linux-kernel - Re: PPC_FPU, ALTIVEC: enable_kernel_fp, put_vr, get

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Message-ID: <8b1cb0a2-ed3a-7da0-a73a-febbda528703@csgroup.eu>
Date:   Sun, 18 Apr 2021 18:24:29 +0200
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Randy Dunlap <rdunlap@...radead.org>,
        PowerPC <linuxppc-dev@...ts.ozlabs.org>
Cc:     LKML <linux-kernel@...r.kernel.org>
Subject: Re: PPC_FPU, ALTIVEC: enable_kernel_fp, put_vr, get_vr



Le 17/04/2021 à 22:17, Randy Dunlap a écrit :
> Hi,
> 
> kernel test robot reports:
> 
>>> drivers/cpufreq/pmac32-cpufreq.c:262:2: error: implicit declaration of function 'enable_kernel_fp' [-Werror,-Wimplicit-function-declaration]
>             enable_kernel_fp();
>             ^
> 
> when
> # CONFIG_PPC_FPU is not set
> CONFIG_ALTIVEC=y
> 
> I see at least one other place that does not handle that
> combination well, here:
> 
> ../arch/powerpc/lib/sstep.c: In function 'do_vec_load':
> ../arch/powerpc/lib/sstep.c:637:3: error: implicit declaration of function 'put_vr' [-Werror=implicit-function-declaration]
>    637 |   put_vr(rn, &u.v);
>        |   ^~~~~~
> ../arch/powerpc/lib/sstep.c: In function 'do_vec_store':
> ../arch/powerpc/lib/sstep.c:660:3: error: implicit declaration of function 'get_vr'; did you mean 'get_oc'? [-Werror=implicit-function-declaration]
>    660 |   get_vr(rn, &u.v);
>        |   ^~~~~~
> 
> 
> Should the code + Kconfigs/Makefiles handle that kind of
> kernel config or should ALTIVEC always mean PPC_FPU as well?

As far as I understand, Altivec is completely independant of FPU in Theory. So it should be possible 
to use Altivec without using FPU.

However, until recently, it was not possible to de-activate FPU support on book3s/32. I made it 
possible in order to reduce unneccessary processing on processors like the 832x that has no FPU.
As far as I can see in cputable.h/.c, 832x is the only book3s/32 without FPU, and it doesn't have 
ALTIVEC either.

So we can in the future ensure that Altivec can be used without FPU support, but for the time being 
I think it is OK to force selection of FPU when selecting ALTIVEC in order to avoid build failures.

> 
> I have patches to fix the build errors with the config as
> reported but I don't know if that's the right thing to do...
> 

Lets see them.

Christophe