[Bug 96847] TearFree breaks when playing back videos in Firefox with GPU accelerated windows enabled in Firefox

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=96847

--- Comment #5 from Chris Wilson <chris at chris-wilson.co.uk> ---
(In reply to nw9165-3201 from comment #4)
> (In reply to Chris Wilson from comment #1)
> > Please note that DRI3 overrides TearFree
> 
> Funny, I had already asked about this before but nobody replied, see:
> https://bugs.freedesktop.org/show_bug.cgi?id=37686#c124.
> 
> Also, are you absolutely sure about that?

Absolutely, since I wrote the code, I had to explicitly disable TearFree when
PresentPixmap is called because a client calling PresentPixmap can then treat
that Pixmap as a front buffer and directly render into it bypassing X.

> Because even with:
> 
> Option "DRI" "3"
> 
> Xorg.0.log says:
> 
> [    16.253] (**) intel(0): TearFree enabled
> 
> So it seems like TearFree can be enabled even when DRI3 is enabled.
> 
> Also, when enabling DRI3 and leaving TearFree disabled, there's still
> tearing. So why should DRI3 override TearFree when DRI3 itself does not
> provide a tear free experience?

Who said that DRI3 was meant to provide a TearFree experience? That's not what
we put together in the specification or the code. Also note that DRI3 *cannot*
provide TearFree updates in a variety of configurations and due to many
limitations in the current implementation is often inferior to DRI2 (such as
multi-monitor rendering).

> (In reply to Chris Wilson from comment #1)
> > since the client is given direct
> > access to the scanout there is nothing the driver can do (until the client
> > relinquishes control of the scanout).
> 
> Well, I've also tried it with DRI2 via:
> 
> Option "DRI" "2"
> 
> but the issue is the same.
> 
> (In reply to Chris Wilson from comment #2)
> > first attach Xorg.0.log
> 
> Attached:
> 
> https://bugs.freedesktop.org/attachment.cgi?id=124946
> 
> (In reply to Chris Wilson from comment #2)
> > double
> > check with xf86-video-intel.git. If you can compile with ./configure
> > --enable-debug that should help catch the easiest problems (in particular
> > losing tracking of DRI3/TearFree switch-over).
> 
> Is it really necessary to compile xf86-video-intel with a custom build
> option to be able to get a verbose (debug) log? Or what does --enable-debug
> do?

Yes. Doing so does 2 things; verify that you know how to build and apply
patches; and secondly it enables extra assertions and many runtime checks that
could be handled without crashing and instead causes X to crash instead.

> By the way, if you want to reproduce the issue:
> 
> As far as I can tell, the issue is triggered by switching the YouTube video
> player to fullscreen.

Worksforme.

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