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Subject: Re: [PATCH] powerpc: Fix missing declaration of
 [en/dis]able_kernel_vsx()
To:     Geert Uytterhoeven <geert@linux-m68k.org>
Cc:     Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
        amd-gfx list <amd-gfx@lists.freedesktop.org>,
        =?UTF-8?Q?Christian_K=c3=b6nig?= <christian.koenig@amd.com>,
        Alex Deucher <alexdeucher@gmail.com>
References: <8d7d285a027e9d21f5ff7f850fa71a2655b0c4af.1615279170.git.christophe.leroy@csgroup.eu>
 <CAMuHMdW0Cn1So8ckvhsT+N+p2hiPiksmCS32jzM0xCUYU4UAdQ@mail.gmail.com>
 <b12f9128-790b-7d8b-5f3c-e0912f5bec0a@csgroup.eu>
 <CAMuHMdXM0qg23UN6VBqbb0Vm2bg3tRSM=OCD5r7U2K1brpnJAg@mail.gmail.com>
 <b64111a3-df3f-bf59-20ce-0af57715ad53@csgroup.eu>
 <CAMuHMdUQcE7+O9NWH4Xxxv+r7ZFnTGqtHuteOMiSPY_gK5xkZw@mail.gmail.com>
From:   Christophe Leroy <christophe.leroy@csgroup.eu>
Message-ID: <5b579a54-e596-bcf2-b003-5c28345447b7@csgroup.eu>
Date:   Wed, 10 Mar 2021 14:33:25 +0100
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Hi Geert,

Le 09/03/2021 à 11:55, Geert Uytterhoeven a écrit :
> Hi Christophe,
> 
> On Tue, Mar 9, 2021 at 10:58 AM Christophe Leroy
> <christophe.leroy@csgroup.eu> wrote:
>> Le 09/03/2021 à 10:16, Geert Uytterhoeven a écrit :
>>> On Tue, Mar 9, 2021 at 9:52 AM Christophe Leroy
>>> <christophe.leroy@csgroup.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@csgroup.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@vger.kernel.org
>>>>>> Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.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.
> 
> OK, convinced.
> 

Note that following build test performed on kisskb, with gcc 4.9 the following change is required in 
addition: 
https://patchwork.ozlabs.org/project/linuxppc-dev/patch/b231dfa040ce4cc37f702f5c3a595fdeabfe0462.1615378209.git.christophe.leroy@csgroup.eu/

Christophe