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

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Message-ID: <CANiq72=BSnom-nQgzLvv-cqwSknK1uJ=CXGP51r0WRj1Y553Ew@mail.gmail.com>
Date: Fri, 20 Jun 2025 15:35:14 +0200
From: Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
To: Alexandre Courbot <acourbot@...dia.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 <lossin@...nel.org>, 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, nouveau@...ts.freedesktop.org
Subject: Re: [PATCH 1/3] rust: add `num` module with `PowerOfTwo` type

On Fri, Jun 20, 2025 at 3:15 PM Alexandre Courbot <acourbot@...dia.com> wrote:
>
> Introduce the `num` module, featuring the `PowerOfTwo` unsigned wrapper
> that guarantees (at build-time or runtime) that a value is a power of
> two.
>
> Such a property is often useful to maintain. In the context of the
> kernel, powers of two are often used to align addresses or sizes up and
> down, or to create masks. These operations are provided by this type.

Before I forget: the other day in a call we discussed powers of two
and I mentioned that there is `Alignment` in the standard library:

    https://doc.rust-lang.org/std/ptr/struct.Alignment.html

    "A type storing a `usize` which is a power of two"

So it would be nice to ask upstream the following if they have plans
to stabilize it, and whether they have considered a generic
`PowerOfTwo<T>` type like this one, rather than one just for alignment
purposes (possibly with an alias or newtype for `Alignment` if
needed).

Similarly, if they stabilize the `Alignment` one (only) and we end up
only using our `PowerOfTwo<T>` for `usize` and those use cases, then
we should consider using the upstream one (and adding any/all methods
that we need).

So I will ask them the next time we meet. I have added
`ptr_alignment_type` to our list (in the "nice to have" section).

(Apologies if this was already discussed!)

Cheers,
Miguel