linux-kernel - Re: [PATCH] rust: add C FFI types to the prelude

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Message-ID: <CANiq72ngfTJ5PTYaJOOQ2NedERrLYzA62hL8Fgw5C4QkuZ2R-Q@mail.gmail.com>
Date: Mon, 14 Apr 2025 15:22:44 +0200
From: Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
To: Alice Ryhl <aliceryhl@...gle.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 <benno.lossin@...ton.me>, Andreas Hindborg <a.hindborg@...nel.org>, 
	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: add C FFI types to the prelude

On Mon, Apr 14, 2025 at 10:47 AM Alice Ryhl <aliceryhl@...gle.com> wrote:
>
> I wonder if it would make more sense to rephrase this section to first
> say that rfl has type aliases for the C integer types called c_int and
> so on, then mention that they are available in the prelude, and then
> at the end of the section have a note that we don't use the type
> aliases from core::ffi. I think focusing on how to use C integer
> types, rather than technical details about how they are defined, is
> more relevant for a reader who is just looking for coding guidelines.

Hmm... I see what you mean. In other places in the document, we start
comparing with userspace Rust in order to establish a bit of context.
But, more importantly, in this particular guideline I think it is
quite important to say "do not use the usual ones", because they are
actually different types, i.e. it is not just a style thing.

I have thought a couple times about perhaps changing the overall style
of the document to have a 1-liner short summary on each guideline --
some books do something like that consistently. And perhaps an
example, before a longer description. So something like the following,
which is closer to what you are suggesting:

    To refer to C types, use the FFI types (aliases) available from
the `kernel` prelude, e.g. ``c_int``.

    For instance:

        ...

    Refer to them with a single segment path: e.g. ``c_char`` instead
of ``ffi::c_char`` or ``kernel::ffi::c_char``.

    Do not use the `core::ffi::*` types -- they are different and some
do not map to the correct C type.

But even with this style, I think it is important putting the last
sentence in the "1-liner summary". Otherwise, someone may skip the
guideline thinking "oh, OK, it is just a style thing about using just
small paths, I will fix it later", without realizing they are actually
different sets of types unless they read the entire section, no?

Thanks!

Cheers,
Miguel