drm/nouveau/kms/nv04-nv40: fix pageflip events via special case.

Cards with nv04 display engine can't reliably use vblank
counts and timestamps computed via drm_handle_vblank(), as
the function gets invoked after sending the pageflip events.

Fix this by defaulting to the old crtcid = -1 fallback path
on <= NV-50 cards, and only using the precise path on NV-50
and later.
Signed-off-by: Mario Kleiner <mario.kleiner.de@gmail.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Cc: <stable@vger.kernel.org> # 3.13+

drivers/gpu/drm/nouveau/nouveau_display.c

@@ -798,6 +798,7 @@ nouveau_finish_page_flip(struct nouveau_channel *chan,
 	struct drm_device *dev = drm->dev;
 	struct nouveau_page_flip_state *s;
 	unsigned long flags;
+	int crtcid = -1;
 
 	spin_lock_irqsave(&dev->event_lock, flags);
 
@@ -808,8 +809,13 @@ nouveau_finish_page_flip(struct nouveau_channel *chan,
 	}
 
 	s = list_first_entry(&fctx->flip, struct nouveau_page_flip_state, head);
-	if (s->event)
-		drm_send_vblank_event(dev, s->crtc, s->event);
+	if (s->event) {
+		/* Vblank timestamps/counts are only correct on >= NV-50 */
+		if (nv_device(drm->device)->card_type >= NV_50)
+			crtcid = s->crtc;
+
+		drm_send_vblank_event(dev, crtcid, s->event);
+	}
 
 	list_del(&s->head);
 	if (ps)