ARM: 8677/1: boot/compressed: fix decompressor header layout for v7-M

As reported by Patrice, the header layout of the decompressor is
incorrect when building for v7-M. In this case, the __nop macro
resolves to 'mov r0, r0', which is emitted as a narrow encoding,
resulting in the header data fields to end up at lower offsets than
required.

Given the variety of targets we need to support with the same code,
the startup sequence is a bit of a jumble, and uses instructions
and macros whose encoding widths cannot be specified (badr), or only
exist in a narrow encoding (bx)

So force the use of a wide encoding in __nop, and replace the start
sequence with a simple jump to the label marking the start of code,
preceded by a Thumb2 mode switch if required (using explicit wide
encodings where appropriate). The label itself can be moved to the
start of code [where it belongs] due to the larger range of branch
instructions as compared to adr instructions.
Reported-by: NPatrice CHOTARD <patrice.chotard@st.com>
Acked-by: NNicolas Pitre <nico@linaro.org>
Signed-off-by: NArd Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: NRussell King <rmk+kernel@armlinux.org.uk>

arch/arm/boot/compressed/efi-header.S

@@ -17,14 +17,12 @@
 		@ there.
 		.inst	'M' | ('Z' << 8) | (0x1310 << 16)   @ tstne r0, #0x4d000
 #else
-		mov	r0, r0
+		W(mov)	r0, r0
 #endif
 		.endm
 
 		.macro	__EFI_HEADER
 #ifdef CONFIG_EFI_STUB
-		b	__efi_start
-
 		.set	start_offset, __efi_start - start
 		.org	start + 0x3c
 		@

arch/arm/boot/compressed/head.S

@@ -130,19 +130,22 @@ start:
 		.rept	7
 		__nop
 		.endr
-   ARM(		mov	r0, r0		)
-   ARM(		b	1f		)
- THUMB(		badr	r12, 1f		)
- THUMB(		bx	r12		)
+#ifndef CONFIG_THUMB2_KERNEL
+		mov	r0, r0
+#else
+ AR_CLASS(	sub	pc, pc, #3	)	@ A/R: switch to Thumb2 mode
+  M_CLASS(	nop.w			)	@ M: already in Thumb2 mode
+		.thumb
+#endif
+		W(b)	1f
 
 		.word	_magic_sig	@ Magic numbers to help the loader
 		.word	_magic_start	@ absolute load/run zImage address
 		.word	_magic_end	@ zImage end address
 		.word	0x04030201	@ endianness flag
 
- THUMB(		.thumb			)
-1:		__EFI_HEADER
-
+		__EFI_HEADER
+1:
  ARM_BE8(	setend	be		)	@ go BE8 if compiled for BE8
  AR_CLASS(	mrs	r9, cpsr	)
 #ifdef CONFIG_ARM_VIRT_EXT