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

Andrew Jones
2022-11-29T16:12:51Z

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.

/*
* 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