linux-kernel - Re: [PATCH v2 0/4] rust: add `Alignment` type

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Message-ID: <CANiq72mEDhT_OvSo1b=z4Z4VhND8+DFzeGBY_NNfXhq9jy5GhA@mail.gmail.com>
Date: Mon, 4 Aug 2025 16:16:25 +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 v2 0/4] rust: add `Alignment` type

On Mon, Aug 4, 2025 at 1:45 PM Alexandre Courbot <acourbot@...dia.com> wrote:
>
> - The `last_set_bit` function is dropped, with the recommendation to use
>   the standard library's `checked_ilog2` which does essentially the same
>   thing.

Yeah, let's see what people think about this one on the kernel side.

I don't mind either way, i.e. to have a few wrappers with slightly
different semantics if that is more common/understandable.

> The upstream `Alignment` is more constrained than the `PowerOfTwo` of
> the last revision: it uses `usize` internally instead of a generic
> value, and does not provide `align_down` or `align_up` methods.

`PowerOfTwo` seemed fine to me as well (or even implementing one in
terms of the other).

> These two shortcomings come together very nicely to gift us with a nice
> headache: we need to align values potentially larger than `usize`, thus
> need to make `align_down` and `align_up` generic. The generic parameter
> needs to be constrained on the operations used to perform the alignment
> (e.g. `BitAnd`, `Not`, etc) and there is one essential operation for
> which no trait exists in the standard library: `checked_add`. Thus the
> first patch of this series introduces a trait for it in the `num` module
> and implements it for all integer types. I suspect we will need
> something alongside these lines for other purposes anyway, and probably
> other traits too.

This part could be avoided implementing them the other way around,
right? i.e. as an extension trait on the other side.

It may also be also a bit easier to understand on the call site, too,
since value would be first.

> This generic nature also restricts these methods to being non-const,
> unfortunately. I have tried to implement them as macros instead, but
> quickly hit a wall due to the inability to convert `Alignment`'s `usize`
> into the type of the value to align.

I guess we could also just have one per type like for other ones to
have them `const`, like we do for other similar things like
`bit`/`genmask`.

Cheers,
Miguel