[Mesa-dev] [PATCH 3/3] glx/dri3: Request non-vsynced Present for swapinterval zero.

[Mario Kleiner]

On 12/15/2014 06:46 AM, Keith Packard wrote:
> Mario Kleiner <mario.kleiner.de at gmail.com> writes:
>
>> Restores proper immediate tearing swap behaviour for
>> OpenGL bufferswap under DRI3/Present.
> Hrm. I'd love for this to be controlled by the GLX_EXT_swap_control_tear
> extension, but that one uses negative interval values to indicate
> tearing, instead of providing a new API, and we can't tell the
> difference between 0 and -0.
>
> Are you sure you don't want GLX_EXT_swap_control_tear and an interval of
> -1 instead of an interval of 0 with tearing?
>

Yes. GLX_EXT_swap_control_tear It's a different use case. Useful for 
games which want to avoid tearing if possible, but don't want to get in 
a "tremor" of switching frequently between say 60 fps and 30 fps if they 
only almost manage to do 60 fps, but not with reliable headroom. Would 
be also nice to have support for that, but only as an addition for 
swapinterval -1, not a replacement.

The 0 case is good for benchmarking.

In my specific case i always want vsync'ed swap for actual visual 
stimulation in neuroscience/medical settings, with no frame skipped 
ever. The bonus use for me, except for benchmarking how fast the system 
can go, is if one has a multi-display setup, e.g., dual-display for 
stereoscopic stimulation - one display per eye, or some CAVE like setup 
for VR with more than 2 displays. You want display updates and scanout 
on all of them synchronized, so the scene stays coherent. One simple way 
for visually testing multi-display sync is to intentionally swap all of 
them without vsync, e.g., timed to swap in the middle of the scanout. If 
the tear-lines on all displays are roughly at the same vertical position 
and stay there then that's a good visual test if stuff works. There are 
other ways to do it, but this is the one method that seems to work 
cross-platform, without lots of mental context switching depending on 
what os/gpu/server/driver combo with what settings one uses, and much 
more easy to grasp for scientists with no graphics background. You can 
see at a glance if stuff is roughly correct or not.

-mario