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
--- a/arch/arm/boot/compressed/head.S
+++ b/arch/arm/boot/compressed/head.S
@@ -168,9 +168,26 @@
.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