On Fri, May 15, 2020 at 02:07:06PM +0900, David Stevens wrote: > 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. Hm ok. I guess if we go with the older patch, where this all is a lot more just code in virtio, doing an extra function to allocate the uuid sounds fine. Then synchronization is entirely up to the virtio subsystem and not a dma-buf problem (and hence not mine). You can use dma_resv_lock or so, but no need to. But with callbacks potentially going both ways things always get a bit interesting wrt locking - this is what makes peer2peer dma-buf so painful right now. Hence I'd like to avoid that if needed, at least at the dma-buf level. virtio code I don't mind what you do there :-) Cheers, Daniel -- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch
