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

Ignacio Encinas Rubio
2025-03-20T22:36:45Z

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@iencinas.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!