linux-kernel - Re: [PATCH RFC] riscv: introduce asm/swab.h

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Message-ID: <8729ccb4-cdd2-48f7-a8d4-f55a24964bc4@iencinas.com>
Date: Mon, 17 Mar 2025 21:29:20 +0100
From: Ignacio Encinas Rubio <ignacio@...cinas.com>
To: Alexandre Ghiti <alex@...ti.fr>, Paul Walmsley
 <paul.walmsley@...ive.com>, Palmer Dabbelt <palmer@...belt.com>
Cc: Eric Biggers <ebiggers@...nel.org>, linux-riscv@...ts.infradead.org,
 linux-kernel@...r.kernel.org, linux-kernel-mentees@...ts.linux.dev,
 skhan@...uxfoundation.org, Zhihang Shao <zhihang.shao.iscas@...il.com>,
 Björn Töpel <bjorn@...nel.org>
Subject: Re: [PATCH RFC] riscv: introduce asm/swab.h

On 17/3/25 17:36, Alexandre Ghiti wrote:
>> We need a default implementation to fall back on, but there isn't any in
>> `asm-generic/swab.h`. Should I introduce a first patch moving
>> ___constant_swab<XX> into include/uapi/asm-generic/swab.h?
> 
> Yes, that or something else to avoid the code duplication.

Thanks for double checking. I figured as much but I ended up getting a
bit confused with the include directories.

>> +#if defined(CONFIG_RISCV_ISA_ZBB) && !defined(NO_ALTERNATIVE)
> 
> NO_ALTERNATIVE does not exist to my knowledge, and you don't need to check for ALTERNATIVE as alternative-macros.h will do the right thing in that case.

It's a define hardcoded into drivers/firmware/efi/libstub/Makefile. It
was introduced in [1]. It helps avoid these types of warnings [2]:

  riscv64-unknown-linux-gnu-ld: warning: orphan section 
  `.init.alternative' from `./drivers/firmware/efi/libstub/riscv.stub.o' 
  being placed in section `.init.alternative'

Let me know if you think I should drop it.

[1] https://lore.kernel.org/all/20231031064553.2319688-3-xiao.w.wang@intel.com/
[2] https://lore.kernel.org/all/DM8PR11MB5751DA69DEEAAC9B06623E0BB80FA@DM8PR11MB5751.namprd11.prod.outlook.com/

> It would be nice to have some perf numbers too, maybe Bjorn will chime in :)
> 
> Thanks for your patch, looking forward the v2,

Thank you!