drm/vc4: Fix race between page flip completion event and clean-up

There was a small window where a userspace program could submit
a pageflip after receiving a pageflip completion event yet still
receive EBUSY.
Signed-off-by: Derek Foreman <derekf@osg.samsung.com>
Signed-off-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Daniel Stone <daniels@collabora.com>

drivers/gpu/drm/vc4/vc4_crtc.c

@@ -669,6 +669,14 @@ void vc4_disable_vblank(struct drm_device *dev, unsigned int crtc_id)
 	CRTC_WRITE(PV_INTEN, 0);
 }
 
+/* Must be called with the event lock held */
+bool vc4_event_pending(struct drm_crtc *crtc)
+{
+	struct vc4_crtc *vc4_crtc = to_vc4_crtc(crtc);
+
+	return !!vc4_crtc->event;
+}
+
 static void vc4_crtc_handle_page_flip(struct vc4_crtc *vc4_crtc)
 {
 	struct drm_crtc *crtc = &vc4_crtc->base;

drivers/gpu/drm/vc4/vc4_drv.h

@@ -442,6 +442,7 @@ int vc4_bo_stats_debugfs(struct seq_file *m, void *arg);
 extern struct platform_driver vc4_crtc_driver;
 int vc4_enable_vblank(struct drm_device *dev, unsigned int crtc_id);
 void vc4_disable_vblank(struct drm_device *dev, unsigned int crtc_id);
+bool vc4_event_pending(struct drm_crtc *crtc);
 int vc4_crtc_debugfs_regs(struct seq_file *m, void *arg);
 int vc4_crtc_get_scanoutpos(struct drm_device *dev, unsigned int crtc_id,
 			    unsigned int flags, int *vpos, int *hpos,

drivers/gpu/drm/vc4/vc4_kms.c

@@ -119,17 +119,34 @@ static int vc4_atomic_commit(struct drm_device *dev,
 
 	/* Make sure that any outstanding modesets have finished. */
 	if (nonblock) {
-		ret = down_trylock(&vc4->async_modeset);
-		if (ret) {
+		struct drm_crtc *crtc;
+		struct drm_crtc_state *crtc_state;
+		unsigned long flags;
+		bool busy = false;
+
+		/*
+		 * If there's an undispatched event to send then we're
+		 * obviously still busy.  If there isn't, then we can
+		 * unconditionally wait for the semaphore because it
+		 * shouldn't be contended (for long).
+		 *
+		 * This is to prevent a race where queuing a new flip
+		 * from userspace immediately on receipt of an event
+		 * beats our clean-up and returns EBUSY.
+		 */
+		spin_lock_irqsave(&dev->event_lock, flags);
+		for_each_crtc_in_state(state, crtc, crtc_state, i)
+			busy |= vc4_event_pending(crtc);
+		spin_unlock_irqrestore(&dev->event_lock, flags);
+		if (busy) {
 			kfree(c);
 			return -EBUSY;
 		}
-	} else {
-		ret = down_interruptible(&vc4->async_modeset);
-		if (ret) {
-			kfree(c);
-			return ret;
-		}
+	}
+	ret = down_interruptible(&vc4->async_modeset);
+	if (ret) {
+		kfree(c);
+		return ret;
 	}
 
 	ret = drm_atomic_helper_prepare_planes(dev, state);