linux-kernel - Re: [PATCH 5.10 267/290] powerpc: Fix missing declaration of [en/dis]able_kernel

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Message-ID: <fed84796-e3b4-47fc-7f5f-57d53565aa73@csgroup.eu>
Date:   Mon, 22 Mar 2021 07:16:25 +0100
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>,
        Greg KH <gregkh@...uxfoundation.org>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        stable <stable@...r.kernel.org>,
        kernel test robot <lkp@...el.com>,
        Michael Ellerman <mpe@...erman.id.au>
Subject: Re: [PATCH 5.10 267/290] powerpc: Fix missing declaration of
 [en/dis]able_kernel_vsx()



Le 15/03/2021 à 15:15, Geert Uytterhoeven a écrit :
> On Mon, Mar 15, 2021 at 3:04 PM <gregkh@...uxfoundation.org> wrote:
>> From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
>>
>> From: Christophe Leroy <christophe.leroy@...roup.eu>
>>
>> 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@csgroup.eu/

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@...r.kernel.org # v5.6+
>> Reported-by: Geert Uytterhoeven <geert@...ux-m68k.org>
>> Reported-by: kernel test robot <lkp@...el.com>
>> Signed-off-by: Christophe Leroy <christophe.leroy@...roup.eu>
>> [mpe: Incorporate some discussion comments into the change log]
>> Signed-off-by: Michael Ellerman <mpe@...erman.id.au>
>> Link: https://lore.kernel.org/r/8d7d285a027e9d21f5ff7f850fa71a2655b0c4af.1615279170.git.christophe.leroy@csgroup.eu
>> Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
>> ---
>>   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
>