libpayload: arm: Pass the coreboot table location to the payload.

To find the coreboot tables, the payload has historically searched for their
signature in a predefined region of memory. This is a little clumsy on x86,
but it works because you can assume certain regions are RAM. Also, there are
areas which are set aside for the firmware by convention. On x86 there's a
forwarding entry which goes in one of those fairly small conventional areas
and which points to the CBMEM area at the end of memory.

On ARM there aren't areas like that, so we've left out the forwarding entry and
gone directly to CBMEM. RAM may not start at the beginning of the address space
or go to its end, and that means there isn't really anywhere fixed you can put
the coreboot tables. That's meant that libpayload has to be configured on a
per board basis to know where to look for CBMEM.

Now that we have boards that don't have fixed amounts of memory, the location
of the end of RAM isn't fixed even on a per board level which means even that
workaround will no longer cut it.

This change makes coreboot pass the location of the coreboot tables to
libpayload using r0, the first argument register. That means we'll be able to
find them no matter where CBMEM is, and we can get rid of the per board search
ranges.

We can extend this mechanism to x86 as well, but there may be more
complications and it's less necessary there. It would be a good thing to do
eventually though.

BUG=None
TEST=Built and booted on nyan. Changed the size of memory and saw that the
payload could still find the coreboot tables where before it couldn't. Built
for pit, snow, and big.
BRANCH=None

Original-Change-Id: I7218afd999da1662b0db8172fd8125670ceac471
Original-Signed-off-by: Gabe Black <gabeblack@google.com>
Original-Reviewed-on: https://chromium-review.googlesource.com/185572
Original-Reviewed-by: Julius Werner <jwerner@chromium.org>
Original-Commit-Queue: Gabe Black <gabeblack@chromium.org>
Original-Tested-by: Gabe Black <gabeblack@chromium.org>
(cherry picked from commit ca88f39c21158b59abe3001f986207a292359cf5)
Signed-off-by: Marc Jones <marc.jones@se-eng.com>

Change-Id: Iab14e9502b6ce7a55f0a72e190fa582f89f11a1e
Reviewed-on: http://review.coreboot.org/7655
Tested-by: build bot (Jenkins)
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>

1 files changed, 7 insertions, 1 deletions

diff --git a/payloads/libpayload/arch/arm/head.S b/payloads/libpayload/arch/arm/head.S
index 54fdb5defd..c5c96ea38f 100644
--- a/payloads/libpayload/arch/arm/head.S
+++ b/payloads/libpayload/arch/arm/head.S
@@ -34,12 +34,16 @@
  */
 ENTRY(_entry)
 
+	/* Save off the location of the coreboot tables */
+	ldr r1, 1f
+	str r0, [r1]
+
 	/* TODO: disable interrupts */
 
 	/* TODO: Clear BSS */
 
 	/* Setup new stack */
-	ldr sp, 1f
+	ldr sp, 2f
 
 	/* TODO: Save old stack pointer and link register */
 
@@ -56,4 +60,6 @@ ENDPROC(_entry)
 
 .align 4
 1:
+.word	cb_header_ptr
+2:
 .word	_stack