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

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Message-Id: <DDO06754OMN5.G0AN9OCWTFLW@kernel.org>
Date: Tue, 21 Oct 2025 14:35:34 +0200
From: "Danilo Krummrich" <dakr@...nel.org>
To: "FUJITA Tomonori" <fujita.tomonori@...il.com>
Cc: <aliceryhl@...gle.com>, <daniel.almeida@...labora.com>,
 <a.hindborg@...nel.org>, <alex.gaynor@...il.com>, <ojeda@...nel.org>,
 <anna-maria@...utronix.de>, <bjorn3_gh@...tonmail.com>,
 <boqun.feng@...il.com>, <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>
Subject: Re: [PATCH v2 2/2] rust: Add read_poll_count_atomic function

On Tue Oct 21, 2025 at 9:11 AM CEST, FUJITA Tomonori wrote:
> +/// Polls periodically until a condition is met, an error occurs,
> +/// or the attempt limit 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 attempt limit specified by `count` is reached.
> +///
> +/// # Errors
> +///
> +/// If `op` returns an error, then that error is returned directly.
> +///
> +/// If the attempt limit specified by `count` is reached, then
> +/// `Err(ETIMEDOUT)` is returned.
> +///
> +/// # Examples
> +///
> +/// ```no_run
> +/// use kernel::io::{Io, poll::read_poll_count_atomic};
> +/// use kernel::time::Delta;
> +///
> +/// const HW_READY: u16 = 0x01;
> +///
> +/// fn wait_for_hardware<const SIZE: usize>(io: &Io<SIZE>) -> Result {
> +///     match read_poll_count_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),
> +///         1000,
> +///     ) {
> +///         Ok(_) => {
> +///             // The hardware is ready. The returned value of the `op` closure
> +///             // isn't used.
> +///             Ok(())
> +///         }
> +///         Err(e) => Err(e),
> +///     }

Please replace the match statement with map().

	read_poll_count_atomic(
	    ...
	)
	.map(|_| ())

> +/// }
> +/// ```
> +pub fn read_poll_count_atomic<Op, Cond, T>(

I understand why you renamed the function, but read_poll_timeout_atomic() would
still be accurate -- it does perform a timeout in every iteration. Let's keep
the original name please.

> +    mut op: Op,
> +    mut cond: Cond,
> +    delay_delta: Delta,
> +    count: usize,

Maybe retry would be a slightly better fit compared to count. If we want to be a
bit more verbose, I suggest retry_count. :)

> +) -> Result<T>
> +where
> +    Op: FnMut() -> Result<T>,
> +    Cond: FnMut(&T) -> bool,
> +{
> +    for _ in 0..count {
> +        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: Just like in read_poll_timeout() I think this comment does not carry much
value, but I'm fine if you want to keep it.

> +            return Ok(val);
> +        }
> +
> +        if !delay_delta.is_zero() {
> +            udelay(delay_delta);
> +        }
> +
> +        cpu_relax();
> +    }
> +
> +    Err(ETIMEDOUT)
> +}
> -- 
> 2.43.0