Randy Dunlap <rdun...@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
