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

["Alexandre Courbot" <acourbot@nvidia.com>]

On Fri Oct 30, 2026 at 1:32 PM JST, Eliot Courtney wrote:
> 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@nvidia.com>
>> ---
>>  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@nvidia.com>
>
> 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.

Anything works for me as long as we reduce the spinning-induced latency
to something reasonable. I'll resping with `from_micros(1)`.