[PATCH 1/3] drm: Send pending vblank events before disabling vblank.

Christopher James Halse Rogers christopher.halse.rogers at canonical.com
Wed Apr 27 01:58:39 PDT 2011

On Wed, 2011-04-27 at 10:32 +0200, Michel Dänzer wrote:
> On Mit, 2011-04-27 at 16:10 +1000, christopher.halse.rogers at canonical.com wrote:
> > From: Christopher James Halse Rogers <christopher.halse.rogers at canonical.com>
> > 
> > This is the least-bad behaviour.  It means that we signal the
> > vblank event before it actually happens, but since we're disabling
> > vblanks there's no guarantee that it will *ever* happen otherwise.
> This may indeed be the best we can do for events that are pending when
> the CRTC is disabled[0], but I can't see anything that would prevent new
> events from getting scheduled (or synchronous vblank waits from timing
> out) while the CRTC is disabled?
> [0] Though it might unnecessarily send events prematurely when the CRTC
> is just disabled temporarily, e.g. as part of a modeset.
> Also, this patch won't seem to help at all for other drivers which don't
> call drm_vblank_off() directly when disabling a CRTC.

This is true.  On the other hand, the other drivers don't wedge the
vblank code into a state where vblanks cannot be re-enabled.  So it's
only a problem when disabling one of 2+ monitors on those drivers,
whereas it's easily triggerable on single monitor systems on intel. 

> Maybe it would be possible to move those calls to core code, and/or only
> force sending out events when the CRTC isn't just getting disabled
> temporarily.

As in: have the core modesetting code call drm_vblank_off before making
the driver-specific calls when disabling a crtc?  I'll look into it -
that would appear to be a more general solution.

It would be nice if the OML_sync_control had been written with a less
than laser-like focus on the single monitor case, too. :/
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