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Commit 0a6a78b8 authored by Russell King's avatar Russell King
Browse files

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: default avatarRussell King <rmk+kernel@arm.linux.org.uk>
parent 89cfdb19

arch/arm/boot/compressed/head.S

+18 −1
Original line number Diff line number Diff line
@@ -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