[PATCH Linux] powerpc: add documentation for HWCAPs

[Nicholas Piggin]

Excerpts from Segher Boessenkool's message of May 25, 2022 4:32 am:
> On Tue, May 24, 2022 at 11:52:00AM +0200, Florian Weimer wrote:
>> * Nicholas Piggin:
>> 
>> > +2. Facilities
>> > +-------------
>> > +The Power ISA uses the term "facility" to describe a class of instructions,
>> > +registers, interrupts, etc. The presence or absence of a facility indicates
>> > +whether this class is available to be used, but the specifics depend on the
>> > +ISA version. For example, if the VSX facility is available, the VSX
>> > +instructions that can be used differ between the v3.0B and v3.1B ISA
>> > +verstions.
>> 
>> The 2.07 ISA manual also has categories.  ISA 3.0 made a lot of things
>> mandatory.  It may make sense to clarify that feature bits for mandatory
>> aspects of the ISA are still set, to help with backwards compatibility.
> 
> Linux runs on ISA 1.xx and ISA 2.01 machines still.  "Category" wasn't
> invented for either yet either, but similar concepts did exist of
> course.

Not sure what to say about this. It now also has "Compliancy Subset"
although maybe that's more like a set of features rather than
incompatible features or modes such as some of the category stuff
seems to be. I'll try add something.

Thanks,
Nick