On Thu, May 14, 2020 at 9:30 PM Daniel Vetter <daniel@xxxxxxxx> wrote: > On Thu, May 14, 2020 at 05:19:40PM +0900, David Stevens wrote: > > Sorry for the duplicate reply, didn't notice this until now. > > > > > Just storing > > > the uuid should be doable (assuming this doesn't change during the > > > lifetime of the buffer), so no need for a callback. > > > > Directly storing the uuid doesn't work that well because of > > synchronization issues. The uuid needs to be shared between multiple > > virtio devices with independent command streams, so to prevent races > > between importing and exporting, the exporting driver can't share the > > uuid with other drivers until it knows that the device has finished > > registering the uuid. That requires a round trip to and then back from > > the device. Using a callback allows the latency from that round trip > > registration to be hidden. > > Uh, that means you actually do something and there's locking involved. > Makes stuff more complicated, invariant attributes are a lot easier > generally. Registering that uuid just always doesn't work, and blocking > when you're exporting? Registering the id at creation and blocking in gem export is feasible, but it doesn't work well for systems with a centralized buffer allocator that doesn't support batch allocations (e.g. gralloc). In such a system, the round trip latency would almost certainly be included in the buffer allocation time. At least on the system I'm working on, I suspect that would add 10s of milliseconds of startup latency to video pipelines (although I haven't benchmarked the difference). Doing the blocking as late as possible means most or all of the latency can be hidden behind other pipeline setup work. In terms of complexity, I think the synchronization would be basically the same in either approach, just in different locations. All it would do is alleviate the need for a callback to fetch the UUID. -David _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/dri-devel
