linux-kernel - Re: [PATCH v1 2/2] rust: Add read_poll_timeout

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Message-Id: <5C851069-5E1E-4DE9-9E1F-0DF2C86C266C@collabora.com>
Date: Tue, 26 Aug 2025 11:02:18 -0300
From: Daniel Almeida <daniel.almeida@...labora.com>
To: FUJITA Tomonori <fujita.tomonori@...il.com>
Cc: a.hindborg@...nel.org,
 alex.gaynor@...il.com,
 ojeda@...nel.org,
 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
Subject: Re: [PATCH v1 2/2] rust: Add read_poll_timeout_atomic function

Hi Fujita,

> On 21 Aug 2025, at 00:57, FUJITA Tomonori <fujita.tomonori@...il.com> wrote:
> 
> Add read_poll_timeout_atomic function which polls periodically until a
> condition is met, an error occurs, or the timeout is reached.
> 
> The C's read_poll_timeout_atomic (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 delay_before_read argument isn't supported since there is no user
> for now. It's rarely used in the C version.
> 
> Signed-off-by: FUJITA Tomonori <fujita.tomonori@...il.com>
> ---
> rust/kernel/io/poll.rs | 90 +++++++++++++++++++++++++++++++++++++++++-
> 1 file changed, 89 insertions(+), 1 deletion(-)
> 
> diff --git a/rust/kernel/io/poll.rs b/rust/kernel/io/poll.rs
> index 7af1934e397a..71c2c0e0d8b4 100644
> --- a/rust/kernel/io/poll.rs
> +++ b/rust/kernel/io/poll.rs
> @@ -8,7 +8,10 @@
>     error::{code::*, Result},
>     processor::cpu_relax,
>     task::might_sleep,
> -    time::{delay::fsleep, Delta, Instant, Monotonic},
> +    time::{
> +        delay::{fsleep, udelay},
> +        Delta, Instant, Monotonic,
> +    },
> };
> 
> /// Polls periodically until a condition is met, an error occurs,
> @@ -102,3 +105,88 @@ pub fn read_poll_timeout<Op, Cond, T>(
>         cpu_relax();
>     }
> }
> +
> +/// Polls periodically until a condition is met, an error occurs,
> +/// or the 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 performs a busy wait for a duration specified by `delay_delta`
> +/// before executing `op` again.
> +///
> +/// This process continues until either `op` returns an error, `cond`
> +/// returns `true`, or the timeout specified by `timeout_delta` is
> +/// reached.
> +///
> +/// # Errors
> +///
> +/// If `op` returns an error, then that error is returned directly.
> +///
> +/// If the timeout specified by `timeout_delta` is reached, then
> +/// `Err(ETIMEDOUT)` is returned.
> +///
> +/// # Examples
> +///
> +/// ```no_run
> +/// use kernel::io::{Io, poll::read_poll_timeout_atomic};
> +/// use kernel::time::Delta;
> +///
> +/// const HW_READY: u16 = 0x01;
> +///
> +/// fn wait_for_hardware<const SIZE: usize>(io: &Io<SIZE>) -> Result<()> {

Just “Result”.

> +///     match read_poll_timeout_atomic(
> +///         // 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_micros(50),
> +///         Delta::from_micros(300),
> +///     ) {
> +///         Ok(_) => {
> +///             // The hardware is ready. The returned value of the `op` closure
> +///             // isn't used.
> +///             Ok(())
> +///         }
> +///         Err(e) => Err(e),
> +///     }
> +/// }
> +/// ```
> +pub fn read_poll_timeout_atomic<Op, Cond, T>(
> +    mut op: Op,
> +    mut cond: Cond,
> +    delay_delta: Delta,
> +    timeout_delta: Delta,
> +) -> Result<T>
> +where
> +    Op: FnMut() -> Result<T>,
> +    Cond: FnMut(&T) -> bool,
> +{
> +    let mut left_ns = timeout_delta.as_nanos();
> +    let delay_ns = delay_delta.as_nanos();
> +
> +    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 left_ns < 0 {
> +            // 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 !delay_delta.is_zero() {
> +            udelay(delay_delta);
> +            left_ns -= delay_ns;
> +        }
> +
> +        cpu_relax();
> +        left_ns -= 1;

A comment on the line above would be nice.

Also, is timeout_delta == 0 an intended use-case?

> +    }
> +}
> -- 
> 2.43.0
> 
>