linux-kernel - Re: [PATCH 1/3] rust: add `num` module with `PowerOfTwo` type

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Message-Id: <DBV2Y6TJGVZZ.1XZG9FEIWYDBU@nvidia.com>
Date: Wed, 06 Aug 2025 14:02:52 +0900
From: "Alexandre Courbot" <acourbot@...dia.com>
To: "Alexandre Courbot" <acourbot@...dia.com>, "Miguel Ojeda"
 <miguel.ojeda.sandonis@...il.com>
Cc: "Alice Ryhl" <aliceryhl@...gle.com>, "Miguel Ojeda" <ojeda@...nel.org>,
 "Alex Gaynor" <alex.gaynor@...il.com>, "Boqun Feng" <boqun.feng@...il.com>,
 "Gary Guo" <gary@...yguo.net>, Björn Roy Baron
 <bjorn3_gh@...tonmail.com>, "Benno Lossin" <lossin@...nel.org>, "Andreas
 Hindborg" <a.hindborg@...nel.org>, "Trevor Gross" <tmgross@...ch.edu>,
 "Danilo Krummrich" <dakr@...nel.org>, <linux-kernel@...r.kernel.org>,
 <rust-for-linux@...r.kernel.org>, <nouveau@...ts.freedesktop.org>,
 "Nouveau" <nouveau-bounces@...ts.freedesktop.org>
Subject: Re: [PATCH 1/3] rust: add `num` module with `PowerOfTwo` type

On Mon Aug 4, 2025 at 4:32 PM JST, Alexandre Courbot wrote:
> On Mon Aug 4, 2025 at 12:15 AM JST, Miguel Ojeda wrote:
>> On Sun, Aug 3, 2025 at 3:13 PM Alexandre Courbot <acourbot@...dia.com> wrote:
>>>
>>> We got some interesting feedback on the ACP already. I have been pointed
>>> to `checked_ilog2` as an equivalent of `last_set_bit`, and it *does*
>>> indeed work well as a replacement - with the caveat that the name is
>>> not very natural to me (or anyone familiar with the C interface). Is
>>> this something we can live with? If we decide to go with the existing
>>> standard library method, how can we make sure that folks looking for an
>>> equivalent of `fls` find `checked_ilog2`?
>>
>> One option is using the `doc(alias = ...)` attribute, which makes it
>> appear in the search in the rendered docs, and would show easily in
>> greps too.
>>
>> Another option is simply wrapping it in an `inline(always)`, I guess,
>> but I think we can just use the upstream ones, unless we want slightly
>> different semantics.
>
> That would be useful - let's see what the Rust lib folks say, as you
> brought up that question on the ACP as well.
>
> In any case, since we have reasonable alternatives for both `fls`
> (`checked_ilog2`) and `ffs` (`NonZero::trailing_zeros`), I guess this
> means we want to use these directly in the kernel and can drop patch
> 2 of this series?

I didn't expect that, but it looks like the Rust folks want these
methods after all:

https://github.com/rust-lang/libs-team/issues/631#issuecomment-3156000663

I'll proceed with sending a PR, and I guess we can have temporary
implementations in the kernel as well?