[RFC] arm: pick the r2 passed DTB over the appended one

[Valentin Longchamp]

Thanks to the APPENDED_DTB option, it is possible to append the dtb to
the kernel's binary image. This is very convenient to accomodate old
bootloaders that do not pass a dtb.

We have updated a product with a new version of the bootloader that now
passed the dtb properly in r2. However, we still want to support with
the same software (i.e. kernel) the old bootloader, where the dtb must
be appended, but give the priority to to the one passed in r2.

This patch tries cover the above use case. It works for the case a dtb
is passed in r2 and one is appended (the one pointed by r2 is picked).
However, when there is only an appended dtb and r2 points to an ATAG
list, this fails. Somehow, the presumably updated r2 does not point to a
valid dtb and it gets zeroed by __vet_atags. Below is an example of
this (end of bootloader, start of kernel):

at 02000000 ...
  Image Name:   Linux-3.10.60-7.2.2-00002-gc6acc
  Image Type:   ARM Linux Kernel Image (uncompressed)
  Data Size:    2904550 Bytes = 2.8 MiB
  Load Address: 00008000
  Entry Point:  00008000
  Verifying Checksum ... OK
  Loading Kernel Image ... OK
OK
r2 (0x00000100) points to (size, header): 0x00000005, 0x54410001

Starting kernel ...

Uncompressing Linux... done, booting the kernel.
e5943000
trying to parse the FDT

dt_phys (r2) is NULL !

Error: unrecognized/unsupported machine ID (r1 = 0x000008cf).

Available machine support:

ID (hex)	NAME
ffffffff	Marvell Kirkwood (Flattened Device Tree)

Please check your kernel config and/or bootloader.

Is this patch a correct approach to implement the above use case ? If
yes, does anyone has an idea why this fails ?

Note: this patch is for the 3.10 stable branch.

Signed-off-by: Valentin Longchamp <valentin.longchamp at keymile.com>
---
 arch/arm/boot/compressed/head.S | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/arch/arm/boot/compressed/head.S b/arch/arm/boot/compressed/head.S
index 032a8d9..a43c8a0 100644
--- a/arch/arm/boot/compressed/head.S
+++ b/arch/arm/boot/compressed/head.S
@@ -244,7 +244,7 @@ restart:	adr	r0, LC0
  * dtb data will get relocated along with the kernel if necessary.
  */
 
-		ldr	lr, [r6, #0]
+		ldr	lr, [r6, #0]		@ potential appended dtb ?
 #ifndef __ARMEB__
 		ldr	r1, =0xedfe0dd0		@ sig is 0xd00dfeed big endian
 #else
@@ -253,6 +253,10 @@ restart:	adr	r0, LC0
 		cmp	lr, r1
 		bne	dtb_check_done		@ not found
 
+		ldr	lr, [r8, #0] 		@ conventionaly passed dtb ?
+		cmp	lr, r1
+		beq	dtb_check_done		@ yes, do not manage it
+
 #ifdef CONFIG_ARM_ATAG_DTB_COMPAT
 		/*
 		 * OK... Let's do some funky business here.
-- 
1.8.0.1