[Intel-gfx] [PATCH] Xv: free tearing on textured video

[Kristian Høgsberg]

On Wed, Feb 25, 2009 at 12:24 PM, Keith Packard <keithp at keithp.com> wrote:
> On Wed, 2009-02-25 at 12:05 -0500, Kristian Høgsberg wrote:
>
>> I'll have to try it out - I'm not sure what the exact effect of
>> syncing everything to vblank might be.  For all I know it may not be
>> noticable at all, except for cases where you do a lot of individual
>> read backs per frame... time for me to stop waving the hands and try
>> it.
>
> We block the chip until vblank, once the command to blt the video is
> queued. So, any other rendering will back-up behind the video blt. If
> anything needs to wait for the GPU (for a fallback, or ring full,
> or...), you'll block the CPU as well.

That's the theory; what I've been saying is that we're only increasing
the latency, not the throughput.  And even though the server may block
waiting to read back from a buffer, there's still a window between one
Xv copy and the next where it's not waiting for vsync and can work
though a back log of requests.

But enough speculation, I'll try it out now.

Kristian