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

[Christophe Leroy]

Le 09/03/2021 à 09:45, Geert Uytterhoeven a écrit :
> Hi Christophe,
>
> 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.

> What about adding "depends on !POWERPC || VSX" instead, to prevent
> the issue from happening in the first place?

CONFIG_VSX is not required as pointed by the DC_FP_START() macro above and the matching DC_FP_END() 
macro.

> Gr{oetje,eeting}s,
>
>                          Geert


Christophe