[Development] QtWayland synchronisation between GL and compositor

[Tomek Bury]

Hi Gunnar,

Thanks for your replay.

I've spend quite some time on #wayland IRC channel yesterday discussing
this matter. It turns out that the compositor implementation assumes
implicit synchronisation.

The "submit" operation assumes that whatever dependencies were present on
the client side are carried over to the server side. This way there's no
need for an explicit CPU-GPU synchronisation on either side. Of course in
order to do that one needs some sort of cross-process synchronisation
either implicitly provided by the driver or via some sort of sync objects
that can safely travel between client and server hidden in the Wayland
platform implementation.

The "release" operation is trickier and it's based on a specific
implementation detail. Here's the wording from the spec (
https://www.khronos.org/registry/gles/extensions/OES/OES_EGL_image_external.txt
):

        Sampling an external texture which has been modified since it was
        bound will return samples which may correspond to image values either
        before, during, or after the modification.  Binding (or re-binding if
        already bound) an external texture by calling BindTexture after all
        modifications are complete guarantees that sampling done in future
        draw calls will return values corresponding to the values in the
        buffer at or after the time that BindTexture is called.

Now, spec says "at or after" while Weston (and QtWayland as far as I can
tell) assume that it's always "at" and never "after". If the driver honours
that assumption it's safe to release the buffer to the client without
waiting for the GL pipeline to finish drawing. Of course GL drivers are not
required to honour that assumption and it's perfectly valid for the client
to start updating the released buffer while compositor still uses it.
The biggest problem is that there's no cross process synchronisation
available in GL/EGL so the Wayland would have to take a performance hit if
it wanted to be perfectly compliant with the GL spec. So technically it is
a bug, but cost of fixing it is to high.

The explicit synchronisation is in the works for quite some time now but
don't hold your breath.

Cheers,
Tomek


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