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

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Message-ID: <CAMuHMdXM0qg23UN6VBqbb0Vm2bg3tRSM=OCD5r7U2K1brpnJAg@mail.gmail.com>
Date:   Tue, 9 Mar 2021 10:16:24 +0100
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Christophe Leroy <christophe.leroy@...roup.eu>
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()

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?

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds