:
+----------------------------------------------
+
+ /* multiple-master IOMMU */
+ iommu {
+ /*
+ * Masters are statically associated with this IOMMU and share
+ * the same address translations because the IOMMU does not
+ * have sufficient information to distinguish between masters.
+ *
+ * Consequently address translation is always on or off for
+ * all masters at any given point in time.
+ */
+ #iommu-cells = <0>;
+ };
+
+ /* static association with IOMMU */
+ master@1 {
+ reg = <1>;
+ iommus = <&{/iommu}>;
+ };
+
+ /* static association with IOMMU */
+ master@2 {
+ reg = <2>;
+ iommus = <&{/iommu}>;
+ };
+
+Multiple-master IOMMU:
+----------------------
+
+ iommu {
+ /* the specifier represents the ID of the master */
+ #iommu-cells = <1>;
+ };
+
+ master@1 {
+ /* device has master ID 42 in the IOMMU */
+ iommus = <&{/iommu} 42>;
+ };
+
+ master@2 {
+ /* device has master IDs 23 and 24 in the IOMMU */
+ iommus = <&{/iommu} 23>, <&{/iommu} 24>;
+ };
+
+Multiple-master IOMMU with configurable DMA window:
+---------------------------------------------------
+
+ / {
+ iommu {
+ /*
+ * One cell for the master ID and one cell for the
+ * address of the DMA window. The length of the DMA
+ * window is encoded in two cells.
+ *
+ * The DMA window is the range addressable by the
+ * master (i.e. the I/O virtual address space).
+ */
+ #iommu-cells = <4>;
+ };
+
+ master {
+ /* master ID 42, 4 GiB DMA window starting at 0 */
+ iommus = <&{/iommu} 42 0 0x1 0x0>;
+ };
+ };
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