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: <4d45df0c-d44e-4bb6-8daa-0dba1b834bc4@iencinas.com>
Date: Wed, 19 Mar 2025 22:37:36 +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 22:12, 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.

I couldn't find compiler builtins for ___constant_swahb32 nor 
___constant_swahw32, so I guess I'll leave them as they are.

Thank you!