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>

2 files changed, 11 insertions, 4 deletions

diff --git a/arch/s390/boot/compressed/head.S b/arch/s390/boot/compressed/head.S
index f86a4eef28a9..28c4f96a2d9c 100644
--- 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 56e4d8234ef2..4431905f8cfa 100644
--- 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: