ARM: 8914/1: NOMMU: Fix exc_ret for XIP

It was reported that 72cd4064 "NOMMU: Toggle only bits in
EXC_RETURN we are really care of" breaks NOMMU+XIP combination.
It happens because saved EXC_RETURN gets overwritten when data
section is relocated.

The fix is to propagate EXC_RETURN via register and let relocation
code to commit that value into memory.

Fixes: 72cd4064 ("ARM: 8830/1: NOMMU: Toggle only bits in EXC_RETURN we are really care of")
Reported-by: afzal mohammed <afzal.mohd.ma@gmail.com>
Tested-by: afzal mohammed <afzal.mohd.ma@gmail.com>
Signed-off-by: Vladimir Murzin <vladimir.murzin@arm.com>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>

arch/arm/kernel/head-common.S

@@ -68,7 +68,7 @@ ENDPROC(__vet_atags)
  * The following fragment of code is executed with the MMU on in MMU mode,
  * and uses absolute addresses; this is not position independent.
  *
- *  r0  = cp#15 control register
+ *  r0  = cp#15 control register (exc_ret for M-class)
  *  r1  = machine ID
  *  r2  = atags/dtb pointer
  *  r9  = processor ID
@@ -137,7 +137,8 @@ __mmap_switched_data:
 #ifdef CONFIG_CPU_CP15
 	.long	cr_alignment			@ r3
 #else
-	.long	0				@ r3
+M_CLASS(.long	exc_ret)			@ r3
+AR_CLASS(.long	0)				@ r3
 #endif
 	.size	__mmap_switched_data, . - __mmap_switched_data
 

arch/arm/kernel/head-nommu.S

@@ -201,6 +201,8 @@ M_CLASS(streq	r3, [r12, #PMSAv8_MAIR1])
 	bic	r0, r0, #V7M_SCB_CCR_IC
 #endif
 	str	r0, [r12, V7M_SCB_CCR]
+	/* Pass exc_ret to __mmap_switched */
+	mov	r0, r10
 #endif /* CONFIG_CPU_CP15 elif CONFIG_CPU_V7M */
 	ret	lr
 ENDPROC(__after_proc_init)

arch/arm/mm/proc-v7m.S

@@ -136,9 +136,8 @@ __v7m_setup_cont:
 	cpsie	i
 	svc	#0
 1:	cpsid	i
-	ldr	r0, =exc_ret
-	orr	lr, lr, #EXC_RET_THREADMODE_PROCESSSTACK
-	str	lr, [r0]
+	/* Calculate exc_ret */
+	orr	r10, lr, #EXC_RET_THREADMODE_PROCESSSTACK
 	ldmia	sp, {r0-r3, r12}
 	str	r5, [r12, #11 * 4]	@ restore the original SVC vector entry
 	mov	lr, r6			@ restore LR