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

[klaus.goger at theobroma-systems.com]

> On 20.04.2018, at 23:55, Marek Vasut <marex at denx.de> wrote:
> 
> On 04/20/2018 09:51 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>
>> 
>> ---
>> 
>> Changes in v2:
>> - use bl instead of blx to call lowlevel_init
>> - remove mxs tag as it apply to all arm926ejs platforms
> 
> What is lowlevel_init() is implemented as a C code and therefore thumb ?
> :-) doesn't ie. armv7a handle this somehow already ?
 
lowlevel_init() is implemented in C (you wrote it :)) and therefore thumb if
compiled as thumb.
armv7 lowlevel_init is implemented in assembly.

As the compiler will generate the necessary interwork code there was no need
to change the branch instruction to blx. So I changed it back to bl as the original
code as this will work with both generated codes (arm and thumb) and also
if someone decides to reimplement lowlevel_init as arm assembly.

> 
>> 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 */
>> #endif
>> -	mov	pc, lr		/* back to my caller */
>> +	bx	lr		/* back to my caller */
>> #endif /* CONFIG_SKIP_LOWLEVEL_INIT */
>> 
> 
> 
> -- 
> Best regards,
> Marek Vasut