[U-Boot] [PATCH v1] arm: mxs: make startup code thumb compatible

[Marek Vasut]

On 04/18/2018 06:25 PM, Klaus Goger wrote:
> 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>
> 
> ---
> 
>  arch/arm/cpu/arm926ejs/start.S | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/arm/cpu/arm926ejs/start.S b/arch/arm/cpu/arm926ejs/start.S
> index 959d1ed86d..103bd15914 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 */
> -	bl	lowlevel_init	/* go setup pll,mux,memory */
> -	mov	lr, ip		/* restore link */
> +	mov	sl, lr		/* perserve link reg across call */
> +	blx	lowlevel_init	/* go setup pll,mux,memory */
> +	mov	lr, sl		/* restore link */
>  #endif
> -	mov	pc, lr		/* back to my caller */
> +	bx	lr		/* back to my caller */
>  #endif /* CONFIG_SKIP_LOWLEVEL_INIT */

Are you sure this doesn't break the non-thumb operation ?
Also, is this really MXS specific or not ?

-- 
Best regards,
Marek Vasut