linux-kernel - Re: [PATCH v5] rust: kernel: add support for bits/genmask macros

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Message-Id: <DA2FBPO5OL2I.1DJAT7E7JOSFJ@nvidia.com>
Date: Thu, 22 May 2025 14:01:28 +0900
From: "Alexandre Courbot" <acourbot@...dia.com>
To: "Daniel Almeida" <daniel.almeida@...labora.com>, "Miguel Ojeda"
 <miguel.ojeda.sandonis@...il.com>
Cc: "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>, <linux-kernel@...r.kernel.org>,
 <rust-for-linux@...r.kernel.org>, "Fiona Behrens" <me@...enk.dev>
Subject: Re: [PATCH v5] rust: kernel: add support for bits/genmask macros

On Wed May 14, 2025 at 3:52 AM JST, Daniel Almeida wrote:
> Hi Miguel,
>
>> On 27 Mar 2025, at 18:27, Miguel Ojeda <miguel.ojeda.sandonis@...il.com> wrote:
>> 
>> Hi Daniel,
>> 
>> My usual docs-only review... I hope that helps!
>> 
>> On Wed, Mar 26, 2025 at 3:07 PM Daniel Almeida
>> <daniel.almeida@...labora.com> wrote:
>>> 
>>> +/// Equivalent to the kernel's `BIT` macro.
>> 
>> "To the C `BIT` macro" or "The C side ..." or similar -- these one
>> would be also the kernel's :)
>> 
>>> +/// Create a contiguous bitmask starting at bit position `l` and ending at
>>> +/// position `h`, where `h >= l`.
>> 
>> The first paragraph is a "short description" / title -- you may want
>> to leave the details to a second sentence, i.e. in a second paragraph.
>> Please check in any case how it looks in the rendered docs -- it may
>> be fine to have all in the title.
>> 
>> In fact, given you `assert!`, we should probably mention that very
>> prominently e.g. in a `# Panics` section. Or, better, avoid the panics
>> to begin with if it makes sense.
>
> I have been staring at this for a little while.
>
> I wonder what is everyone's opinions on an extra set of:
>
> // e.g.: for u32
> const fn const_genmask_u32<const H: u32, const L: u32>() -> u32 {
>   crate::build_assert!(H >= L);
>   ...
> }
>
> ..on top of the current genmask functions we already have?

Do we need to make this generic? IIUC `build_assert` should enforce that
the condition can be guaranteed at build time, even without the const
generics (which a quick test seems to confirm).