[PATCH v1 1/3] RISC-V: clarify ISA string ordering rules in cpu.c

[Conor Dooley]

On Thu, Dec 01, 2022 at 09:27:43AM +0100, Andrew Jones wrote:
> On Wed, Nov 30, 2022 at 11:41:24PM +0000, Conor Dooley wrote:
> > From: Conor Dooley <conor.dooley at microchip.com>
> > 
> > While the current list of rules may have been accurate when created
> > it now lacks some clarity in the face of isa-manual updates. Instead of
> > trying to continuously align this rule-set with the one in the
> > specifications, change the role of this comment.
> > 
> > This particular comment is important, as the array it "decorates"
> > defines the order in which the ISA string appears to userspace in
> > /proc/cpuinfo.
> > 
> > Re-jig and strengthen the wording to provide contributors with a set
> > order in which to add entries & note why this particular struct needs
> > more attention than others.
> > 
> > While in the area, add some whitespace and tweak some wording for
> > readability's sake.
> > 
> > Suggested-by: Andrew Jones <ajones at ventanamicro.com>
> > Signed-off-by: Conor Dooley <conor.dooley at microchip.com>
> > ---
> >  arch/riscv/kernel/cpu.c | 49 ++++++++++++++++++++++++++++++-----------
> >  1 file changed, 36 insertions(+), 13 deletions(-)
> > 
> > diff --git a/arch/riscv/kernel/cpu.c b/arch/riscv/kernel/cpu.c
> > index 852ecccd8920..68b2bd0cc3bc 100644
> > --- a/arch/riscv/kernel/cpu.c
> > +++ b/arch/riscv/kernel/cpu.c
> > @@ -120,22 +120,45 @@ device_initcall(riscv_cpuinfo_init);
> >  		.uprop = #UPROP,				\
> >  		.isa_ext_id = EXTID,				\
> >  	}
> > +
> >  /*
> > - * Here are the ordering rules of extension naming defined by RISC-V
> > - * specification :
> > - * 1. All extensions should be separated from other multi-letter extensions
> > - *    by an underscore.
> > - * 2. The first letter following the 'Z' conventionally indicates the most
> > + * The canonical order of ISA extension names in the ISA string is defined in
> > + * chapter 27 of the unprivileged specification.
> > + *
> > + * Ordinarily, for in-kernel data structures, this order is unimportant but
> > + * isa_ext_arr defines the order of the ISA string in /proc/cpuinfo.
> > + *
> > + * The specification uses vague wording, such as should, when it comes to
> > + * ordering so for our purposes the following rules apply:
> > + *
> > + * 1. All multi-letter extensions must be separated from other multi-letter
> 
> 1. All multi-letter extensions must be separated from other extensions by an
> underscore.
> 
> (Because we always lead multi-letter extensions with underscore, even the
> first one, which follows the single-letter extensions.)

Yah, I need to think as if I am using De Morgan's... The DT ABI requires
"should" and permits this. The uAPI is "must"/"will" and always has an
_. I'll propagate that change to the docs patch too.

> > + *    extensions by an underscore.
> > + *
> > + * 2. Additional standard extensions (starting with 'Z') must be sorted after
> > + *    single-letter extensions and before any higher-privileged extensions.
> > +
> > + * 3. The first letter following the 'Z' conventionally indicates the most
> >   *    closely related alphabetical extension category, IMAFDQLCBKJTPVH.
> > - *    If multiple 'Z' extensions are named, they should be ordered first
> > - *    by category, then alphabetically within a category.
> > - * 3. Standard supervisor-level extensions (starts with 'S') should be
> > - *    listed after standard unprivileged extensions.  If multiple
> > - *    supervisor-level extensions are listed, they should be ordered
> > + *    If multiple 'Z' extensions are named, they should be ordered first by
> > + *    category, then alphabetically within a category.
> > + *
> > + * 3. Standard supervisor-level extensions (starting with 'S') must be listed
> > + *    after standard unprivileged extensions.  If multiple
> > + *    supervisor-level extensions are listed, they must be ordered
> >   *    alphabetically.
> > - * 4. Non-standard extensions (starts with 'X') must be listed after all
> > - *    standard extensions. They must be separated from other multi-letter
> > - *    extensions by an underscore.
> > + *
> > + * 4. Standard machine-level extensions (starting with 'Zxm') must be listed
> > + *    after any lower-privileged, standard extensions.  If multiple
> > + *    machine-level extensions are listed, they must be ordered
> > + *    alphabetically.
> > + *
> > + * 5. Non-standard extensions (starts with 'X') must be listed after all
> > + *    standard extensions.
>                             ^and alphabetically.

"If multiple non-standard extensions are listed, they must be ordered
alphabetically." I'll also propagate this to the doc one, if I have not
already.

> Otherwise,
> 
> Reviewed-by: Andrew Jones <ajones at ventanamicro.com>

Cool. I'll give it a bit before respinning, but I think we are at least
getting less ambiguous as time goes on..

Thanks,
Conor.