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: <07b8051b-9d5e-440e-b74d-1ca97402fe2a@app.fastmail.com>
Date: Wed, 19 Mar 2025 22:49:57 +0100
From: "Arnd Bergmann" <arnd@...db.de>
To: "Ignacio Encinas" <ignacio@...cinas.com>,
 "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 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 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.

      Arnd