drm/radeon: Track the status of a page flip more explicitly

This prevents a panic: radeon_crtc_handle_page_flip() could run before
radeon_flip_work_func(), triggering the BUG_ON() in drm_vblank_put().

Tested-by: Dieter Nützel <[e-mail]>
Reviewed-by: Christian König <[e-mail]>
Signed-off-by: Michel Dänzer <[e-mail]>
Signed-off-by: Alex Deucher <[e-mail]>

drivers/gpu/drm/radeon/radeon_display.c

···
 void radeon_crtc_handle_vblank(struct radeon_device *rdev, int crtc_id)
 {
 	struct radeon_crtc *radeon_crtc = rdev->mode_info.crtcs[crtc_id];
-	struct radeon_flip_work *work;
 	unsigned long flags;
 	u32 update_pending;
 	int vpos, hpos;
···
 		return;
 
 	spin_lock_irqsave(&rdev->ddev->event_lock, flags);
-	work = radeon_crtc->flip_work;
-	if (work == NULL) {
+	if (radeon_crtc->flip_status != RADEON_FLIP_SUBMITTED) {
+		DRM_DEBUG_DRIVER("radeon_crtc->flip_status = %d != "
+				 "RADEON_FLIP_SUBMITTED(%d)\n",
+				 radeon_crtc->flip_status,
+				 RADEON_FLIP_SUBMITTED);
 		spin_unlock_irqrestore(&rdev->ddev->event_lock, flags);
 		return;
 	}
···
 
 	spin_lock_irqsave(&rdev->ddev->event_lock, flags);
 	work = radeon_crtc->flip_work;
-	if (work == NULL) {
+	if (radeon_crtc->flip_status != RADEON_FLIP_SUBMITTED) {
+		DRM_DEBUG_DRIVER("radeon_crtc->flip_status = %d != "
+				 "RADEON_FLIP_SUBMITTED(%d)\n",
+				 radeon_crtc->flip_status,
+				 RADEON_FLIP_SUBMITTED);
 		spin_unlock_irqrestore(&rdev->ddev->event_lock, flags);
 		return;
 	}
 
 	/* Pageflip completed. Clean up. */
+	radeon_crtc->flip_status = RADEON_FLIP_NONE;
 	radeon_crtc->flip_work = NULL;
 
 	/* wakeup userspace */
···
 	/* do the flip (mmio) */
 	radeon_page_flip(rdev, radeon_crtc->crtc_id, base);
 
+	radeon_crtc->flip_status = RADEON_FLIP_SUBMITTED;
 	spin_unlock_irqrestore(&crtc->dev->event_lock, flags);
 	up_read(&rdev->exclusive_lock);
 
···
 	/* We borrow the event spin lock for protecting flip_work */
 	spin_lock_irqsave(&crtc->dev->event_lock, flags);
 
-	if (radeon_crtc->flip_work) {
+	if (radeon_crtc->flip_status != RADEON_FLIP_NONE) {
 		DRM_DEBUG_DRIVER("flip queue: crtc already busy\n");
 		spin_unlock_irqrestore(&crtc->dev->event_lock, flags);
 		drm_gem_object_unreference_unlocked(&work->old_rbo->gem_base);
···
 		kfree(work);
 		return -EBUSY;
 	}
+	radeon_crtc->flip_status = RADEON_FLIP_PENDING;
 	radeon_crtc->flip_work = work;
 
 	/* update crtc fb */

drivers/gpu/drm/radeon/radeon_mode.h

···
 	uint16_t amount;
 };
 
+enum radeon_flip_status {
+	RADEON_FLIP_NONE,
+	RADEON_FLIP_PENDING,
+	RADEON_FLIP_SUBMITTED
+};
+
 struct radeon_crtc {
 	struct drm_crtc base;
 	int crtc_id;
···
 	/* page flipping */
 	struct workqueue_struct *flip_queue;
 	struct radeon_flip_work *flip_work;
+	enum radeon_flip_status flip_status;
 	/* pll sharing */
 	struct radeon_atom_ss ss;
 	bool ss_enabled;