On 13 April 2017 at 06:39, Chris Wilson <chris at chris-wilson.co.uk> wrote: > On Wed, Apr 12, 2017 at 09:01:32PM +0100, Chris Wilson wrote: >> On Thu, Apr 13, 2017 at 05:05:27AM +1000, Dave Airlie wrote: >> > >> >> > >> Not sure what the best semantics are there, any opinions on barring >> > >> wakeups/polling on semaphore sync_files, and just punting this >> > >> until we need it. >> > > >> > > I think getting it right now will make writing sw_sync-esque (i.e. cpu) >> > > tests easier and more complete. >> > >> > I just don't have any use case for it, so we would be writing code to >> > write tests for it. >> > >> > That doesn't seem smart. >> > >> > If there is a future non-testing use case, the API is expressive >> > enough for someone >> > to add a flag or new sync obj to allow polling and to add support in a >> > nice easily >> > digestible patch. >> >> My first thought was to check the signaled status would be to use >> poll(0), but that can be retrieved from the sync_file_status ioctl. But >> to get that still needs for us to acquire an fd from the syncobj. And if >> we were to want check the flag on a driver syncobj, we would need to be >> able to export one. That doesn't look very promising... > > Hmm, you do export fd to pass syncobj between processes. Let's not start > with syncobj being a second class sync_file. It's not the same semantics as a sync_file. Userspace should never be polling on semaphores. Semaphores are one process signals, one process waits, no semantics for sniffing or use cases. Unless you have a practical use case for an new or proposed API I don't think we should start with adding functionality. Dave.
