Re: [PATCH v5 2/9] gpu: nova-core: gsp: add mechanism to wait for space on command queue

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

On Thu Mar 5, 2026 at 4:37 PM JST, Eliot Courtney wrote:
> On Wed Mar 4, 2026 at 8:39 PM JST, Danilo Krummrich wrote:
>> On Wed Mar 4, 2026 at 2:42 AM CET, Eliot Courtney wrote:
>>> +    fn allocate_command(&mut self, size: usize, timeout: Delta) -> Result<GspCommand<'_>> {
>>> +        read_poll_timeout(
>>> +            || Ok(self.driver_write_area_size()),
>>> +            |available_bytes| *available_bytes >= size_of::<GspMsgElement>() + size,
>>> +            Delta::ZERO,
>>
>> Isn't this either creating unneccessary thrashing of the memory controller or
>> unnecessary contention at the cache-coherency level?
>>
>> I think we should probably add at least a small delay of something around 1us.
>
> This is what nouveau does (specifically `usleep_range(1, 2)`). OTOH,
> openrm just does a busy wait, which is what I replicated here for now.
> GSP command queue not having space IIUC is meant to be very exceptional.
> I am not sure which is best, maybe Alex has an opinion, but also happy
> to change it because that reasoning makes sense to me and I don't know
> enough about the distribution of how often it would actually need
> to wait to know if 0 delay is justified.

We tend to align on OpenRM generally speaking, but since waiting should
be exceptional anyway it shouldn't really matter if we add a very short
delay between poll. Agree that it looks better than a brute busy-loop.