linux-kernel - Re: [PATCH] powerpc: Fix missing declaration of [en/dis]able_kernel

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Date:   Tue, 9 Mar 2021 09:52:11 +0100
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        amd-gfx list <amd-gfx@...ts.freedesktop.org>,
        Christian König <christian.koenig@....com>,
        Alex Deucher <alexdeucher@...il.com>
Subject: Re: [PATCH] powerpc: Fix missing declaration of
 [en/dis]able_kernel_vsx()



Le 09/03/2021 à 09:45, Geert Uytterhoeven a écrit :
> Hi Christophe,
> 
> On Tue, Mar 9, 2021 at 9:39 AM Christophe Leroy
> <christophe.leroy@...roup.eu> wrote:
>> Add stub instances of enable_kernel_vsx() and disable_kernel_vsx()
>> when CONFIG_VSX is not set, to avoid following build failure.
>>
>>    CC [M]  drivers/gpu/drm/amd/amdgpu/../display/dc/calcs/dcn_calcs.o
>> In file included from ./drivers/gpu/drm/amd/amdgpu/../display/dc/dm_services_types.h:29,
>>                   from ./drivers/gpu/drm/amd/amdgpu/../display/dc/dm_services.h:37,
>>                   from drivers/gpu/drm/amd/amdgpu/../display/dc/calcs/dcn_calcs.c:27:
>> drivers/gpu/drm/amd/amdgpu/../display/dc/calcs/dcn_calcs.c: In function 'dcn_bw_apply_registry_override':
>> ./drivers/gpu/drm/amd/amdgpu/../display/dc/os_types.h:64:3: error: implicit declaration of function 'enable_kernel_vsx'; did you mean 'enable_kernel_fp'? [-Werror=implicit-function-declaration]
>>     64 |   enable_kernel_vsx(); \
>>        |   ^~~~~~~~~~~~~~~~~
>> drivers/gpu/drm/amd/amdgpu/../display/dc/calcs/dcn_calcs.c:640:2: note: in expansion of macro 'DC_FP_START'
>>    640 |  DC_FP_START();
>>        |  ^~~~~~~~~~~
>> ./drivers/gpu/drm/amd/amdgpu/../display/dc/os_types.h:75:3: error: implicit declaration of function 'disable_kernel_vsx'; did you mean 'disable_kernel_fp'? [-Werror=implicit-function-declaration]
>>     75 |   disable_kernel_vsx(); \
>>        |   ^~~~~~~~~~~~~~~~~~
>> drivers/gpu/drm/amd/amdgpu/../display/dc/calcs/dcn_calcs.c:676:2: note: in expansion of macro 'DC_FP_END'
>>    676 |  DC_FP_END();
>>        |  ^~~~~~~~~
>> cc1: some warnings being treated as errors
>> make[5]: *** [drivers/gpu/drm/amd/amdgpu/../display/dc/calcs/dcn_calcs.o] Error 1
>>
>> Fixes: 16a9dea110a6 ("amdgpu: Enable initial DCN support on POWER")
>> Cc: stable@...r.kernel.org
>> Signed-off-by: Christophe Leroy <christophe.leroy@...roup.eu>
> 
> Thanks for your patch!
> 
>> --- a/arch/powerpc/include/asm/switch_to.h
>> +++ b/arch/powerpc/include/asm/switch_to.h
>> @@ -71,6 +71,16 @@ static inline void disable_kernel_vsx(void)
>>   {
>>          msr_check_and_clear(MSR_FP|MSR_VEC|MSR_VSX);
>>   }
>> +#else
>> +static inline void enable_kernel_vsx(void)
>> +{
>> +       BUILD_BUG();
>> +}
>> +
>> +static inline void disable_kernel_vsx(void)
>> +{
>> +       BUILD_BUG();
>> +}
>>   #endif
> 
> I'm wondering how this is any better than the current situation: using
> BUILD_BUG() will still cause a build failure?

No it won't cause a failure. In drivers/gpu/drm/amd/display/dc/os_types.h you have:

#define DC_FP_START() { \
	if (cpu_has_feature(CPU_FTR_VSX_COMP)) { \
		preempt_disable(); \
		enable_kernel_vsx(); \
	} else if (cpu_has_feature(CPU_FTR_ALTIVEC_COMP)) { \
		preempt_disable(); \
		enable_kernel_altivec(); \
	} else if (!cpu_has_feature(CPU_FTR_FPU_UNAVAILABLE)) { \
		preempt_disable(); \
		enable_kernel_fp(); \
	} \

When CONFIG_VSX is not selected, cpu_has_feature(CPU_FTR_VSX_COMP) constant folds to 'false' so the 
call to enable_kernel_vsx() is discarded and the build succeeds.

> 
> What about adding "depends on !POWERPC || VSX" instead, to prevent
> the issue from happening in the first place?

CONFIG_VSX is not required as pointed by the DC_FP_START() macro above and the matching DC_FP_END() 
macro.

> 
> Gr{oetje,eeting}s,
> 
>                          Geert
> 


Christophe