Message-Id: <20250820.154535.2021779027770226518.fujita.tomonori@gmail.com>
Date: Wed, 20 Aug 2025 15:45:35 +0900 (JST)
From: FUJITA Tomonori <fujita.tomonori@...il.com>
To: daniel.almeida@...labora.com
Cc: fujita.tomonori@...il.com, 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, me@...enk.dev
Subject: Re: [PATCH v2 2/2] rust: Add read_poll_timeout functions

On Tue, 19 Aug 2025 15:30:51 -0300
Daniel Almeida <daniel.almeida@...labora.com> wrote:

> Thanks for working on this. Definitely going to be needed by a lot of drivers.
> 
> Reviewed-by: Daniel Almeida <daniel.almeida@...labora.com>

Thanks!

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

If we follow the C implementation, it will be different; C's
read_poll_atomic doesn't use ktime to calculate a timeout.

It would look like the following. I think that the read_poll_timeout
patchset is almost complete so I'll send the
read_poll_timeout_atomic() patchset shortly.

diff --git a/rust/kernel/io/poll.rs b/rust/kernel/io/poll.rs
index e6325725d5a3..dc4f1ecdf31f 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 or a timeout is reached.
@@ -94,3 +97,86 @@ pub fn read_poll_timeout<Op, Cond, T>(
         cpu_relax();
     }
 }
+
+/// 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 performs a busy wait for a duration specified by `delay_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.
+///
+/// # 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<()> {
+///     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();
+
+    let timeout_is_zero = timeout_delta.is_zero();
+
+    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 !timeout_is_zero && 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);
+            if !timeout_is_zero {
+                left_ns -= delay_ns;
+            }
+        }
+
+        cpu_relax();
+        if !timeout_is_zero {
+            left_ns -= 1;
+        }
+    }
+}
-- 
2.43.0

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