[PATCH v1 1/3] RISC-V: clarify ISA string ordering rules in cpu.c

[Andrew Jones]

On Wed, Nov 30, 2022 at 11:41:24PM +0000, Conor Dooley wrote:
> From: Conor Dooley <conor.dooley at microchip.com>
> 
> While the current list of rules may have been accurate when created
> it now lacks some clarity in the face of isa-manual updates. Instead of
> trying to continuously align this rule-set with the one in the
> specifications, change the role of this comment.
> 
> This particular comment is important, as the array it "decorates"
> defines the order in which the ISA string appears to userspace in
> /proc/cpuinfo.
> 
> Re-jig and strengthen the wording to provide contributors with a set
> order in which to add entries & note why this particular struct needs
> more attention than others.
> 
> While in the area, add some whitespace and tweak some wording for
> readability's sake.
> 
> Suggested-by: Andrew Jones <ajones at ventanamicro.com>
> Signed-off-by: Conor Dooley <conor.dooley at microchip.com>
> ---
>  arch/riscv/kernel/cpu.c | 49 ++++++++++++++++++++++++++++++-----------
>  1 file changed, 36 insertions(+), 13 deletions(-)
> 
> diff --git a/arch/riscv/kernel/cpu.c b/arch/riscv/kernel/cpu.c
> index 852ecccd8920..68b2bd0cc3bc 100644
> --- a/arch/riscv/kernel/cpu.c
> +++ b/arch/riscv/kernel/cpu.c
> @@ -120,22 +120,45 @@ 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.
> - * 2. The first letter following the 'Z' conventionally indicates the most
> + * The canonical order of ISA extension names in the ISA string is defined in
> + * chapter 27 of the unprivileged specification.
> + *
> + * Ordinarily, for in-kernel data structures, this order is unimportant but
> + * isa_ext_arr defines the order of the ISA string in /proc/cpuinfo.
> + *
> + * The specification uses vague wording, such as should, when it comes to
> + * ordering so for our purposes the following rules apply:
> + *
> + * 1. All multi-letter extensions must be separated from other multi-letter

1. All multi-letter extensions must be separated from other extensions by an
underscore.

(Because we always lead multi-letter extensions with underscore, even the
first one, which follows the single-letter extensions.)

> + *    extensions by an underscore.
> + *
> + * 2. Additional standard extensions (starting with 'Z') must be sorted after
> + *    single-letter extensions and before any higher-privileged extensions.
> +
> + * 3. 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.
> - * 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
> + *    If multiple 'Z' extensions are named, they should be ordered first by
> + *    category, then alphabetically within a category.
> + *
> + * 3. Standard supervisor-level extensions (starting with 'S') must be listed
> + *    after standard unprivileged extensions.  If multiple
> + *    supervisor-level extensions are listed, they must be ordered
>   *    alphabetically.
> - * 4. 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.
> + *
> + * 4. Standard machine-level extensions (starting with 'Zxm') must be listed
> + *    after any lower-privileged, standard extensions.  If multiple
> + *    machine-level extensions are listed, they must be ordered
> + *    alphabetically.
> + *
> + * 5. Non-standard extensions (starts with 'X') must be listed after all
> + *    standard extensions.
                            ^and alphabetically.

> + *
> + * An example string following the order is:
> + *    rv64imadc_zifoo_zigoo_zafoo_sbar_scar_zxmbaz_xqux_xrux
> + *
> + * New entries to this struct should follow the ordering rules described above.
>   */
>  static struct riscv_isa_ext_data isa_ext_arr[] = {
>  	__RISCV_ISA_EXT_DATA(sscofpmf, RISCV_ISA_EXT_SSCOFPMF),
> -- 
> 2.38.1
>

Otherwise,

Reviewed-by: Andrew Jones <ajones at ventanamicro.com>

Thanks,
drew