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ARM: add documentation for finding start of physical memory

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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>
This commit is contained in:
Russell King 2015-03-26 09:41:33 +00:00
parent 89cfdb19a8
commit 0a6a78b8b3
1 changed files with 18 additions and 1 deletions
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19
arch/arm/boot/compressed/head.S
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@ -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