linux-kernel - Re: [PATCH v2 2/2] rust: Add read_poll

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Message-Id: <DC5ZPDQADDJT.32SFK2OHDRGTG@nvidia.com>
Date: Tue, 19 Aug 2025 09:49:38 +0900
From: "Alexandre Courbot" <acourbot@...dia.com>
To: "FUJITA Tomonori" <fujita.tomonori@...il.com>, <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>, <daniel.almeida@...labora.com>, "Fiona Behrens"
 <me@...enk.dev>
Subject: Re: [PATCH v2 2/2] rust: Add read_poll_timeout functions

On Sun Aug 17, 2025 at 1:47 PM JST, FUJITA Tomonori 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@...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>

Tested this with nova-core, and it seems to work fine!

Reviewed-by: Alexandre Courbot <acourbot@...dia.com>
Tested-by: Alexandre Courbot <acourbot@...dia.com>

Just one last comment about the documentation below.

<snip>
> +/// 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`,

For precision: "This process continues until either `op` returns and
error, `cond` returns `true`, or the timeout specified by
`timeout_delta` is reached."

> +/// polling continues indefinitely until `cond` evaluates to `true` or an error occurs.
> +///
> +/// This function can only be used in a nonatomic context.

Here I'd add an errors section:

# 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};
> +/// 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.

nit: this comment looks superfluous to me, this is a different
implementation from the C version anyway.

> +            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.

Same here.