Message-ID: <20250817044724.3528968-3-fujita.tomonori@gmail.com>
Date: Sun, 17 Aug 2025 13:47:23 +0900
From: FUJITA Tomonori <fujita.tomonori@...il.com>
To: a.hindborg@...nel.org,
	alex.gaynor@...il.com,
	ojeda@...nel.org
Cc: aliceryhl@...gle.com,
	anna-maria@...utronix.de,
	bjorn3_gh@...tonmail.com,
	boqun.feng@...il.com,
	dakr@...nel.org,
	frederic@...nel.org,
	gary@...yguo.net,
	jstultz@...gle.com,
	linux-kernel@...r.kernel.org,
	lossin@...nel.org,
	lyude@...hat.com,
	rust-for-linux@...r.kernel.org,
	sboyd@...nel.org,
	tglx@...utronix.de,
	tmgross@...ch.edu,
	acourbot@...dia.com,
	daniel.almeida@...labora.com,
	Fiona Behrens <me@...enk.dev>
Subject: [PATCH v2 2/2] rust: Add read_poll_timeout functions

Add read_poll_timeout function which poll periodically until a
condition is met or a timeout is reached.

The C's read_poll_timeout (include/linux/iopoll.h) is a complicated
macro and a simple wrapper for Rust doesn't work. So this implements
the same functionality in Rust.

The C version uses usleep_range() while the Rust version uses
fsleep(), which uses the best sleep method so it works with spans that
usleep_range() doesn't work nicely with.

The sleep_before_read argument isn't supported since there is no user
for now. It's rarely used in the C version.

Reviewed-by: Andreas Hindborg <a.hindborg@...nel.org>
Reviewed-by: Fiona Behrens <me@...enk.dev>
Tested-by: Daniel Almeida <daniel.almeida@...labora.com>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@...il.com>
---
 rust/kernel/io.rs      |  1 +
 rust/kernel/io/poll.rs | 96 ++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 97 insertions(+)
 create mode 100644 rust/kernel/io/poll.rs

diff --git a/rust/kernel/io.rs b/rust/kernel/io.rs
index 03b467722b86..ee182b0b5452 100644
--- a/rust/kernel/io.rs
+++ b/rust/kernel/io.rs
@@ -8,6 +8,7 @@
 use crate::{bindings, build_assert, ffi::c_void};
 
 pub mod mem;
+pub mod poll;
 pub mod resource;
 
 pub use resource::Resource;
diff --git a/rust/kernel/io/poll.rs b/rust/kernel/io/poll.rs
new file mode 100644
index 000000000000..e6325725d5a3
--- /dev/null
+++ b/rust/kernel/io/poll.rs
@@ -0,0 +1,96 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! IO polling.
+//!
+//! C header: [`include/linux/iopoll.h`](srctree/include/linux/iopoll.h).
+
+use crate::{
+    error::{code::*, Result},
+    processor::cpu_relax,
+    task::might_sleep,
+    time::{delay::fsleep, Delta, Instant, Monotonic},
+};
+
+/// Polls periodically until a condition is met or a timeout is reached.
+///
+/// The function repeatedly executes the given operation `op` closure and
+/// checks its result using the condition closure `cond`.
+///
+/// If `cond` returns `true`, the function returns successfully with the result of `op`.
+/// Otherwise, it waits for a duration specified by `sleep_delta`
+/// before executing `op` again.
+///
+/// This process continues until either `cond` returns `true` or the timeout,
+/// specified by `timeout_delta`, is reached. If `timeout_delta` is `None`,
+/// polling continues indefinitely until `cond` evaluates to `true` or an error occurs.
+///
+/// This function can only be used in a nonatomic context.
+///
+/// # Examples
+///
+/// ```no_run
+/// use kernel::io::{Io, poll::read_poll_timeout};
+/// use kernel::time::Delta;
+///
+/// const HW_READY: u16 = 0x01;
+///
+/// fn wait_for_hardware<const SIZE: usize>(io: &Io<SIZE>) -> Result<()> {
+///     match read_poll_timeout(
+///         // The `op` closure reads the value of a specific status register.
+///         || io.try_read16(0x1000),
+///         // The `cond` closure takes a reference to the value returned by `op`
+///         // and checks whether the hardware is ready.
+///         |val: &u16| *val == HW_READY,
+///         Delta::from_millis(50),
+///         Delta::from_secs(3),
+///     ) {
+///         Ok(_) => {
+///             // The hardware is ready. The returned value of the `op` closure
+///             // isn't used.
+///             Ok(())
+///         }
+///         Err(e) => Err(e),
+///     }
+/// }
+/// ```
+#[track_caller]
+pub fn read_poll_timeout<Op, Cond, T>(
+    mut op: Op,
+    mut cond: Cond,
+    sleep_delta: Delta,
+    timeout_delta: Delta,
+) -> Result<T>
+where
+    Op: FnMut() -> Result<T>,
+    Cond: FnMut(&T) -> bool,
+{
+    let start: Instant<Monotonic> = Instant::now();
+
+    // Unlike the C version, we always call `might_sleep()` unconditionally,
+    // as conditional calls are error-prone. We clearly separate
+    // `read_poll_timeout()` and `read_poll_timeout_atomic()` to aid
+    // tools like klint.
+    might_sleep();
+
+    loop {
+        let val = op()?;
+        if cond(&val) {
+            // Unlike the C version, we immediately return.
+            // We know the condition is met so we don't need to check again.
+            return Ok(val);
+        }
+
+        if start.elapsed() > timeout_delta {
+            // Unlike the C version, we immediately return.
+            // We have just called `op()` so we don't need to call it again.
+            return Err(ETIMEDOUT);
+        }
+
+        if !sleep_delta.is_zero() {
+            fsleep(sleep_delta);
+        }
+
+        // fsleep() could be busy-wait loop so we always call cpu_relax().
+        cpu_relax();
+    }
+}
-- 
2.43.0

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