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

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Message-ID: <20210418174648.GN26583@gate.crashing.org>
Date:   Sun, 18 Apr 2021 12:46:48 -0500
From:   Segher Boessenkool <segher@...nel.crashing.org>
To:     Christophe Leroy <christophe.leroy@...roup.eu>
Cc:     Randy Dunlap <rdunlap@...radead.org>,
        PowerPC <linuxppc-dev@...ts.ozlabs.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: PPC_FPU, ALTIVEC: enable_kernel_fp, put_vr, get_vr

On Sun, Apr 18, 2021 at 06:24:29PM +0200, Christophe Leroy wrote:
> Le 17/04/2021 à 22:17, Randy Dunlap a écrit :
> >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. 

And, as far as the hardware is concerned, in practice as well.

> So it should be possible to use Altivec without using FPU.

Yup.

> 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.

The processor has to implement FP to be compliant to any version of
PowerPC, as far as I know?  So that is all done by emulation, including
all the registers?  Wow painful.

> 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.

602 doesn't have double-precision hardware, also no 64-bit FP registers.
But that CPU was never any widely used :-)

> 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.

It is useful to allow MSR[VEC,FP]=1,0 but yeah there are no CPUs that
have VMX (aka AltiVec) but that do not have FP.  I don't see how making
that artificial dependency buys anything, but maybe it does?

> >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...

Neither do we, we cannot see those patches :-)


Segher