linux-kernel - Re: [PATCH 5/7] gpu: nova-core: add extra conversion functions and traits

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Message-ID: <CANiq72mgoW_TyWf9Nv=5t3Qij_dsDjicNpGsa=F1t+sg23vxSA@mail.gmail.com>
Date: Sun, 26 Oct 2025 17:44:08 +0100
From: Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
To: Alexandre Courbot <acourbot@...dia.com>
Cc: Alice Ryhl <aliceryhl@...gle.com>, David Airlie <airlied@...il.com>, 
	Simona Vetter <simona@...ll.ch>, 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>, 
	Trevor Gross <tmgross@...ch.edu>, John Hubbard <jhubbard@...dia.com>, 
	Alistair Popple <apopple@...dia.com>, Joel Fernandes <joelagnelf@...dia.com>, 
	Timur Tabi <ttabi@...dia.com>, Edwin Peer <epeer@...dia.com>, nouveau@...ts.freedesktop.org, 
	dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org, 
	rust-for-linux@...r.kernel.org, Danilo Krummrich <dakr@...nel.org>, 
	Matthew Wilcox <willy@...radead.org>
Subject: Re: [PATCH 5/7] gpu: nova-core: add extra conversion functions and traits

On Sun, Oct 26, 2025 at 3:40 PM Alexandre Courbot <acourbot@...dia.com> wrote:
>
> +/// Infallibly converts a `usize` to `u64`.
> +///
> +/// This conversion is always lossless as Linux only supports 32-bit and 64-bit platforms, thus a
> +/// `usize` is always smaller than or of the same size as a `u64`.
> +///
> +/// Prefer this over the `as` keyword to ensure no lossy conversions are performed.
> +///
> +/// This is for use from a `const` context. For non `const` use, prefer the [`FromAs`] and
> +/// [`IntoAs`] traits.
> +pub(crate) const fn usize_as_u64(value: usize) -> u64 {
> +    kernel::static_assert!(size_of::<u64>() >= size_of::<usize>());
> +
> +    value as u64
> +}

Since you have the static asserts, this is fine today.

However, we may actually get 128-bit architectures in the
not-so-distant future -- Matthew suggests to be ready by 2035:

    https://lwn.net/Articles/908026/

So this one in particular may actually not be true "soon" -- we also
had related discussions about these assumptions, e.g.:

    https://lore.kernel.org/rust-for-linux/CANiq72m9AeqFKHrRniQ5Nr9vPv2MmUMHFTuuj5ydmqo+OYn60A@mail.gmail.com/

So we should consider having the `cfg` already to prevent people from
assuming it will be always available, and likely a note in its docs,
i.e. we may introducing trouble to port later on to new architectures.
Similarly, the docs of the trait may need rewording.

What do you think?

Regarding the `.into_as()` name, it makes sense, but it can be a bit
surprising when reading out of context... The standalone functions are
super clear, in comparison. But I am not sure what could be better.
`into_in_this_arch()` or similar could emphasize that this will only
work in certain architectures, i.e. it is "an `into()` for this arch"
rather than the general one.
That would go well with the idea that you didn't implement it for
other obvious types, which I guess was to avoid developers using this
instead of `into()` by mistake, right?

(By the way, please use intra-doc links on the primitives too.)

Thanks!

Cheers,
Miguel