[PATCH] gpu: nova-core: gsp: cmdq: do not sleep in wait_for_msg

[Alexandre Courbot]

If a message is not immediately available on the queue, `wait_for_msg`
sleeps for 1 millisecond before checking again. This is 3 orders of
magnitude more than Nouveau (which sleeps for 1 microsecond) or OpenRM
(which does not sleep at all and busy-polls) and is way too much, as GSP
commands are expected to be processed much faster than that.

Set the sleep time to `Delta::ZERO` to mirror OpenRM's behavior. This
makes the CPU busy-loop with a call to `cpu_relax` at each iteration,
keeping latency to a minimum.

Slightly higher values like 2us were also considered, but with such low
values `fsleep` also busy-waits, and would thus only add latency for no
benefit.

Signed-off-by: Alexandre Courbot <acourbot@xxxxxxxxxx>
---
 drivers/gpu/nova-core/gsp/cmdq.rs | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/nova-core/gsp/cmdq.rs b/drivers/gpu/nova-core/gsp/cmdq.rs
index e6d9a97130ea..d8c00df012bf 100644
--- a/drivers/gpu/nova-core/gsp/cmdq.rs
+++ b/drivers/gpu/nova-core/gsp/cmdq.rs
@@ -590,9 +590,8 @@ fn send_command_element<E: CommandElement>(&mut self, command: E) -> Result {
     fn wait_for_msg(&self, timeout: Delta) -> Result<GspMessage<'_>> {
         // Wait for a message to arrive from the GSP.
         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.
+            // If `timeout == Delta::ZERO`, check the message queue exactly once instead of calling
+            // `read_poll_timeout`, which would keep polling until the clock moves forward.
             let slices = self.gsp_mem.driver_read_area();
             if !slices.0.is_empty() {
                 Ok(slices)
@@ -603,7 +602,7 @@ fn wait_for_msg(&self, timeout: Delta) -> Result<GspMessage<'_>> {
             read_poll_timeout(
                 || Ok(self.gsp_mem.driver_read_area()),
                 |driver_area| !driver_area.0.is_empty(),
-                Delta::from_millis(1),
+                Delta::ZERO,
                 timeout,
             )
         }

---
base-commit: 9b840ed71d34ff545e3ada8be2ad5daaae542b50
change-id: 20261030-cmdq_wait_msg_timeout-26df11234d10

Best regards,
--  
Alexandre Courbot <acourbot@xxxxxxxxxx>