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

[klaus.goger at theobroma-systems.com]

> On 18.04.2018, at 22:02, Marek Vasut <marex at denx.de> wrote:
> 
> 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 ?

Yes I’m sure. I also tested thumb and non-thumb builds on the same device with
that patch. Both give you a proper u-boot shell.

I think there will be more patches down the road fixing other thumb/non-thumb issues
(like booting a kernel). But they are not related to that patchset.

> Also, is this really MXS specific or not ?

My error, shouldn’t have tagged it mxs as it’s for all arm926ejs platforms.

I guess arm920t, arm946es, arm720t, arm1136 and omap3 will have the same issue as
it has more or less verbatim copies of these functions.
But do the lack of hardware I could not test it.