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

[Eliot Courtney]

On Fri Oct 30, 2026 at 12:21 PM JST, Alexandre Courbot wrote:
> 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>

Previously we had a discussion about Delta::ZERO waits [1] and Danilo
had some concerns about creating unnecessary contention by reading
coherent memory extremely often. Also, AFAICT `fsleep`'s busy wait is a
bit nicer than just spinning. WDYT? I think switching this to
Delta::from_micros(1) is probably better.

[1]: https://lore.kernel.org/all/DGTYVZMRVLLE.3HQVL7ZT6MU7H@xxxxxxxxxx/