[PATCH v2] powerpc: add documentation for HWCAPs

[Nicholas Piggin]

Excerpts from Tulio Magno Quites Machado Filho's message of July 16, 2022 6:17 am:
> Segher Boessenkool <segher at kernel.crashing.org> writes:
> 
>> That is a usability problem.  Can it be fixed, or will that create its
>> own compatibility problems?  In practice I mean.  If it is, the C
>> libraries could fix it up, for new programs, and then after a while the
>> kernel can do the sane thing?
>>
>> How big is the problem, anyway?  Is it only 2.05, or also 2.04, 2.03?
> 
> PPC_FEATURE_ARCH_2_05 is the first bit referring to an ISA level.
> Before that, AT_HWCAP used to have bits for specific processors, e.g.
> PPC_FEATURE_CELL and PPC_FEATURE_POWER4.
> 
> Notice that glibc creates its own hwcap-based information that is used by
> __builtin_cpu_supports().  In this case bits PPC_FEATURE_ARCH_2_05,
> PPC_FEATURE_POWER5_PLUS, PPC_FEATURE_POWER5 and PPC_FEATURE_POWER4 are enabled
> whenever if the processor is compatible with the features provided by any of
> the previous processors [1].
> AT_HWCAP and AT_HWCAP2 are kept intact, though.
> 
> [1] https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/powerpc/hwcapinfo.c;h=afde05f86382413ce1f0c38e33c9bdd38d6b7e9d;hb=HEAD#l45

Hmm, this doesn't seem very nice. That said, before possibly changing 
that in the kernel, documenting existing unexpected behaviour is 
probably a good idea. Good catch, I obviously wasn't careful enough
reviewing these bits.

I'll send out a final patch with this adjustment in a week or so in
case any more comments come in the meantime.

Thanks,
Nick