linux-kernel - Re: [PATCH] rust: workaround `bindgen` issue with forward references to `enum` types

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Message-ID: <CANiq72kWumOHt=VgV3mb2G4mOE3bCVUCUrKdjk4caFJPVCTBbA@mail.gmail.com>
Date: Tue, 25 Mar 2025 20:48:17 +0100
From: Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
To: Miguel Ojeda <ojeda@...nel.org>
Cc: 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>, rust-for-linux@...r.kernel.org, 
	linux-kernel@...r.kernel.org, patches@...ts.linux.dev
Subject: Re: [PATCH] rust: workaround `bindgen` issue with forward references
 to `enum` types

On Tue, Mar 25, 2025 at 7:43 PM Miguel Ojeda <ojeda@...nel.org> wrote:
>
> Instead, let's have a section at the top of our `bindings_helper.h` that
> `#include`s the headers with the affected types -- hopefully there are
> not many cases and there is a single ordering that covers all cases.

By the way, I was curious and from a quick look at generated bindings
lines matching `type.*i32` for a x86_64 build I have, I see only other
three cases:

  - `key_serial_t` is not an `enum`, i.e. it is correct.

  - `fs_value_type` and `drm_mode_status` are both `enum`s, and they
indeed suffer from this issue. However, we do not use them and I don't
see any patches posted for them either.

Those are good news.

However, there are also bad news: if for instance we actually needed
either of those two latter ones, then we cannot just include the
header that defines them, because that header happens to include
others that in turn use forward references to that `enum` the root one
defines.

So in some cases, if we start to use them before we have a fix, would
require C header changes to avoid the forward references. Thus, for
those, I think it may be simpler to instead use a cast and mark them
with a comment so that we know we should clean them up.

Cheers,
Miguel