Waiting for vertical refresh on Intel 965 and 945 chips in recent git

[Xavier Bestel]

On Fri, 2007-06-08 at 08:27 -0700, Jesse Barnes wrote:
> > Why can't vblank be enabled only when the DRM_IOCTL_WAIT_VBLANK ioctl is
> > called ? Is that because OpenGL hangs on to it ?
> 
> The problem is that GL exposes an absolute vblank event count to clients.  So 
> we have to keep an accurate count or applications could hang or refresh on 
> the wrong event.  Fortunately, some devices have internal vblank counters, so 
> we can disable the interrupt and only enable it when some client is actively 
> waiting, and use the register to keep track of the absolute number of events 
> when interrupts were disabled.

Perfectly clear. I understand now the discussions about using some time-
based counters for some hardware (obviously the other kind of hw).

Thanks,
	Xav