drm: Always reject drm_vblank_get() after drm_vblank_off()

Make sure drm_vblank_get() never succeeds when called between
drm_vblank_off() and drm_vblank_on(). Borrow a trick from the
old drm_vblank_{pre,post}_modeset() functions and just bump
the refcount in drm_vblank_off() and drop it in drm_vblank_on().

When drm_vblank_get() encounters a >0 refcount and the vblank
interrupt is already disabled it will simply return -EINVAL.

Hopefully the use of inmodeset won't conflict badly with
drm_vblank_{pre,post}_modeset().

For i915 there's a window between drm_vblank_off() and marking the
crtc as inactive where the current code still allows drm_vblank_get().

v2: Describe what drm_vblank_get() does to explain how
    a simple refcount bump manages to fix things (Daniel)

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>

1 files changed, 14 insertions, 0 deletions

diff --git a/drivers/gpu/drm/drm_irq.c b/drivers/gpu/drm/drm_irq.c
index 0de123afdb34..b16a63622bad 100644
--- a/drivers/gpu/drm/drm_irq.c
+++ b/drivers/gpu/drm/drm_irq.c
@@ -1039,6 +1039,15 @@ void drm_vblank_off(struct drm_device *dev, int crtc)
         }
         spin_unlock(&dev->event_lock);
 
+        /*
+         * Prevent subsequent drm_vblank_get() from re-enabling
+         * the vblank interrupt by bumping the refcount.
+         */
+        if (!dev->vblank[crtc].inmodeset) {
+                atomic_inc(&dev->vblank[crtc].refcount);
+                dev->vblank[crtc].inmodeset = 1;
+        }
+
         spin_unlock_irqrestore(&dev->vbl_lock, irqflags);
 }
 EXPORT_SYMBOL(drm_vblank_off);
@@ -1079,6 +1088,11 @@ void drm_vblank_on(struct drm_device *dev, int crtc)
         unsigned long irqflags;
 
         spin_lock_irqsave(&dev->vbl_lock, irqflags);
+        /* Drop our private "prevent drm_vblank_get" refcount */
+        if (dev->vblank[crtc].inmodeset) {
+                atomic_dec(&dev->vblank[crtc].refcount);
+                dev->vblank[crtc].inmodeset = 0;
+        }
         /* re-enable interrupts if there's are users left */
         if (atomic_read(&dev->vblank[crtc].refcount) != 0)
                 WARN_ON(drm_vblank_enable(dev, crtc));