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Subject: Re: PPC_FPU, ALTIVEC: enable_kernel_fp, put_vr, get_vr
To: Segher Boessenkool <segher@kernel.crashing.org>,
 Christophe Leroy <christophe.leroy@csgroup.eu>
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From: Randy Dunlap <rdunlap@infradead.org>
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Date: Sun, 18 Apr 2021 10:59:42 -0700
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On 4/18/21 10:46 AM, Segher Boessenkool wrote:
> 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 :-)

Sure.  I'll post them later today.
They keep FPU and ALTIVEC as independent (build) features.

-- 
~Randy