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From: Alice Ryhl <aliceryhl@google.com>
Date: Mon, 8 Apr 2024 11:26:23 +0200
Message-ID: <CAH5fLgjr57qB2qp+b7xQ3wsKGyT0zsOkhQj0Q-z0KLo2jhCsTQ@mail.gmail.com>
Subject: Re: [PATCH v4 4/4] rust: add abstraction for `struct page`
To: Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>
Cc: Benno Lossin <benno.lossin@proton.me>, Miguel Ojeda <ojeda@kernel.org>, 
	Matthew Wilcox <willy@infradead.org>, Al Viro <viro@zeniv.linux.org.uk>, 
	Andrew Morton <akpm@linux-foundation.org>, Kees Cook <keescook@chromium.org>, 
	Alex Gaynor <alex.gaynor@gmail.com>, Wedson Almeida Filho <wedsonaf@gmail.com>, 
	Boqun Feng <boqun.feng@gmail.com>, Gary Guo <gary@garyguo.net>, 
	=?UTF-8?Q?Bj=C3=B6rn_Roy_Baron?= <bjorn3_gh@protonmail.com>, 
	Andreas Hindborg <a.hindborg@samsung.com>, Greg Kroah-Hartman <gregkh@linuxfoundation.org>, 
	=?UTF-8?B?QXJ2ZSBIasO4bm5ldsOlZw==?= <arve@android.com>, 
	Todd Kjos <tkjos@android.com>, Martijn Coenen <maco@android.com>, 
	Joel Fernandes <joel@joelfernandes.org>, Carlos Llamas <cmllamas@google.com>, 
	Suren Baghdasaryan <surenb@google.com>, Arnd Bergmann <arnd@arndb.de>, linux-mm@kvack.org, 
	linux-kernel@vger.kernel.org, rust-for-linux@vger.kernel.org, 
	Christian Brauner <brauner@kernel.org>
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On Mon, Apr 8, 2024 at 11:19=E2=80=AFAM Miguel Ojeda
<miguel.ojeda.sandonis@gmail.com> wrote:
>
> On Mon, Apr 8, 2024 at 9:54=E2=80=AFAM Alice Ryhl <aliceryhl@google.com> =
wrote:
> >
> > From bindgen's point of view, this constant is just an integer literal
> > with no type information. So I don't see how we can expect it to
> > always be generated as a usize?
>
> In the case of `PAGE_SIZE`, there is type information (`size_t`),
> since it comes from the constant helper:
>
>     const size_t RUST_CONST_HELPER_PAGE_SIZE =3D PAGE_SIZE;
>
> For the other one, `PAGE_SHIFT`, there is also type information
> (`int`), but bindgen currently picks a type based on the value for
> those (but ideally/eventually bindgen should respect it instead).
>
> So for the former, the allow and the cast are not needed since we are
> already handling it explicitly. And for the latter, if we want to have
> it as `usize`, we should have the cast but not the allow, because now
> it does `u32`, but if it respected the type, it would be `c_int` or
> `i32`. So either way we would need the cast.

Good point. Will fix this for next version.

Alice