linux-kernel - Re: [PATCH v2 1/2] include/uapi/linux/swab.h: move default implementation for swab macros into asm-generic

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Message-ID: <ac0b1f3e-6abe-4de2-bf5a-a4b3207a22c3@iencinas.com>
Date: Thu, 20 Mar 2025 23:36:36 +0100
From: Ignacio Encinas Rubio <ignacio@...cinas.com>
To: Arnd Bergmann <arnd@...db.de>, Paul Walmsley <paul.walmsley@...ive.com>,
 Palmer Dabbelt <palmer@...belt.com>, Alexandre Ghiti <alex@...ti.fr>
Cc: Eric Biggers <ebiggers@...nel.org>, linux-riscv@...ts.infradead.org,
 linux-kernel@...r.kernel.org, linux-kernel-mentees@...ts.linux.dev,
 Shuah Khan <skhan@...uxfoundation.org>,
 Zhihang Shao <zhihang.shao.iscas@...il.com>, Björn Töpel
 <bjorn@...nel.org>, Linux-Arch <linux-arch@...r.kernel.org>
Subject: Re: [PATCH v2 1/2] include/uapi/linux/swab.h: move default
 implementation for swab macros into asm-generic



On 19/3/25 22:49, Arnd Bergmann wrote:
> On Wed, Mar 19, 2025, at 22:37, Ignacio Encinas Rubio wrote:
>> On 19/3/25 22:12, Arnd Bergmann wrote:
>>> On Wed, Mar 19, 2025, at 22:09, Ignacio Encinas wrote:
>>>> Move the default byteswap implementation into asm-generic so that it can
>>>> be included from arch code.
>>>>
>>>> This is required by RISC-V in order to have a fallback implementation
>>>> without duplicating it.
>>>>
>>>> Signed-off-by: Ignacio Encinas <ignacio@...cinas.com>
>>>> ---
>>>>  include/uapi/asm-generic/swab.h | 32 ++++++++++++++++++++++++++++++++
>>>>  include/uapi/linux/swab.h       | 33 +--------------------------------
>>>>  2 files changed, 33 insertions(+), 32 deletions(-)
>>>>
>>>
>>> I think we should just remove these entirely in favor of the
>>> compiler-povided built-ins.
>>
>> Got it. I assumed they existed to explicitly avoid relying on
>> __builtin_bswap as they might not exist. However, I did a quick grep and
>> found that there are some uses in the wild.
> 
> Right, I do remember when we had a discussion about this maybe
> 15 years ago when gcc didn't have the builtins on all architectures
> yet, but those versions are long gone, and we never cleaned it up.

I just had a chance to look at this and it looks a bit more complex than
I initially thought. ___constant_swab macros are used in more places
than I expected, and {little,big}_endian.h define their own macros that
are used elsewhere, ...

It is not clear to me how to proceed here. I could:

  1) Just remove ___constant_swab macros and replace them with
  __builtin_swap everywhere

  2) Go a step further and evaluate removing __constant_htonl and
  relatives

Let me know what you think is the best option :)

I'll resend this series without this patch (and make the RISC-V use
fall back into __builtin_bswap)
 
>> I couldn't find compiler builtins for ___constant_swahb32 nor 
>> ___constant_swahw32, so I guess I'll leave them as they are.
> 
> Correct. There are also 24-bit and 48-bit swap functions
> in include/linux/unaligned.h that have no corresponding builtins.

Thanks for clarifying!