On Sat, 25 Jun 2016 21:20:59 +0200 Eugen Friedrich <[email protected]> wrote:
> Von: "Eugen Friedrich" <[email protected]> > Datum: 24.06.2016 10:59 nachm. > Betreff: Re: How to create client in child thread > An: "Pekka Paalanen" <[email protected]> > Cc: > > Hello Pekka, > you mentions some interesting points for me in this email not really > related to the original topic, > therefore I'm just jumping in between. Hi Eugen! > > > eglSwapBuffers() counts as a Wayland protocol call, because it is > > > guaranteed to call wl_surface.commit before it returns and with the > > totally agree with that, but is there any official specification for this? Unfortunately no. It has been a dream for a good while, and one idea was that it might be possible to write the text into EGL_KHR_platform_wayland. Even better would be to write a EGL Wayland platform conformance test suite. I think it will need to excercise both server- and client-side at the same time, so that it can trigger the possible races, corner cases and detect bad shortcuts like calling wl_surface.commit from another thread as reliably as possible. But that would be a huge undertaking. > Argument: because it is implemented in Mesa is not always accepted by > suppliers. Also if compositor and clients use different hardware for > rendering synchronization might become difficult. Sync is a two-fold problem. Certainly EGL Wayland platform spec should say something about it, even if just for the same-device case. Syncing between different devices is only emerging on the FOSS side. There is a lot of work already done in the kernel, but we also need a lot of work in other places. I am very much looking forward to explicit fencing. > I already know two different EGL providers where wl_surface.commit is > not called during eglSwapBuffer execution, there is a thread inside > the EGL which is doing this. Yes, I have heard of that, it is horribly broken. You can quote me on that, in lack of any better references. Applications *must* be able to rely on eglSwapBuffers() issuing a wl_surface.commit *before* it returns, so that the surface state submission can be reliable. That can cause performance issues when you do not have the implicit fencing we have with FOSS drivers for the same-device case, we know that. The proper fix will be found through the work on fences. > > > newly rendered buffer attached, and it will also wait for > > > wl_buffer.release events if necessary, and by default it also waits for > > > wl_surface.frame callbacks to return. > > > > > > Here is what I wrote about EGL elsewhere: > > > > > > eglSwapBuffers will be waiting for the previous eglSwapBuffers' > > > frame callback and only if it has not arrived already for the > > > particular surface. If you call eglSwapBuffers as a response to > > > receiving your own copy of the frame callback or later, > > > eglSwapBuffers will never block. > > > > > > To be more precise, wait for the frame callback happens on the > > > eglSwapBuffers *after* the current one as I explained above, > > > and waits for wl_buffer.releases happen at the first draw call > > > that would need a buffer to draw into if necessary. > > > > > > Those are the two orthogonal throttling mechanisms in Mesa. > > > Setting swap interval to 0 will prevent waits for the frame > > > callbacks, but not for buffer releases, because Mesa does not > > > want to potentially allocate an unlimited number of buffers in > > > case the server is slow to send out releases (which would imply > > > your whole system is already hosed anyway, so putting even more > > > pressure to it would only make things worse). > > > > > > The above describes the expected behaviour of EGL. Bugs, which we know > > > to be around, are another matter. > > > > > > Do note, that I am only talking about Mesa and what a proper EGL > > > implementation would do. If you are using any properietary EGL > > > implementation, especially Vivante, you might be in trouble. > > > > > > Other things you must take care of are: > > > > > > - every thread where you want Wayland events dispatched must have its > > > own wl_event_queue (Mesa EGL creates one for itself) > > > > > > - you must use the wl_display_prepare_read() API of libwayland-client > > > in all callers of libwayland-client properly > > > > > > - Pay attention to all bugs linked from > > > https://bugs.freedesktop.org/show_bug.cgi?id=91769 , > > > particularly https://bugs.freedesktop.org/show_bug.cgi?id=91273 which > > > also requires a fix in the EGL implementation (for Mesa, see > > > https://lists.freedesktop.org/archives/mesa-dev/2016-May/115617.html > > > for a patch that does not seem to have been even merged yet). > > > > > > In general, Wayland functions are *not* meant for controlling things > > > across thread boundaries. It is safe to send requests from multiple > > > threads, but you are still responsible for ordering them correctly > > > yourself in cases where the order matters. > > > > > > If this gives you the impression that using threads with EGL/Wayland in > > > particular is rare and not really tested, you are right. I do not know > > > of any good code examples, maybe others have some? > > don't have an example by hand but I think with wayland 1.11 there is a > proper way to implement thread save handling, right? > important points are already mentioned above only one to add: > - use proxy wrappers for creation of wayland objects We have thought we got the thread-safeness right several times, until someone found one more case that explodes painfully. I hope for the best, but will not be surprised if there are still some corner cases left. Thanks, pq
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