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

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Date:   Tue, 9 Mar 2021 10:57:44 +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 10:16, Geert Uytterhoeven a écrit :
> Hi Christophe,
> 
> On Tue, Mar 9, 2021 at 9:52 AM Christophe Leroy
> <christophe.leroy@...roup.eu> wrote:
>> Le 09/03/2021 à 09:45, Geert Uytterhoeven a écrit :
>>> 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.
> 
> IC. So you might as well have an empty (dummy) function instead?
> 

But with an empty function, you take the risk that one day, someone calls it without checking that 
CONFIG_VSX is selected. Here if someone does that, build will fail.

Another solution is to declare a non static prototype of it, like __put_user_bad() for instance. In 
that case, the link will fail.

I prefer the BUILD_BUG() approach as I find it cleaner and more explicit, and also because it breaks 
the build at compile time, you don't have to wait link time to catch the error.

Christophe