[U-Boot] printf and R9 in ARM architecture

[U-Boot] printf and R9 in ARM architecture

[Da Shi Cao]

In standalone applications, it seems that the execution of printf
function in U-Boot is dependent on the value of register R9, which is
setup in app_startup. But r9 can be used by your program at the
decision of gcc compilation which is not aware of this dependence.
This is especially the case if +O2 is used!

My version is U-Boot 2014.10

Regards,
Dashi Cao

[U-Boot] printf and R9 in ARM architecture

[Albert ARIBAUD]

Hello Dashi Cao,

On Tue, 20 Jan 2015 14:12:35 +0800, Da Shi Cao <dscao999@gmail.com>
wrote:
> In standalone applications, it seems that the execution of printf
> function in U-Boot is dependent on the value of register R9, which is
> setup in app_startup. But r9 can be used by your program at the
> decision of gcc compilation which is not aware of this dependence.
> This is especially the case if +O2 is used!

U-Boot follows the ARM EABI, in which ""The role of register r9 is
platform specific". In U-Boot r9 is specifically used for the global
data structure, and the standalone programs must follow this EABI.

If your standalone program is built with the same options as U-Boot
example standalone programs are (see hellow_world), then it should
have gcc option -ffixed-r9 which will prevent the compiler from ever
touching r9.

How exactly do you build your standalone application?

> My version is U-Boot 2014.10
> 
> Regards,
> Dashi Cao

Amicalement,
-- 
Albert.

[U-Boot] printf and R9 in ARM architecture

[Da Shi Cao]

No, I didn't use that option. This option should fix it.

Thanks a lot.
Thomas

On Tue, Jan 20, 2015 at 5:37 PM, Albert ARIBAUD
<albert.u.boot@aribaud.net> wrote:
> Hello Dashi Cao,
>
> On Tue, 20 Jan 2015 14:12:35 +0800, Da Shi Cao <dscao999@gmail.com>
> wrote:
>> In standalone applications, it seems that the execution of printf
>> function in U-Boot is dependent on the value of register R9, which is
>> setup in app_startup. But r9 can be used by your program at the
>> decision of gcc compilation which is not aware of this dependence.
>> This is especially the case if +O2 is used!
>
> U-Boot follows the ARM EABI, in which ""The role of register r9 is
> platform specific". In U-Boot r9 is specifically used for the global
> data structure, and the standalone programs must follow this EABI.
>
> If your standalone program is built with the same options as U-Boot
> example standalone programs are (see hellow_world), then it should
> have gcc option -ffixed-r9 which will prevent the compiler from ever
> touching r9.
>
> How exactly do you build your standalone application?
>
>> My version is U-Boot 2014.10
>>
>> Regards,
>> Dashi Cao
>
> Amicalement,
> --
> Albert.

[U-Boot] printf and R9 in ARM architecture

[Albert ARIBAUD]

Hello Thomas,

On Tue, 20 Jan 2015 19:34:32 +0800, Da Shi Cao <dscao999@gmail.com>
wrote:
> No, I didn't use that option. This option should fix it.

Please also look at other options used for hello_world, as you might hit
other compatibility-related issues if you're not using the same ones.
 
> Thanks a lot.

No problem.

> Thomas

Amicalement,
-- 
Albert.

[U-Boot] printf and R9 in ARM architecture

[Marek Vasut]

On Tuesday, January 20, 2015 at 07:12:35 AM, Da Shi Cao wrote:
> In standalone applications, it seems that the execution of printf
> function in U-Boot is dependent on the value of register R9, which is
> setup in app_startup. But r9 can be used by your program at the
> decision of gcc compilation which is not aware of this dependence.
> This is especially the case if +O2 is used!

+CC the experts who probably broke it ;-)

Shouldn't you compile your standalone application with something like -ffixed-r9 
?

Best regards,
Marek Vasut