ARM: add documentation for finding start of physical memory

Occasionally, there's a question about the method we use to find the
start of physical memory.  Add some documentation so we don't have to
keep repeating outselves on the mailing list.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

diff --git a/arch/arm/boot/compressed/head.S b/arch/arm/boot/compressed/head.S
index c41a793..55a3532 100644 (file)
--- a/arch/arm/boot/compressed/head.S
+++ b/arch/arm/boot/compressed/head.S
@@ -168,9 +168,26 @@ not_angel:
                .text
 
 #ifdef CONFIG_AUTO_ZRELADDR
-               @ determine final kernel image address
+               /*
+                * Find the start of physical memory.  As we are executing
+                * without the MMU on, we are in the physical address space.
+                * We just need to get rid of any offset by aligning the
+                * address.
+                *
+                * This alignment is a balance between the requirements of
+                * different platforms - we have chosen 128MB to allow
+                * platforms which align the start of their physical memory
+                * to 128MB to use this feature, while allowing the zImage
+                * to be placed within the first 128MB of memory on other
+                * platforms.  Increasing the alignment means we place
+                * stricter alignment requirements on the start of physical
+                * memory, but relaxing it means that we break people who
+                * are already placing their zImage in (eg) the top 64MB
+                * of this range.
+                */
                mov     r4, pc
                and     r4, r4, #0xf8000000
+               /* Determine final kernel image address. */
                add     r4, r4, #TEXT_OFFSET
 #else
                ldr     r4, =zreladdr