[Intel-gfx] Video tearing on Sandybridge

Chris Wilson chris at chris-wilson.co.uk
Mon Apr 2 12:03:50 CEST 2012

On Sat, 31 Mar 2012 19:52:09 +0200, Oliver Seitz <info at vtnd.de> wrote:
> If, however, two screens are next to or on top of each other, playing 
> video on one of the screens can show some tearing. It is not always seen 
> with normal video, a special test pattern is helpful. Also, it does only 
> tear in one out of ten tries, you can hit pause again and again, sooner 
> or later the tearing will appear, and disappear again after the next 
> pause/unpause cycle. While the tearing is there, it stays at about the 
> same height of the video until the next pause/unpause cycle. Most times 
> it is seen in the top third of the video. It does not matter if the 
> video is fullscreen or not.
> Is it likely that this is a bug, or am I doing something wrong?

Video tearing in windows is a known issue on SandyBridge as they removed
the commands to perform vsync'ed updates and hid the logic behind a
baffling sequence of registers that we have not yet got working.

The only workaround is to force pageflipping (ie exchange the whole
scanout for each frame) either by using a compositing WM (and hoping
for the best) or to only use fullscreen DRI video applications
(OpenGL or libva decoders) that use SwapBuffers.

Chris Wilson, Intel Open Source Technology Centre

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