[PATCH v2 14/21] gpu: nova-core: add helper function to wait on condition

[Alexandre Courbot <acourbot@nvidia.com>]

While programming the hardware, we frequently need to busy-wait until
a condition (like a given bit of a register to switch value) happens.

Add a basic `wait_on` helper function to wait on such conditions
expressed as a closure, with a timeout argument.

This is temporary as we will switch to `read_poll_timeout` [1] once it
is available.

[1] https://lore.kernel.org/lkml/20250220070611.214262-8-fujita.tomonori@gmail.com/

Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
---
 drivers/gpu/nova-core/util.rs | 30 ++++++++++++++++++++++++++++++
 1 file changed, 30 insertions(+)

diff --git a/drivers/gpu/nova-core/util.rs b/drivers/gpu/nova-core/util.rs
index 332a64cfc6a9d7d787fbdc228887c0be53a97160..afb525228431a2645afe7bb34988e9537757b1d7 100644
--- a/drivers/gpu/nova-core/util.rs
+++ b/drivers/gpu/nova-core/util.rs
@@ -1,5 +1,10 @@
 // SPDX-License-Identifier: GPL-2.0
 
+use core::time::Duration;
+
+use kernel::prelude::*;
+use kernel::time::Ktime;
+
 pub(crate) const fn to_lowercase_bytes<const N: usize>(s: &str) -> [u8; N] {
     let src = s.as_bytes();
     let mut dst = [0; N];
@@ -19,3 +24,28 @@ pub(crate) const fn const_bytes_to_str(bytes: &[u8]) -> &str {
         Err(_) => kernel::build_error!("Bytes are not valid UTF-8."),
     }
 }
+
+/// Wait until `cond` is true or `timeout` elapsed.
+///
+/// When `cond` evaluates to `Some`, its return value is returned.
+///
+/// `Err(ETIMEDOUT)` is returned if `timeout` has been reached without `cond` evaluating to
+/// `Some`.
+///
+/// TODO: replace with `read_poll_timeout` once it is available.
+/// (https://lore.kernel.org/lkml/20250220070611.214262-8-fujita.tomonori@gmail.com/)
+#[expect(dead_code)]
+pub(crate) fn wait_on<R, F: Fn() -> Option<R>>(timeout: Duration, cond: F) -> Result<R> {
+    let start_time = Ktime::ktime_get();
+
+    loop {
+        if let Some(ret) = cond() {
+            return Ok(ret);
+        }
+
+        let cur_time = Ktime::ktime_get();
+        if (cur_time - start_time).to_ns() > timeout.as_nanos() as i64 {
+            return Err(ETIMEDOUT);
+        }
+    }
+}

-- 
2.49.0