drm/i915: Clarify event_lock locking, irq&mixed context

Now we tackle the functions also called from interrupt handlers.

- intel_check_page_flip is exclusively called from irq handlers, so a
  plain spin_lock is all we need. In i915_irq.c we have the convention
  to give all such functions an _irq_handler postfix, but that would
  look strange and als be a bit a misleading name. I've opted for a
  WARN_ON(!in_irq()) instead.

- The other two places left are called both from interrupt handlers
  and from our reset work, so need the full irqsave dance. Annotate
  them with a short comment.
Reviewed-by: NJesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_display.c

@@ -9328,6 +9328,10 @@ static void do_intel_finish_page_flip(struct drm_device *dev,
 	if (intel_crtc == NULL)
 		return;
 
+	/*
+	 * This is called both by irq handlers and the reset code (to complete
+	 * lost pageflips) so needs the full irqsave spinlocks.
+	 */
 	spin_lock_irqsave(&dev->event_lock, flags);
 	work = intel_crtc->unpin_work;
 
@@ -9409,7 +9413,12 @@ void intel_prepare_page_flip(struct drm_device *dev, int plane)
 		to_intel_crtc(dev_priv->plane_to_crtc_mapping[plane]);
 	unsigned long flags;
 
-	/* NB: An MMIO update of the plane base pointer will also
+
+	/*
+	 * This is called both by irq handlers and the reset code (to complete
+	 * lost pageflips) so needs the full irqsave spinlocks.
+	 *
+	 * NB: An MMIO update of the plane base pointer will also
 	 * generate a page-flip completion irq, i.e. every modeset
 	 * is also accompanied by a spurious intel_prepare_page_flip().
 	 */
@@ -9866,18 +9875,19 @@ void intel_check_page_flip(struct drm_device *dev, int pipe)
 	struct drm_i915_private *dev_priv = dev->dev_private;
 	struct drm_crtc *crtc = dev_priv->pipe_to_crtc_mapping[pipe];
 	struct intel_crtc *intel_crtc = to_intel_crtc(crtc);
-	unsigned long flags;
+
+	WARN_ON(!in_irq());
 
 	if (crtc == NULL)
 		return;
 
-	spin_lock_irqsave(&dev->event_lock, flags);
+	spin_lock(&dev->event_lock);
 	if (intel_crtc->unpin_work && __intel_pageflip_stall_check(dev, crtc)) {
 		WARN_ONCE(1, "Kicking stuck page flip: queued at %d, now %d\n",
 			 intel_crtc->unpin_work->flip_queued_vblank, drm_vblank_count(dev, pipe));
 		page_flip_completed(intel_crtc);
 	}
-	spin_unlock_irqrestore(&dev->event_lock, flags);
+	spin_unlock(&dev->event_lock);
 }
 
 static int intel_crtc_page_flip(struct drm_crtc *crtc,