On Thu, Jun 19, 2014 at 1:48 PM, Thierry Reding <thierry.red...@gmail.com> wrote: >> > With these changes, can we pull the android sync logic out of >> > drivers/staging/ now? >> >> Afaik the google guys never really looked at this and acked it. So I'm not >> sure whether they'll follow along. The other issue I have as the >> maintainer of gfx driver is that I don't want to implement support for two >> different sync object primitives (once for dma-buf and once for android >> syncpts), and my impression thus far has been that even with this we're >> not there. >> >> I'm trying to get our own android guys to upstream their i915 syncpts >> support, but thus far I haven't managed to convince them to throw people's >> time at this. > > This has been discussed a fair bit internally recently and some of our > GPU experts have raised concerns that this may result in seriously > degraded performance in our proprietary graphics stack. Now I don't care > very much for the proprietary graphics stack, but by extension I would > assume that the same restrictions are relevant for any open-source > driver as well. > > I'm still trying to fully understand all the implications and at the > same time get some of the people who raised concerns to join in this > discussion. As I understand it the concern is mostly about explicit vs. > implicit synchronization and having this mechanism in the kernel will > implicitly synchronize all accesses to these buffers even in cases where > it's not needed (read vs. write locks, etc.). In one particular instance > it was even mentioned that this kind of implicit synchronization can > lead to deadlocks in some use-cases (this was mentioned for Android > compositing, but I suspect that the same may happen for Wayland or X > compositors).
Well the implicit fences here actually can't deadlock. That's the entire point behind using ww mutexes. I've also heard tons of complaints about implicit enforced syncing (especially from opencl people), but in the end drivers and always expose unsynchronized access for specific cases. We do that in i915 for upload buffers and other fun stuff. This is about shared stuff across different drivers and different processes. I also expect that i915 will loose implicit syncing in a few upcoming hw modes because explicit syncing is a more natural fit there. All that isn't about the discussion at hand imo since no matter what i915 needs to have on internal representation for a bit of gpu work, and afaics right now we don't have that. With this patch android syncpts use Maarten's fences internally, but I can't freely exchange one for the other. So in i915 I still expect to get stuck with both of them, which is one too many. The other issue (and I haven't dug into details that much) I have with syncpts are some of the interface choices. Apparently you can commit a fence after creation (or at least the hw composer interface works like that) which means userspace can construct deadlocks with android syncpts if I'm not super careful in my driver. I haven't seen any generic code to do that, so I presume everyone just blindly trusts surface-flinger to not do that. Speaks of the average quality of an android gfx driver if the kernel is less trusted than the compositor in userspace ... There's a few other things like exposing timestamps (which are tricky to do right, our driver is littered with wrong attempts) and other details. Finally I've never seen anyone from google or any android product guy push a real driver enabling for syncpts to upstream, and google itself has a bit a history of constantly exchanging their gfx framework for the next best thing. So I really doubt this is worthwhile to pursue in upstream with our essentially eternal api guarantees. At least until we see serious uptake from vendors and gfx driver guys. Unfortunately the Intel android folks are no exception here and haven't pushed anything like this in my direction yet at all. Despite multiple pokes from my side. So from my side I think we should move ahead with Maarten's work and figure the android side out once there's real interest. -Daniel -- Daniel Vetter Software Engineer, Intel Corporation +41 (0) 79 365 57 48 - http://blog.ffwll.ch -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
