ARM: nommu: avoid deprecated source register on mov

Commit Message

Stefan Agner May 22, 2015, 10:46 p.m. UTC

In Thumb2 mode, the stack register r13 is deprecated if the
destination register is the program counter (r15). Similar to
head.S, head-nommu.S uses r13 to store the return address used
after configuring the CPU's CP15 register. However, since we do
not enable a MMU, there will be no address switch and it is
possible to use branch with link instruction to call
__after_proc_init.

Avoid using r13 completely by using bl to call __after_proc_init
and get rid of __secondary_switched.

Beside removing unnecessary complexity, this also fixes a
compiler warning when compiling a !MMU kernel:
Warning: Use of r13 as a source register is deprecated when r15
is the destination register.

Suggested-by: Russell King <[e-mail]>
Signed-off-by: Stefan Agner <[e-mail]>
---
The patch takes the BSYM() to badr change into account and hence
applies cleanly on Russell's for-next branch. Tested using a
single core Cortex-M4 configuration.

 arch/arm/kernel/head-nommu.S | 22 ++++++++--------------
 1 file changed, 8 insertions(+), 14 deletions(-)

Comments

Russell King - ARM Linux May 22, 2015, 11:04 p.m. UTC | #1

On Sat, May 23, 2015 at 12:46:52AM +0200, Stefan Agner wrote:
> @@ -106,32 +106,26 @@ ENTRY(secondary_startup)
>  	movs	r10, r5				@ invalid processor?
>  	beq	__error_p			@ yes, error 'p'
>  
> -	adr	r4, __secondary_data
> -	ldmia	r4, {r7, r12}
> -
>  #ifdef CONFIG_ARM_MPU
>  	/* Use MPU region info supplied by __cpu_up */
> +	ldr	r7, __secondary_data

Almost, you want this above the #ifdef though, as r7 is used below.
("set up the stack pointer").  Apart from that, I don't see any
obvious problems, thanks.

>  	ldr	r6, [r7]			@ get secondary_data.mpu_szr
>  	bl      __setup_mpu			@ Initialize the MPU
>  #endif
>  
> -	badr	lr, __after_proc_init		@ return address
> -	mov	r13, r12			@ __secondary_switched address
> +	badr	lr, 1f				@ return (PIC) address
>  	ldr	r12, [r10, #PROCINFO_INITFUNC]
>  	add	r12, r12, r10
>  	ret	r12
> -ENDPROC(secondary_startup)
> -
> -ENTRY(__secondary_switched)
> +1:	bl	__after_proc_init
>  	ldr	sp, [r7, #12]			@ set up the stack pointer

Patch

diff --git a/arch/arm/kernel/head-nommu.S b/arch/arm/kernel/head-nommu.S
index c966016..570d89a 100644
--- a/arch/arm/kernel/head-nommu.S
+++ b/arch/arm/kernel/head-nommu.S
@@ -77,13 +77,13 @@  ENTRY(stext)
 	orr	r6, r6, #(1 << MPU_RSR_EN)	@ Set region enabled bit
 	bl	__setup_mpu
 #endif
-	ldr	r13, =__mmap_switched		@ address to jump to after
-						@ initialising sctlr
+
 	badr	lr, 1f				@ return (PIC) address
 	ldr	r12, [r10, #PROCINFO_INITFUNC]
 	add	r12, r12, r10
 	ret	r12
- 1:	b	__after_proc_init
+1:	bl	__after_proc_init
+	b	__mmap_switched
 ENDPROC(stext)
 
 #ifdef CONFIG_SMP
@@ -106,32 +106,26 @@  ENTRY(secondary_startup)
 	movs	r10, r5				@ invalid processor?
 	beq	__error_p			@ yes, error 'p'
 
-	adr	r4, __secondary_data
-	ldmia	r4, {r7, r12}
-
 #ifdef CONFIG_ARM_MPU
 	/* Use MPU region info supplied by __cpu_up */
+	ldr	r7, __secondary_data
 	ldr	r6, [r7]			@ get secondary_data.mpu_szr
 	bl      __setup_mpu			@ Initialize the MPU
 #endif
 
-	badr	lr, __after_proc_init		@ return address
-	mov	r13, r12			@ __secondary_switched address
+	badr	lr, 1f				@ return (PIC) address
 	ldr	r12, [r10, #PROCINFO_INITFUNC]
 	add	r12, r12, r10
 	ret	r12
-ENDPROC(secondary_startup)
-
-ENTRY(__secondary_switched)
+1:	bl	__after_proc_init
 	ldr	sp, [r7, #12]			@ set up the stack pointer
 	mov	fp, #0
 	b	secondary_start_kernel
-ENDPROC(__secondary_switched)
+ENDPROC(secondary_startup)
 
 	.type	__secondary_data, %object
 __secondary_data:
 	.long	secondary_data
-	.long	__secondary_switched
 #endif /* CONFIG_SMP */
 
 /*
@@ -164,7 +158,7 @@  __after_proc_init:
 #endif
 	mcr	p15, 0, r0, c1, c0, 0		@ write control reg
 #endif /* CONFIG_CPU_CP15 */
-	ret	r13
+	ret	lr
 ENDPROC(__after_proc_init)
 	.ltorg