Re: [PATCH v2 2/2] rust: Add read_poll_timeout functions

[Daniel Almeida <daniel.almeida@collabora.com>]

Hi Fujita,

> On 17 Aug 2025, at 01:47, FUJITA Tomonori <fujita.tomonori@gmail.com> wrote:
> 
> 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@kernel.org>
> Reviewed-by: Fiona Behrens <me@kloenk.dev>
> Tested-by: Daniel Almeida <daniel.almeida@collabora.com>
> Signed-off-by: FUJITA Tomonori <fujita.tomonori@gmail.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
> 
> 

Thanks for working on this. Definitely going to be needed by a lot of drivers.

Reviewed-by: Daniel Almeida <daniel.almeida@collabora.com>

How is the atomic version going to look like? The same, except for
might_sleep() and without the sleep_delta argument?