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

[Marek Vasut]

On 04/19/2018 10:02 AM, klaus.goger at theobroma-systems.com wrote:
> 
>> 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.

Can lowlevel_init be ARM code if U-Boot is compiled in Thumb mode ? I
guess it can if lowlevel_init is assembler code, in which case doing
blx/bx to it would call it in Thumb mode and crash ?

I might be wrong though

>> 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.


-- 
Best regards,
Marek Vasut