linux-kernel - Re: [PATCH v2 2/3] RISC-V: resort all extensions in consistent orders

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Date:   Fri, 20 Jan 2023 14:56:32 +0100
From:   Andrew Jones <ajones@...tanamicro.com>
To:     Conor Dooley <conor.dooley@...rochip.com>
Cc:     linux-riscv@...ts.infradead.org,
        Palmer Dabbelt <palmer@...belt.com>, aou@...s.berkeley.edu,
        conor@...nel.org, corbet@....net, guoren@...nel.org,
        heiko@...ech.de, paul.walmsley@...ive.com,
        linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
        Heiko Stuebner <heiko.stuebner@...ll.eu>
Subject: Re: [PATCH v2 2/3] RISC-V: resort all extensions in consistent orders

On Mon, Dec 05, 2022 at 02:45:25PM +0000, Conor Dooley wrote:
> Ordering between each and every list of extensions is wildly
> inconsistent. Per discussion on the lists pick the following policy:
> 
> - The array defining order in /proc/cpuinfo follows a narrow
>   interpretation of the ISA specifications, described in a comment
>   immediately presiding it.
> 
> - All other lists of extensions are sorted alphabetically.
> 
> This will hopefully allow for easier review & future additions, and
> reduce conflicts between patchsets as the number of extensions grows.
> 
> Link: https://lore.kernel.org/all/20221129144742.2935581-2-conor.dooley@microchip.com/
> Suggested-by: Andrew Jones <ajones@...tanamicro.com>
> Reviewed-by: Andrew Jones <ajones@...tanamicro.com>
> Reviewed-by: Heiko Stuebner <heiko.stuebner@...ll.eu>
> Signed-off-by: Conor Dooley <conor.dooley@...rochip.com>
> ---
>  arch/riscv/include/asm/hwcap.h | 12 +++++++-----
>  arch/riscv/kernel/cpu.c        |  4 ++--
>  arch/riscv/kernel/cpufeature.c |  6 ++++--
>  3 files changed, 13 insertions(+), 9 deletions(-)
> 
> diff --git a/arch/riscv/include/asm/hwcap.h b/arch/riscv/include/asm/hwcap.h
> index b22525290073..ce522aad641a 100644
> --- a/arch/riscv/include/asm/hwcap.h
> +++ b/arch/riscv/include/asm/hwcap.h
> @@ -51,14 +51,15 @@ extern unsigned long elf_hwcap;
>   * RISCV_ISA_EXT_MAX. 0-25 range is reserved for single letter
>   * extensions while all the multi-letter extensions should define the next
>   * available logical extension id.
> + * Entries are sorted alphabetically.
>   */
>  enum riscv_isa_ext_id {
>  	RISCV_ISA_EXT_SSCOFPMF = RISCV_ISA_EXT_BASE,
> +	RISCV_ISA_EXT_SSTC,
> +	RISCV_ISA_EXT_SVINVAL,
>  	RISCV_ISA_EXT_SVPBMT,
>  	RISCV_ISA_EXT_ZICBOM,
>  	RISCV_ISA_EXT_ZIHINTPAUSE,
> -	RISCV_ISA_EXT_SSTC,
> -	RISCV_ISA_EXT_SVINVAL,
>  	RISCV_ISA_EXT_ID_MAX = RISCV_ISA_EXT_MAX,

Hi Conor,

I'm digging this back up because I'm basing Zicboz on it.

If we take "riscv: improve boot time isa extensions handling", then this
becomes a bunch of manually enumerated defines

 #define RISCV_ISA_EXT_SSCOFPMF         26
 #define RISCV_ISA_EXT_SVPBMT           27
 #define RISCV_ISA_EXT_ZICBOM           28
 #define RISCV_ISA_EXT_ZIHINTPAUSE      29
 #define RISCV_ISA_EXT_SSTC             30
 #define RISCV_ISA_EXT_SVINVAL          31

Keeping those in alphabetical order would either require manually
reenumerating them or to allow the numbers to be out of order as
we add more extensions. I think I'd prefer we just add new
extensions at the bottom and keep the numbers in order.

Thanks,
drew