linux-kernel - Re: [PATCH v4 6/6] rust: bitfield: Use 'as' operator for setter type conversion

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Message-Id: <DD5D59FH4JTT.2G5WEXF3RBCQJ@nvidia.com>
Date: Mon, 29 Sep 2025 23:44:56 +0900
From: "Alexandre Courbot" <acourbot@...dia.com>
To: "Miguel Ojeda" <miguel.ojeda.sandonis@...il.com>, "Joel Fernandes"
 <joelagnelf@...dia.com>
Cc: <linux-kernel@...r.kernel.org>, <rust-for-linux@...r.kernel.org>,
 <dri-devel@...ts.freedesktop.org>, <dakr@...nel.org>,
 <acourbot@...dia.com>, "Alistair Popple" <apopple@...dia.com>, "Miguel
 Ojeda" <ojeda@...nel.org>, "Alex Gaynor" <alex.gaynor@...il.com>, "Boqun
 Feng" <boqun.feng@...il.com>, "Gary Guo" <gary@...yguo.net>,
 <bjorn3_gh@...tonmail.com>, "Benno Lossin" <lossin@...nel.org>, "Andreas
 Hindborg" <a.hindborg@...nel.org>, "Alice Ryhl" <aliceryhl@...gle.com>,
 "Trevor Gross" <tmgross@...ch.edu>, "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>, "John Hubbard"
 <jhubbard@...dia.com>, "Timur Tabi" <ttabi@...dia.com>,
 <joel@...lfernandes.org>, "Elle Rhumsaa" <elle@...thered-steel.dev>, "Yury
 Norov" <yury.norov@...il.com>, "Daniel Almeida"
 <daniel.almeida@...labora.com>, <nouveau@...ts.freedesktop.org>
Subject: Re: [PATCH v4 6/6] rust: bitfield: Use 'as' operator for setter
 type conversion

On Mon Sep 29, 2025 at 10:59 PM JST, Miguel Ojeda wrote:
> On Sat, Sep 20, 2025 at 8:23 PM Joel Fernandes <joelagnelf@...dia.com> wrote:
>>
>> The bitfield macro's setter currently uses the From trait for type
>> conversion, which is overly restrictive and prevents use cases such as
>> narrowing conversions (e.g., u32 storage size to u8 field size) which
>> aren't supported by From.
>
> Being restrictive is a good thing

On that note, I have been wondering whether we should not push the
restriction up to having bounded primitive types with only a set number
of bits valid, e.g. `bound_u8::<2>` is guaranteed to only contain values
in the range `0..=3`.

Getters and setters would use these types depending on the number of
bits of the field, meaning that a caller would have to validate the
value they want to write if it does not implement e.g.
`Into<bound_u8<2>>`.

A bit radical maybe, but correcness ensues. :)