linux-kernel - [PATCH 2/3] rust: error: improve `to

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Message-ID: <20250829192243.678079-3-ojeda@kernel.org>
Date: Fri, 29 Aug 2025 21:22:42 +0200
From: Miguel Ojeda <ojeda@...nel.org>
To: Miguel Ojeda <ojeda@...nel.org>,
	Alex Gaynor <alex.gaynor@...il.com>
Cc: 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,
	patches@...ts.linux.dev
Subject: [PATCH 2/3] rust: error: improve `to_result` documentation

Core functions like `to_result` should have good documentation.

Thus improve it, including adding an example of how to perform early
returns with it.

Signed-off-by: Miguel Ojeda <ojeda@...nel.org>
---
 rust/kernel/error.rs | 39 +++++++++++++++++++++++++++++++++++++--
 1 file changed, 37 insertions(+), 2 deletions(-)

diff --git a/rust/kernel/error.rs b/rust/kernel/error.rs
index c415c3d3a3b6..1ebdb798fd5d 100644
--- a/rust/kernel/error.rs
+++ b/rust/kernel/error.rs
@@ -390,8 +390,43 @@ fn from(e: core::convert::Infallible) -> Error {
 /// [Rust documentation]: https://doc.rust-lang.org/book/ch09-02-recoverable-errors-with-result.html
 pub type Result<T = (), E = Error> = core::result::Result<T, E>;
 
-/// Converts an integer as returned by a C kernel function to an error if it's negative, and
-/// `Ok(())` otherwise.
+/// Converts an integer as returned by a C kernel function to a [`Result`].
+///
+/// If the integer is negative, an [`Err`] with an [`Error`] as given by [`Error::from_errno`] is
+/// returned. This means the integer must be `>= -MAX_ERRNO`.
+///
+/// Otherwise, it returns [`Ok`].
+///
+/// It is a bug to pass an out-of-range negative integer. `Err(EINVAL)` is returned in such a case.
+///
+/// # Examples
+///
+/// This function may be used to easily perform early returns with the [`?`] operator when working
+/// with C APIs within Rust abstractions:
+///
+/// ```
+/// # use kernel::error::to_result;
+/// # mod bindings {
+/// #     #![expect(clippy::missing_safety_doc)]
+/// #     use kernel::prelude::*;
+/// #     pub(super) unsafe fn f1() -> c_int { 0 }
+/// #     pub(super) unsafe fn f2() -> c_int { EINVAL.to_errno() }
+/// # }
+/// fn f() -> Result {
+///     // SAFETY: ...
+///     to_result(unsafe { bindings::f1() })?;
+///
+///     // SAFETY: ...
+///     to_result(unsafe { bindings::f2() })?;
+///
+///     // ...
+///
+///     Ok(())
+/// }
+/// # assert_eq!(f(), Err(EINVAL));
+/// ```
+///
+/// [`?`]: https://doc.rust-lang.org/reference/expressions/operator-expr.html#the-question-mark-operator
 pub fn to_result(err: crate::ffi::c_int) -> Result {
     if err < 0 {
         Err(Error::from_errno(err))
-- 
2.51.0