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>

1 files changed, 18 insertions, 1 deletions

diff --git a/arch/arm/boot/compressed/head.S b/arch/arm/boot/compressed/head.S
index c41a793b519c..55a353243a90 100644
--- 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