[Bug 18542] [PATCH] Textured Video (XV) tearing.

[bugzilla-daemon at freedesktop.org]

http://bugs.freedesktop.org/show_bug.cgi?id=18542





--- Comment #13 from Roland Scheidegger <sroland at tungstengraphics.com>  2008-11-20 10:39:33 PST ---
(In reply to comment #12)
> Basically what the patch does is at the start of an acceleration call, I stall
> the engine until we get to the rising or falling edge or a particular vline. 
> The vline I chose was the last one of the visible portion of the scanout
> buffer.  At that point the display controller should be at the start of the
> blanking period; the stall drops and rendering continues.  Hopefully it
> finishes before the blanking period ends.  If not you get tearing again;
> possibly in a different place.
Are you sure it really needs to finish before scanout starts again? Since
vblank is something like 4% of total time (with non-reduced blanking), that
would mean those blits would need to be fast enough for a sustained rate of
1500 fps. I doubt  (at least for the slower chips) that's possible at all (for
reasonably sized full-screen video). I thought it should be sufficient if
rendering is just ahead of scanout, since AFAIK rasterization happens (roughly)
top to bottom. So those 4% should pretty much always be enough for no tearing
to happen I think in theory... Though of course it won't do much if compositing
is used, at least not without the 2d parts of the patch I guess...


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