[Intel-gfx] [PATCH 1/2] drm: fix order of event_lock wrt. vblank_time_clock

[Imre Deak]

drm_vblank_off() requires callers to hold the event_lock, while itself
locking vbl_time and vblank_time_lock. This defines a dependency chain
that conflicts with the one in drm_handle_vblank() where we first lock
vblank_time_lock and then the event_lock. Fix this by reversing the
locking order in drm_handle_vblank().

This should've triggered a lockdep warning in the exynos driver, the
rest of the drivers were ok, since they are not using drm_vblank_off(),
or as in the case of the intel driver were not holding the event_lock
when calling drm_vblank_off(). This latter issue is addressed in the
next patch.

Signed-off-by: Imre Deak <imre.deak at intel.com>
---
 drivers/gpu/drm/drm_irq.c |   32 ++++++++++++++++++++------------
 1 file changed, 20 insertions(+), 12 deletions(-)

Tested with i915, the rest of the drivers should be checked with
lockdep enabled.

diff --git a/drivers/gpu/drm/drm_irq.c b/drivers/gpu/drm/drm_irq.c
index 3a3d0ce..2193d4a 100644
--- a/drivers/gpu/drm/drm_irq.c
+++ b/drivers/gpu/drm/drm_irq.c
@@ -1236,17 +1236,21 @@ done:
 	return ret;
 }
 
+/**
+ * drm_handle_vblank_events - send pending vblank events
+ * @dev: DRM device
+ * @crtc: crtc where the vblank(s) happened
+ *
+ * Must be called with @dev->event_lock held.
+ */
 static void drm_handle_vblank_events(struct drm_device *dev, int crtc)
 {
 	struct drm_pending_vblank_event *e, *t;
 	struct timeval now;
-	unsigned long flags;
 	unsigned int seq;
 
 	seq = drm_vblank_count_and_time(dev, crtc, &now);
 
-	spin_lock_irqsave(&dev->event_lock, flags);
-
 	list_for_each_entry_safe(e, t, &dev->vblank_event_list, base.link) {
 		if (e->pipe != crtc)
 			continue;
@@ -1266,8 +1270,6 @@ static void drm_handle_vblank_events(struct drm_device *dev, int crtc)
 						 e->event.sequence);
 	}
 
-	spin_unlock_irqrestore(&dev->event_lock, flags);
-
 	trace_drm_vblank_event(crtc, seq);
 }
 
@@ -1285,21 +1287,22 @@ bool drm_handle_vblank(struct drm_device *dev, int crtc)
 	s64 diff_ns;
 	struct timeval tvblank;
 	unsigned long irqflags;
+	bool ret = false;
 
 	if (!dev->num_crtcs)
 		return false;
 
+	spin_lock_irqsave(&dev->event_lock, irqflags);
+
 	/* Need timestamp lock to prevent concurrent execution with
 	 * vblank enable/disable, as this would cause inconsistent
 	 * or corrupted timestamps and vblank counts.
 	 */
-	spin_lock_irqsave(&dev->vblank_time_lock, irqflags);
+	spin_lock(&dev->vblank_time_lock);
 
 	/* Vblank irq handling disabled. Nothing to do. */
-	if (!dev->vblank_enabled[crtc]) {
-		spin_unlock_irqrestore(&dev->vblank_time_lock, irqflags);
-		return false;
-	}
+	if (!dev->vblank_enabled[crtc])
+		goto unlock_ret;
 
 	/* Fetch corresponding timestamp for this vblank interval from
 	 * driver and store it in proper slot of timestamp ringbuffer.
@@ -1340,7 +1343,12 @@ bool drm_handle_vblank(struct drm_device *dev, int crtc)
 	DRM_WAKEUP(&dev->vbl_queue[crtc]);
 	drm_handle_vblank_events(dev, crtc);
 
-	spin_unlock_irqrestore(&dev->vblank_time_lock, irqflags);
-	return true;
+	ret = true;
+
+unlock_ret:
+	spin_unlock(&dev->vblank_time_lock);
+	spin_unlock_irqrestore(&dev->event_lock, irqflags);
+
+	return ret;
 }
 EXPORT_SYMBOL(drm_handle_vblank);
-- 
1.7.9.5