[PATCH v2.1] ARM: Thumb-2: Restore sensible zImage header layout for CONFIG_THUMB2_KERNEL

[Dave Martin]

The code which makes up the zImage header intends to leave a
32-byte gap followed by a branch to the real entry point, a
magic number, and a word containing the absolute entry point
address.

This gets messed up with with CONFIG_THUMB2_KERNEL, because the
size of the initial padding NOPs changes.

Instead, the header can be made fully compatible by restoring
it to ARM.

In the Thumb-2 case, we can replace the initial NOPs with a
sequence which switches to Thumb and jumps to the real entry
point.

As a consequence, the zImage entry point is now always ARM, so
no special magic is needed any more for the uImage rules in the
Thumb-2 case.

Signed-off-by: Dave Martin <dave.martin at linaro.org>
Acked-by: Catalin Marinas <catalin.marinas at arm.com>
Acked-by: Nicolas Pitre <nicolas.pitre at linaro.org>
---

v2.1:

   * Stop replacing "b 1f" with a nop in the zImage header.
	This stops ARM kernels booting (though Thumb-2 kernel
	still work, since the bx instruction bypasses this branch.)

 arch/arm/boot/Makefile          |    5 -----
 arch/arm/boot/compressed/head.S |    7 ++++++-
 2 files changed, 6 insertions(+), 6 deletions(-)

diff --git a/arch/arm/boot/Makefile b/arch/arm/boot/Makefile
index 4a590f4..4d26f2c 100644
--- a/arch/arm/boot/Makefile
+++ b/arch/arm/boot/Makefile
@@ -70,12 +70,7 @@ else
 $(obj)/uImage: LOADADDR=$(ZRELADDR)
 endif
 
-ifeq ($(CONFIG_THUMB2_KERNEL),y)
-# Set bit 0 to 1 so that "mov pc, rx" switches to Thumb-2 mode
-$(obj)/uImage: STARTADDR=$(shell echo $(LOADADDR) | sed -e "s/.$$/1/")
-else
 $(obj)/uImage: STARTADDR=$(LOADADDR)
-endif
 
 $(obj)/uImage:	$(obj)/zImage FORCE
 	$(call if_changed,uimage)
diff --git a/arch/arm/boot/compressed/head.S b/arch/arm/boot/compressed/head.S
index 6825c34..57f7634 100644
--- a/arch/arm/boot/compressed/head.S
+++ b/arch/arm/boot/compressed/head.S
@@ -125,9 +125,13 @@ wait:		mrc	p14, 0, pc, c0, c1, 0
  * sort out different calling conventions
  */
 		.align
+		.arm				@ Always enter in ARM state
 start:
 		.type	start,#function
-		.rept	8
+ THUMB(		adr	r12, BSYM(1f)	)
+ THUMB(		bx	r12		)
+ THUMB(		.rept	6		)
+ ARM(		.rept	8		)
 		mov	r0, r0
 		.endr
 
@@ -135,6 +139,7 @@ start:
 		.word	0x016f2818		@ Magic numbers to help the loader
 		.word	start			@ absolute load/run zImage address
 		.word	_edata			@ zImage end address
+ THUMB(		.thumb			)
 1:		mov	r7, r1			@ save architecture ID
 		mov	r8, r2			@ save atags pointer
 
-- 
1.7.1