[U-Boot] [PATCH v2 1/2] arm: make arm926ejs startup code thumb compatible

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Klaus Goger <klaus.goger at theobroma-systems.com> writes:

> When building the mxs platform in thumb mode gcc generates code using
> the intra procedure call scratch register (ip/r12) for the calling the
> lowlevel_init function. This modifies the lr in flush_dcache which
> causes u-boot proper to end in an endless loop.
>
> 40002334:       e1a0c00e        mov     ip, lr
> 40002338:       eb00df4c        bl      4003a070
> <__lowlevel_init_from_arm>
> 4000233c:       e1a0e00c        mov     lr, ip
> 40002340:       e1a0f00e        mov     pc, lr
> [...]
> 4003a070 <__lowlevel_init_from_arm>:
> 4003a070:       e59fc004        ldr     ip, [pc, #4]    ; 4003a07c
> <__lowlevel_init_from_arm+0xc>
> 4003a074:       e08fc00c        add     ip, pc, ip
> 4003a078:       e12fff1c        bx      ip
> 4003a07c:       fffc86cd        .word   0xfffc86cd
>
> Instead of using the the ip/r12 register we use sl/r10 to preserve the
> link register.
>
> According to "Procedure Call Standard for the ARM Architecture" by ARM
> subroutines have to preserve the contents of register r4-r8, r10, r11
> and SP. So using r10 instead of r12 should be save.
>
> Signed-off-by: Klaus Goger <klaus.goger at theobroma-systems.com>
> Signed-off-by: Christoph Muellner <christoph.muellner at theobroma-systems.com>

This problem isn't specific to Thumb mode.  An ARM build would also
break if the lowlevel_init function happened to clobber r12, which it is
permitted to do.  It's just dumb luck that this hasn't happened yet.

> ---
>
> Changes in v2:
> - use bl instead of blx to call lowlevel_init
> - remove mxs tag as it apply to all arm926ejs platforms
>
>  arch/arm/cpu/arm926ejs/start.S | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/arch/arm/cpu/arm926ejs/start.S b/arch/arm/cpu/arm926ejs/start.S
> index 959d1ed86d..317df5c401 100644
> --- a/arch/arm/cpu/arm926ejs/start.S
> +++ b/arch/arm/cpu/arm926ejs/start.S
> @@ -105,9 +105,9 @@ flush_dcache:
>  	/*
>  	 * Go setup Memory and board specific bits prior to relocation.
>  	 */
> -	mov	ip, lr		/* perserve link reg across call */
> +	mov	sl, lr		/* perserve link reg across call */
>  	bl	lowlevel_init	/* go setup pll,mux,memory */
> -	mov	lr, ip		/* restore link */
> +	mov	lr, sl		/* restore link */

I prefer to use plain register names (r10) rather than the aliases (sl)
when not using them for the special functions indicated by the latter.

>  #endif
> -	mov	pc, lr		/* back to my caller */
> +	bx	lr		/* back to my caller */

This change seems unrelated.  Yes, bx is the preferred instruction, but
using mov here isn't breaking anything.  If it bothers you, feel free to
make a separate patch fixing all the instances of mov to the pc
register, not just this one.

-- 
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