[RFC 1/2] RISC-V: clarify ISA string ordering rules in cpu.c

[Conor Dooley]

On Tue, Nov 29, 2022 at 05:12:23PM +0100, Andrew Jones wrote:
> On Tue, Nov 29, 2022 at 02:47:42PM +0000, Conor Dooley wrote:
> > While the list of rules may have been accurate when created, it now
> > lacks some clarity in the face of isa-manual updates. Specifically:
> > 
> > - there is no mention here of a distinction between regular 'Z'
> >   extensions which are "Additional Standard Extensions" and "Zxm"
> >   extensions which are "Standard Machine-Level Extensions"
> > 
> > - there is also no explicit mention of where either should be sorted in
> >   the list
> > 
> > - underscores are only required between two *multi-letter* extensions but
> >   the list of rules implies that this is required between a multi-letter
> >   extension and any other extension. IOW "rv64imafdzicsr_zifencei" is a
> >   valid string
> > 
> > Attempt to clean up the list of rules, by adding information on the
> > above & sprinkling in some white space for readability.
> > 
> > Signed-off-by: Conor Dooley <conor.dooley at microchip.com>
> > ---
> >  arch/riscv/kernel/cpu.c | 22 +++++++++++++++++-----
> >  1 file changed, 17 insertions(+), 5 deletions(-)
> > 
> > diff --git a/arch/riscv/kernel/cpu.c b/arch/riscv/kernel/cpu.c
> > index 852ecccd8920..5e42c92a8456 100644
> > --- a/arch/riscv/kernel/cpu.c
> > +++ b/arch/riscv/kernel/cpu.c
> > @@ -120,20 +120,32 @@ 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.
> > + *
> > + * 1. All multi-letter extensions should be separated from other multi-letter
> > + *    extensions by an underscore.
> > + *
> >   * 2. 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.
> > + *    'Z' extensions should be sorted after single-letter extensions and before
> > + *    any higher-privileged extensions.
> > + *    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
> >   *    alphabetically.
> > - * 4. Non-standard extensions (starts with 'X') must be listed after all
> > + *
> > + * 4  Standard machine-level extensions (starts with 'Zxm') should be
> > + *    listed after any lower-privileged, standard extensions.  If multiple
> > + *    machine-level extensions are listed, they should be ordered
> > + *    alphabetically.
> > + *
> > + * 5. 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.
> >   */
> > -- 
> > 2.38.1
> >
> 
> Alternatively, we could change the comment to just point out the spec
> chapter and provide an example, e.g.

IDK, maybe add the reference & the example but keep the summary?

> /*
>  * The canonical order of ISA extension names in the ISA string is defined in
>  * chapter 27 of the unprivileged spec. An example string following the
>  * order is
>  *
>  *   rv64imadc_zifoo_zigoo_zafoo_sbar_scar_zxmbaz_xqux_xrux
>  *
>  * Notice how Z-extensions are first sorted by category using the canonical
>  * order of the first letter following the Z. Extension groups are in the
>  * order specified in chapter 27. Extensions within each group are sorted
>  * alphabetically.
>  */
> 
> 
> Thanks,
> drew