On 3/4/26 11:37 PM, 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.
Open RM has some ancient bad habits!
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.
Almost never! There is a big big difference in how the OS behaves,
between 0 delay and non-zero delay. Sleeping, if you can afford to
(and we can, here, IIUC) is far better than a hard spin loop: the
scheduler can do something reasonable, for one thing.
thanks,
--
John Hubbard
