[Wayland-bugs] [Bug 98731] EGL client behaviour too undefined

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=98731

--- Comment #11 from Pekka Paalanen <ppaalanen at gmail.com> ---
If it is impossible to support atomic state changes and have optimized
performance, maybe we'd need two modes then: normal and streaming.

In normal mode (default) EGL operation is deterministic and window state
changes can be executed atomically. This is what I described in comment 9.

In streaming mode, EGL operation can asynchronous and non-deterministic, and
window state changes are "forbidden". The application cannot identify which
frame is going to the screen or is being displayed at any time. Presentation
extension is useless because there is no guarantee which frame submission it
corresponds to. This mode must be explicitly opt-in, so probably it would be an
EGL/Wayland extension with API to switch the mode.

I presume only games would ever be interested in the streaming mode, as they
tend to repaint regardless of the compositor frame cycle anyway and do not care
which frame(s) happen to end up on screen. I cannot imagine anything else to be
like that. E.g. video players care about audio/video sync, so they care about
which frame was shown when.

I wouldn't want to go in this direction, though. It seems like a step
backwards, towards unpredictable behaviour.

Also "modes" are hard, we already learnt that with the sub-surface extension
which has "synchronized" and "desynchronized" modes. It's a pain to spec and
implement.

With the streaming mode there is also a risk of wanting "just one more thing
supported while streaming", like first a frame callback, then Presentation
feedback, size change, and then it's a never-ending slippery slope of
extensions duplicating what Wayland itself already had...

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