[PATCH v6 11/15] gpu: nova-core: gsp: cmdq: do not sleep when waiting for a message with timeout 0

[Alexandre Courbot <acourbot@nvidia.com>]

`read_poll_timeout` can potentially sleep for the specified
`sleep_delta` if `timeout_delta` is `Delta::ZERO` and the system clock
is coarse enough to not register a tick between the time the function
has been called and the time the loop test is done. `wait_for_msg` has a
short test condition and is particularly vulnerable to this condition.

Add a dedicated branch when `timeout` is `Delta::ZERO` to perform the
test without calling `read_poll_timeout` and avoid potentially sleeping
for 1ms.

Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
---
 drivers/gpu/nova-core/gsp/cmdq.rs | 24 ++++++++++++++++++------
 1 file changed, 18 insertions(+), 6 deletions(-)

diff --git a/drivers/gpu/nova-core/gsp/cmdq.rs b/drivers/gpu/nova-core/gsp/cmdq.rs
index 9e678715c624..9c2c8fbad045 100644
--- a/drivers/gpu/nova-core/gsp/cmdq.rs
+++ b/drivers/gpu/nova-core/gsp/cmdq.rs
@@ -909,12 +909,24 @@ fn send_command<M>(&mut self, command: M) -> Result
     /// - `EIO` if a message fails framing or checksum validation.
     fn wait_for_msg(&self, timeout: Delta) -> Result<GspMessage<'_>> {
         // Wait for a message to arrive from the GSP.
-        let (slice_1, slice_2) = read_poll_timeout(
-            || Ok(self.gsp_mem.driver_read_area()),
-            |driver_area| !driver_area.0.is_empty(),
-            Delta::from_millis(1),
-            timeout,
-        )
+        let (slice_1, slice_2) = if timeout == Delta::ZERO {
+            // If `timeout == Delta::ZERO`, check the message queue once as `read_poll_timeout` can
+            // potentially sleep if the clock hasn't moved forward while the condition was being
+            // checked.
+            let slices = self.gsp_mem.driver_read_area();
+            if !slices.0.is_empty() {
+                Ok(slices)
+            } else {
+                Err(ETIMEDOUT)
+            }
+        } else {
+            read_poll_timeout(
+                || Ok(self.gsp_mem.driver_read_area()),
+                |driver_area| !driver_area.0.is_empty(),
+                Delta::from_millis(1),
+                timeout,
+            )
+        }
         .map(|(slice_1, slice_2)| (slice_1.as_flattened(), slice_2.as_flattened()))?;
 
         // Extract the `GspMsgElement`.

-- 
2.56.0