[PATCH v2 2/2] rust: Add read_poll_count_atomic function

[FUJITA Tomonori <fujita.tomonori@gmail.com>]

Add read_poll_count_atomic function which polls periodically until a
condition is met, an error occurs, or the attempt limit is reached.

The C's read_poll_timeout_atomic() is used for the similar purpose.
In atomic context the timekeeping infrastructure is unavailable, so
reliable time-based timeouts cannot be implemented. So instead, the
helper accepts a maximum number of attempts and busy-waits (udelay +
cpu_relax) between tries.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@gmail.com>
---
 rust/kernel/io/poll.rs | 80 +++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 79 insertions(+), 1 deletion(-)

diff --git a/rust/kernel/io/poll.rs b/rust/kernel/io/poll.rs
index 613eb25047ef..c7dd0816205f 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 +105,78 @@ pub fn read_poll_timeout<Op, Cond, T>(
         cpu_relax();
     }
 }
+
+/// 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),
+///     }
+/// }
+/// ```
+pub fn read_poll_count_atomic<Op, Cond, T>(
+    mut op: Op,
+    mut cond: Cond,
+    delay_delta: Delta,
+    count: usize,
+) -> 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.
+            return Ok(val);
+        }
+
+        if !delay_delta.is_zero() {
+            udelay(delay_delta);
+        }
+
+        cpu_relax();
+    }
+
+    Err(ETIMEDOUT)
+}
-- 
2.43.0