From: Randy Dunlap <rdunlap@infradead.org>
To: 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: Sun, 18 Apr 2021 10:59:42 -0700 [thread overview]
Message-ID: <bf119bfe-7db1-e7f3-d837-f910635eeebb@infradead.org> (raw)
In-Reply-To: <20210418174648.GN26583@gate.crashing.org>
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
next prev parent reply other threads:[~2021-04-18 18:00 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 [this message]
2021-04-19 13:16 ` Michael Ellerman
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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