On Wed, 22 Jan 2025 18:36:12 +0000
Gary Guo [off-list ref] wrote:

quoted

+#[track_caller]
+pub fn read_poll_timeout<Op, Cond, T: Copy>(

I wonder if we can lift the `T: Copy` restriction and have `Cond` take
`&T` instead. I can see this being useful in many cases.

I know that quite often `T` is just an integer so you'd want to pass by
value, but I think almost always `Cond` is a very simple closure so
inlining would take place and they won't make a performance difference.

Yeah, we can. More handy for the users of this function. I'll do.

quoted

+    mut op: Op,
+    cond: Cond,
+    sleep_delta: Delta,
+    timeout_delta: Delta,
+) -> Result<T>
+where
+    Op: FnMut() -> Result<T>,
+    Cond: Fn(T) -> bool,
+{
+    let start = Instant::now();
+    let sleep = !sleep_delta.is_zero();
+    let timeout = !timeout_delta.is_zero();
+
+    might_sleep(Location::caller());

This should only be called if `timeout` is true?

Oops, I messed up this in v6 somehow. I'll fix.

quoted

+    let val = loop {
+        let val = op()?;
+        if cond(val) {
+            // Unlike the C version, we immediately return.
+            // We know a condition is met so we don't need to check again.
+            return Ok(val);
+        }
+        if timeout && start.elapsed() > timeout_delta {
+            // Should we return Err(ETIMEDOUT) here instead of call op() again
+            // without a sleep between? But we follow the C version. op() could
+            // take some time so might be worth checking again.
+            break op()?;

Maybe the reason is `ktime_get` can take some time (due to its use of
seqlock and thus may require retrying?) Although this logic breaks down
when `read_poll_timeout_atomic` also has this extra `op(args)` despite
the condition being trivial.

ktime_get() might do retrying (read_seqcount) but compared to the op
function, I think that ktime_get() is fast (usually an op function
waits for hardware).

So I really can't convince myself that this additional `op()` call is
needed. I can't think of any case where this behaviour would be
depended on by a driver, so I'd be tempted just to return ETIMEOUT
straight.

As I commented in the code, I just mimic the logic of the C version,
which has been used for a long time. But as you said, looks like we
can return Err(ETIMEOUT) immediately here. I'll do that in the next
version.