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

[Christophe Leroy]

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@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.

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