drm/gma500: Fix crtc_vblank reference leak when userspace queues mult…

…iple events

The gma500 page-flip code kinda assume that userspace never queues more
then 1 vblank event. So basically it assume that userspace does:

- page-flip
- wait for vblank event
- render
- page-flip
- etc.

In the case where userspace would submit 2 page-flips without waiting
for the first to finish, the current code will just overwrite
gma_crtc->page_flip_event with the event from the 2nd page-flip.

Before this patch if page-flips are submitted then drm_crtc_vblank_get()
will get called twice, where drm_crtc_vblank_put(crtc) will only be run
once, since only 1 event will get reported (the last event set in
gma_crtc->page_flip_event).

Fix the crtc_vblank reference leak by not calling drm_crtc_vblank_get()
when replacing a still set gma_crtc->page_flip_event with a new one.

And while at it add a warning for when userspace tries to queue
multiple page-flips with events attached which gma500 currently
does not support.

Note this is not a real fix for the issue of the gma500 code not
supporting multiple page-flips events being pending, but it at least
improves the situation a bit.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>

drivers/gpu/drm/gma500/gma_display.c

@@ -528,10 +528,13 @@ int gma_crtc_page_flip(struct drm_crtc *crtc,
 
 	if (event) {
 		spin_lock_irqsave(&dev->event_lock, flags);
-
-		WARN_ON(drm_crtc_vblank_get(crtc) != 0);
-
-		gma_crtc->page_flip_event = event;
+		if (!gma_crtc->page_flip_event) {
+			WARN_ON(drm_crtc_vblank_get(crtc) != 0);
+			gma_crtc->page_flip_event = event;
+		} else {
+			drm_warn_once(dev, "page_flip_event already set, gma500 does not support queuing multiple events\n");
+			gma_crtc->page_flip_event = event;
+		}
 		spin_unlock_irqrestore(&dev->event_lock, flags);
 
 		/* Call this locked if we want an event at vblank interrupt. */