ARM: 6503/1: Thumb-2: Restore sensible zImage header layout for CONFIG_THUMB2_KERNEL

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 <[e-mail]>
Acked-by: Catalin Marinas <[e-mail]>
Acked-by: Nicolas Pitre <[e-mail]>
Signed-off-by: Russell King <[e-mail]>

arch/arm/boot/Makefile

···
 $(obj)/uImage: LOADADDR=$(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)

arch/arm/boot/compressed/head.S

···
  * 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
 
···
 		.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