From: Michael Ellerman <mpe@ellerman.id.au>
To: Randy Dunlap <rdunlap@infradead.org>,
Segher Boessenkool <segher@kernel.crashing.org>,
Christophe Leroy <christophe.leroy@csgroup.eu>
Cc: PowerPC <linuxppc-dev@lists.ozlabs.org>,
LKML <linux-kernel@vger.kernel.org>
Subject: Re: PPC_FPU, ALTIVEC: enable_kernel_fp, put_vr, get_vr
Date: Mon, 19 Apr 2021 23:16:18 +1000 [thread overview]
Message-ID: <87sg3mct3x.fsf@mpe.ellerman.id.au> (raw)
In-Reply-To: <bf119bfe-7db1-e7f3-d837-f910635eeebb@infradead.org>
Randy Dunlap <rdunlap@infradead.org> writes:
> 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.
Those patches look OK.
But I don't think it makes sense to support that configuration, FPU=n
ALTVEC=y. No one is ever going to make a CPU like that. We have enough
testing surface due to configuration options, without adding artificial
combinations that no one is ever going to use.
IMHO :)
So I'd rather we just make ALTIVEC depend on FPU.
cheers
next prev parent reply other threads:[~2021-04-19 13:16 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-04-17 20:17 PPC_FPU, ALTIVEC: enable_kernel_fp, put_vr, get_vr Randy Dunlap
2021-04-18 16:24 ` Christophe Leroy
2021-04-18 17:46 ` Segher Boessenkool
2021-04-18 17:59 ` Randy Dunlap
2021-04-19 13:16 ` Michael Ellerman [this message]
2021-04-19 17:59 ` Randy Dunlap
2021-04-19 21:39 ` Randy Dunlap
2021-04-20 4:55 ` Christophe Leroy
2021-04-20 13:15 ` Michael Ellerman
2021-04-20 18:25 ` Randy Dunlap
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