[PATCH v2] powerpc: add documentation for HWCAPs

[Segher Boessenkool]

On Fri, Jul 15, 2022 at 11:26:36AM +1000, Nicholas Piggin wrote:
> Take the arm64 HWCAP documentation file and adjust it for powerpc.
> 
> Signed-off-by: Nicholas Piggin <npiggin at gmail.com>

Reviewed-by: Segher Boessenkool <segher at kernel.crashing.org>

> +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
> +versions.

The term "facility" is never defined in the architecture documents, so
the common language meaning applies, which google says is "a place,
amenity, or piece of equipment provided for a particular purpose", which
fits your explanation well.

> +3. Categories
> +-------------
> +
> +The Power ISA before v3.0 uses the term "category" to describe certain
> +classes of instructions and operating modes which may be optional or
> +mutually exclusive, the exact meaning of the HWCAP flag may depend on
> +context, e.g., the presence of the BOOKE feature implies that the server
> +category is not implemented.

Since ISA 3.0 the preface says
  The resulting architecture included [...] and optional application-
  specific facilities (categories) as extensions to a pervasive base
  architecture.
which fits your explanation.

Thanks for improving the documentation!


Segher