Re: [PATCH 5.10 267/290] powerpc: Fix missing declaration of [en/dis]able_kernel_vsx()

[Christophe Leroy]

Le 15/03/2021 à 15:15, Geert Uytterhoeven a écrit :
> On Mon, Mar 15, 2021 at 3:04 PM <gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
> > From: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> >
> > From: Christophe Leroy <christophe.leroy@xxxxxxxxxx>
> >
> > commit bd73758803c2eedc037c2268b65a19542a832594 upstream.
> >
> > Add stub instances of enable_kernel_vsx() and disable_kernel_vsx()
> > when CONFIG_VSX is not set, to avoid following build failure.
>
> Please note that this is not sufficient, and will just turn the build error
> in another, different build error.

Not exactly, the fix is sufficient in most case, it is only with ancient versions of gcc (eg 4.9) or 
with CONFIG_CC_OPTIMISE_FOR_SIZE that we now get a build bug. Building with gcc 10 now works.

> Waiting for the subsequent fix to enter v5.12-rc4...
> https://lore.kernel.org/lkml/2c123f94-ceae-80c0-90e2-21909795eb76@xxxxxxxxxx/

This has now landed in mainline as commit eed5fae00593ab9d261a0c1ffc1bdb786a87a55a see 
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/arch/powerpc/include/asm/cpu_has_feature.h?h=v5.12-rc4&id=eed5fae00593ab9d261a0c1ffc1bdb786a87a55a

Christophe

> >    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
> >
> > This works because the caller is checking if VSX is available using
> > cpu_has_feature():
> >
> >    #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.
> >
> > Fixes: 16a9dea110a6 ("amdgpu: Enable initial DCN support on POWER")
> > Cc: stable@xxxxxxxxxxxxxxx # v5.6+
> > Reported-by: Geert Uytterhoeven <geert@xxxxxxxxxxxxxx>
> > Reported-by: kernel test robot <lkp@xxxxxxxxx>
> > Signed-off-by: Christophe Leroy <christophe.leroy@xxxxxxxxxx>
> > [mpe: Incorporate some discussion comments into the change log]
> > Signed-off-by: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
> > Link: https://lore.kernel.org/r/8d7d285a027e9d21f5ff7f850fa71a2655b0c4af.1615279170.git.christophe.leroy@xxxxxxxxxx
> > Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> > ---
> >   arch/powerpc/include/asm/switch_to.h |   10 ++++++++++
> >   1 file changed, 10 insertions(+)
> >
> > --- 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(vo
> >   {
> >          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
> >
> >   #ifdef CONFIG_SPE
>
> Gr{oetje,eeting}s,
>
>                          Geert