linux-kernel - Re: [PATCH v4 04/20] rust: add new `num` module with useful integer operations

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Message-Id: <DA2E1BNC668R.MMCARZ3K2NTS@nvidia.com>
Date: Thu, 22 May 2025 13:00:53 +0900
From: "Alexandre Courbot" <acourbot@...dia.com>
To: "Alexandre Courbot" <acourbot@...dia.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"
 <benno.lossin@...ton.me>, "Andreas Hindborg" <a.hindborg@...nel.org>,
 "Alice Ryhl" <aliceryhl@...gle.com>, "Trevor Gross" <tmgross@...ch.edu>,
 "Danilo Krummrich" <dakr@...nel.org>, "David Airlie" <airlied@...il.com>,
 "Simona Vetter" <simona@...ll.ch>, "Maarten Lankhorst"
 <maarten.lankhorst@...ux.intel.com>, "Maxime Ripard" <mripard@...nel.org>,
 "Thomas Zimmermann" <tzimmermann@...e.de>, "Daniel Almeida"
 <daniel.almeida@...labora.com>
Cc: "John Hubbard" <jhubbard@...dia.com>, "Ben Skeggs" <bskeggs@...dia.com>,
 "Joel Fernandes" <joelagnelf@...dia.com>, "Timur Tabi" <ttabi@...dia.com>,
 "Alistair Popple" <apopple@...dia.com>, <linux-kernel@...r.kernel.org>,
 <rust-for-linux@...r.kernel.org>, <nouveau@...ts.freedesktop.org>,
 <dri-devel@...ts.freedesktop.org>
Subject: Re: [PATCH v4 04/20] rust: add new `num` module with useful integer
 operations

On Wed May 21, 2025 at 3:44 PM JST, Alexandre Courbot wrote:
> Introduce the `num` module, featuring the `NumExt` extension trait
> that expands unsigned integers with useful operations for the kernel.
>
> These are to be used by the nova-core driver, but they are so ubiquitous
> that other drivers should be able to take advantage of them as well.
>
> The currently implemented operations are:
>
> - align_down()
> - align_up()
> - fls()
>
> But this trait is expected to be expanded further.

A trait is nice, but prevents any use in const context... After looking
at the genmask patch [1] I am now wondering (again) whether a set of
const functions would not better serve the needs of the kernel.

Either that, or we enable `#![feature(const_trait_impl)]`. I just tried
and with it we could indeed define and implement `NumExt` as const,
which looks like the cleanest way to do this to me.

The functions of [1] could then also be implemented as methods of that
trait, which would allow them to leverage the macro generating the impl
blocks for all supporting types, while having their examples/doc-tests
in the trait declaration.

[1] https://lore.kernel.org/rust-for-linux/20250326-topic-panthor-rs-genmask-v5-1-bfa6140214da@collabora.com/