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

[Andrew Jones]

On Tue, Nov 29, 2022 at 04:54:19PM +0000, Conor Dooley wrote:
> 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?

It risks needing to synchronize the comment with the spec. Also, the
comment doesn't need to reproduce the flexible specifications, since
Linux has a single implementation (it always puts an underscore between
single-letter extensions and the first multi-letter extension, for
example). So, rather than paraphrase too much of the spec, we could just
point out Linux's specific implementation (with the help of an example).

I don't feel that strongly about it though, so we can keep the text
the spec paraphrasing too.

Thanks,
drew

> 
> > /*
> >  * 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