EGL Wayland platform specifications and conformance (Re: How to create client in child thread, egl behavior)

[Pekka Paalanen]

On Sat, 25 Jun 2016 21:20:59 +0200
Eugen Friedrich <friedrix at gmail.com> wrote:

> Von: "Eugen Friedrich" <friedrix at gmail.com>
> Datum: 24.06.2016 10:59 nachm.
> Betreff: Re: How to create client in child thread
> An: "Pekka Paalanen" <ppaalanen at gmail.com>
> 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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