s390: clarify compressed image code path

The way the decompressor is hooked into the start-up code is rather
subtle, with a mix of multiply-defined symbols and hardcoded address
literals. Add some comments at the junction points to clarify how it
works.

Signed-off-by: Sascha Silbe <silbe@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

diff --git a/arch/s390/boot/compressed/head.S b/arch/s390/boot/compressed/head.S
index f86a4ee..28c4f96 100644 (file)
--- a/arch/s390/boot/compressed/head.S
+++ b/arch/s390/boot/compressed/head.S
@@ -21,16 +21,21 @@ ENTRY(startup_continue)
        lg      %r15,.Lstack-.LPG1(%r13)
        aghi    %r15,-160
        brasl   %r14,decompress_kernel
-       # setup registers for memory mover & branch to target
+       # Set up registers for memory mover. We move the decompressed image to
+       # 0x11000, starting at offset 0x11000 in the decompressed image so
+       # that code living at 0x11000 in the image will end up at 0x11000 in
+       # memory.
        lgr     %r4,%r2
        lg      %r2,.Loffset-.LPG1(%r13)
        la      %r4,0(%r2,%r4)
        lg      %r3,.Lmvsize-.LPG1(%r13)
        lgr     %r5,%r3
-       # move the memory mover someplace safe
+       # Move the memory mover someplace safe so it doesn't overwrite itself.
        la      %r1,0x200
        mvc     0(mover_end-mover,%r1),mover-.LPG1(%r13)
-       # decompress image is started at 0x11000
+       # When the memory mover is done we pass control to
+       # arch/s390/kernel/head64.S:startup_continue which lives at 0x11000 in
+       # the decompressed image.
        lgr     %r6,%r2
        br      %r1
 mover:

diff --git a/arch/s390/kernel/head.S b/arch/s390/kernel/head.S
index 56e4d82..4431905 100644 (file)
--- a/arch/s390/kernel/head.S
+++ b/arch/s390/kernel/head.S
@@ -309,7 +309,9 @@ ENTRY(startup_kdump)
        l       %r15,.Lstack-.LPG0(%r13)
        ahi     %r15,-STACK_FRAME_OVERHEAD
        brasl   %r14,verify_facilities
-       /* Continue with startup code in head64.S */
+# For uncompressed images, continue in
+# arch/s390/kernel/head64.S. For compressed images, continue in
+# arch/s390/boot/compressed/head.S.
        jg      startup_continue
 
 .Lstack: