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

Conor Dooley
2022-11-29T16:59:34Z

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@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