Re: [PATCH] powerpc: Fix missing declaration of [en/dis]able_kernel_vsx()

[Geert Uytterhoeven]

Hi Christophe,

On Tue, Mar 9, 2021 at 10:58 AM Christophe Leroy
<christophe.leroy@xxxxxxxxxx> 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@xxxxxxxxxx> 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@xxxxxxxxxx> 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@xxxxxxxxxxxxxxx
> >>>> Signed-off-by: Christophe Leroy <christophe.leroy@xxxxxxxxxx>
> >>>
> >>> 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.

> 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.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx

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