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drm: Avoid random vblank counter jumps if the hardware counter has been reset

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When drm_vblank_on() is called the hardware vblank counter may have
been reset, so we can't trust that the old values sampled prior to
drm_vblank_off() have anything to do with the new values.

So update the .last count in drm_vblank_on() to make the first
drm_vblank_enable() consider that as the reference point. This
will correct the user space visible counter to account for the
time between drm_vblank_on() and the first drm_vblank_enable()
calls.

For extra safety subtract one from the .last count in drm_vblank_on()
to make sure that user space will never see the same counter value
before and after modeset.

Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
This commit is contained in:
Ville Syrjälä 2014-08-06 14:49:49 +03:00 committed by Daniel Vetter
parent 812e7465a7
commit f8ad028cc0
1 changed files with 12 additions and 0 deletions
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12
drivers/gpu/drm/drm_irq.c
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@ -1095,6 +1095,18 @@ void drm_vblank_on(struct drm_device *dev, int crtc)
atomic_dec(&dev->vblank[crtc].refcount);
dev->vblank[crtc].inmodeset = 0;
}
/*
* sample the current counter to avoid random jumps
* when drm_vblank_enable() applies the diff
*
* -1 to make sure user will never see the same
* vblank counter value before and after a modeset
*/
dev->vblank[crtc].last =
(dev->driver->get_vblank_counter(dev, crtc) - 1) &
dev->max_vblank_count;
/* re-enable interrupts if there's are users left */
if (atomic_read(&dev->vblank[crtc].refcount) != 0)
WARN_ON(drm_vblank_enable(dev, crtc));