[Mesa-dev] [PATCH 2/2] glx/dri2: Add support for adaptive vsync

[Eric Anholt]

Lauri Kasanen <cand at gmx.com> writes:

> On Thu, 2 Jan 2014 11:20:13 +0200
> Lauri Kasanen <cand at gmx.com> wrote:
>
>> On Sun, 15 Dec 2013 12:38:28 +0200
>> Lauri Kasanen <cand at gmx.com> wrote:
>> 
>> > There is a GLX extension for this behavior, glx_swap_control_tear, which mesa doesn't
>> > support ATM. But as usual, even after it becomes supported, there will be thousands
>> > of applications that won't add support for it, necessitating the need for a user
>> > override.
>> 
>> Ping. Patch 1 is already merged.
>
> Ping 2.

I don't think this is what people are thinking of when they ask for
GLX_swap_control_tear.  The model behind swap_control_tear, as far as
I've heard, is that you've got a workload that should be hitting refresh
rate, but then something happens (a shader recompile, for example).  So
one frame misses vsync, and you want that frame to get presented as soon
as possible rather than at the next vblank.

Instead, this patch has that swap still presented vsynced next frame,
late, but then the frame after that (that you predict should be back to
hitting refresh rate) ends up tearing even when it finished on time.  It
seems worse than what would have happened without the patch.

There's also the fact that gettimeofday() has only a very limited
relationship to when frames are ready to be flipped.
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