drm/atomic: log when page-flip is requested without CRTCs

Signed-off-by: Simon Ser <contact@emersion.fr>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221019143736.267324-6-contact@emersion.fr

drivers/gpu/drm/drm_atomic_uapi.c

@@ -1265,8 +1265,10 @@ static int prepare_signaling(struct drm_device *dev,
 	 * Having this flag means user mode pends on event which will never
 	 * reach due to lack of at least one CRTC for signaling
 	 */
-	if (c == 0 && (arg->flags & DRM_MODE_PAGE_FLIP_EVENT))
+	if (c == 0 && (arg->flags & DRM_MODE_PAGE_FLIP_EVENT)) {
+		drm_dbg_atomic(dev, "need at least one CRTC for DRM_MODE_PAGE_FLIP_EVENT");
 		return -EINVAL;
+	}
 
 	return 0;
 }