DRI2 is about to land in the Intel driver, so we've been thinking about how to
deal with the last missing piece: vertical blank synchronized buffer swaps. I
think there are a few of problems to solve here:
1) swap buffers should be asynchronous (i.e. it should let the app continue
rendering the next frame, not block until the swap completes, since that
would defeat the whole purpose of double buffering)
2) swap buffers in general shouldn't just let the app run flat out (i.e.
apps typically want to block until at least one of their frames is
displayed, configurable with the swap control extension)
3) in a composited environment the compositor decides when the frames are
actually displayed, so we can't just do direct vblank waits in the app
like we do today
I think the typical use case is a double buffered app doing glXSwapBuffers
between frames, expecting the function to return immediately unless the buffer
being swapped to hasn't been displayed yet. That will allow it to continue
drawing as soon as possible, which should allow it to render another frame
before the next vertical blank period even in the face of scheduling latency.
Currently, DRI2 glXSwapBuffers ends up in the DDX CopyRegion hook, which at
this point simply does a GC CopyArea. Kristian has a patch to allow for swaps
synchronized to vertical blank, using a separate queue in the i915 DRM driver,
but that only addresses problem (1).
To handle problems (2) and (3) it seems like we need a way for the compositing
manager to signal to the server/DDX that the front buffer has been updated
with the most recent buffers from the currently running GL apps. Kristian
thought we might be able to use the compositor's damage notifier for this; if
the DDX knew which one the compositor was using, it could wait until it was
clear before allowing subsequent buffer swaps to continue.
Comments on this? In other environments (for example specific EGL back ends)
some other synchronization mechanism will be necessary. In the case of
Wayland, I think the compositor has its own copy of things, so it can simply
allow the swap to continue once it has a copy of the latest buffer.
--
Jesse Barnes, Intel Open Source Technology Center
-------------------------------------------------------------------------
This SF.Net email is sponsored by the Moblin Your Move Developer's challenge
Build the coolest Linux based applications with Moblin SDK & win great prizes
Grand prize is a trip for two to an Open Source event anywhere in the world
http://moblin-contest.org/redirect.php?banner_id=100&url=/
--
_______________________________________________
Dri-devel mailing list
Dri-devel@lists.sourceforge.net
https://lists.sourceforge.net/lists/listinfo/dri-devel
