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

[Andrew Jones]

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.

/*
 * 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