linux-kernel - Re: [PATCH v2 3/3] rust: Support deriving `AsBytes`/`FromBytes` on bindgen types

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Message-ID: <CAGSQo02KiCBPCFfpv7xLqFM8znN9w+KfGhy62O3YLu2UfVHCTw@mail.gmail.com>
Date: Wed, 17 Dec 2025 10:26:57 -0800
From: Matthew Maurer <mmaurer@...gle.com>
To: Alexandre Courbot <acourbot@...dia.com>
Cc: Miguel Ojeda <ojeda@...nel.org>, 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>, rust-for-linux@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 3/3] rust: Support deriving `AsBytes`/`FromBytes` on
 bindgen types

On Tue, Dec 16, 2025 at 7:16 PM Alexandre Courbot <acourbot@...dia.com> wrote:
>
> On Tue Dec 16, 2025 at 9:44 AM JST, Matthew Maurer wrote:
> > To support this, we need to move the `transmute` module into a separate
> > crate to allow the `bindings` crate to depend on it. Most user code is
> > still expected to address the module as `kernel::transmute`, which is a
> > re-export. `ffi::transmute` is now available for use in `bindings`.
> >
> > Signed-off-by: Matthew Maurer <mmaurer@...gle.com>
>
> Maybe this should be two commits, one for the new crate, another one to
> introduce the ability to use on bindgen types.

I could do this if people think it'd be helpful, but moving it into
the `ffi` crate has no point without using on the main set of bindgen
types.

>
> I have tried this with the Nova bindings, and somehow could not get past
> this error:
>
>     error[E0433]: failed to resolve: could not find `ffi` in the list of imported crates
>       --> ../drivers/gpu/nova-core/gsp/fw/r570_144/bindings.rs:321:54
>         |
>     321 | #[derive(Debug, Default, Copy, Clone, MaybeZeroable, FromBytesFfi)]
>         |                                                      ^^^^^^^^^^^^ could not find `ffi` in the list of imported crates
>         |
>         = note: this error originates in the derive macro `FromBytesFfi` (in Nightly builds, run with -Z macro-backtrace for more info)
>
> I do have `kernel::ffi` imported in the bindings module though, so I am
> not quite sure what this is about. Any idea?

If you have the kernel module available, please set the bindgen flag
to apply `FromBytes` and `AsBytes` rather than `FromBytesFfi` and
`AsBytesFfi`. Those are only for the crates that are used by the
kernel crate, and so do not have access to the `kernel` crate.

The macro itself will generate `::kernel::transmute::FromBytes` when
you use `FromBytes` and `::ffi::transmute::FromBytes` when you use
`FromBytesFfi`, specifically to avoid weirdness around namespaces and
imports.

If for some reason you really wanted to use `FromBytesFfi` (you
shouldn't, but just so you understand the mechanism), you'd need to
add a direct dependency edge on the `ffi` crate, i.e. `--extern crate
ffi`