[Bug 796640] Fix for a synchronization bug in 1.14.1 ext/qt/gstqsgtexture.cc

[GStreamer (GNOME Bugzilla)]

https://bugzilla.gnome.org/show_bug.cgi?id=796640

--- Comment #8 from Jan Schmidt <thaytan at noraisin.net> ---
(In reply to jimmy from comment #7)
> (In reply to Jan Schmidt from comment #6)
> > I've uploaded the patches as 2 separate plain files so they can be more
> > easily reviewed.
> > 
> > 2 comments:
> > * The git commit summary should be a single line less than 76 characters,
> > then leave 1 blank line and write any commit log description that needs more
> > than 76 chars.
> > * 0001 contains some extra unnecessary white-space changes
> 
> Do you need me to submit a new patch with modified git commit summaries?

That would be great. FYI, many of us use the git-bz tool to help with uploading
patches to Bugzilla (https://wiki.gnome.org/Git/WorkingWithPatches)

> > 
> > Does the mutex actually have any effect, or is the
> > gst_gl_sync_meta_wait_cpu() call sufficient?
> 
> In my testing the gst_gl_sync_meta_wait_cpu call was sufficient (appears to
> be the same as glFinish()) BUT
> 
> From reading the comments it seemed as if the gstreamer thread calls
> GstQSGTexture::setBuffer while Qt:s render thread calls on
> GstQSGTexture::bind, this prompted me to add this lock.

Which comment are you thinking of? If I remember correctly, the setBuffer call
is made while blocking the Qt render thread, preventing ::bind from running
simultaneously.

> > 
> > I gather the bug you're seeing is flashing and out-of-order frames, but how
> > are you reproducing it? I haven't seen such behaviour with qmlglsink myself.
> 
> Yes flashing and out-of-order frames.
> 
> The issue is clearly visible on the Tegra X2 platform (most of the time)
> when running the modified qmlglsink example (moving ball pattern), it is
> even more obvious when live streaming from a camera. 

OK. It would be good to mention that it happens on that platform.

> Since this is timing related, it might not be visible or rarely visible on
> other platforms.

You're quite right. I tend to test on platforms with shared GPU/CPU system
memory or simpler GPUs that might not parallelise so much.

> I could snap a video of the issue with my cell-phone and send to you if you
> really want to see it.

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