diff --git a/drivers/gpu/drm/drm_crtc.c b/drivers/gpu/drm/drm_crtc.c
index 75e0beeb3793..bd3c8b243447 100644
--- a/drivers/gpu/drm/drm_crtc.c
+++ b/drivers/gpu/drm/drm_crtc.c
@@ -52,7 +52,10 @@
  * @idx: index of registered CRTC to find for
  *
  * Given a CRTC index, return the registered CRTC from DRM device's
- * list of CRTCs with matching index.
+ * list of CRTCs with matching index. This is the inverse of drm_crtc_index().
+ * It's useful in the vblank callbacks (like &drm_driver.enable_vblank or
+ * &drm_driver.disable_vblank), since that still deals with indices instead
+ * of pointers to &struct drm_crtc."
  */
 struct drm_crtc *drm_crtc_from_index(struct drm_device *dev, int idx)
 {
diff --git a/drivers/gpu/drm/drm_plane.c b/drivers/gpu/drm/drm_plane.c
index 7b7275f0c2df..eed66be18329 100644
--- a/drivers/gpu/drm/drm_plane.c
+++ b/drivers/gpu/drm/drm_plane.c
@@ -254,7 +254,7 @@ EXPORT_SYMBOL(drm_plane_cleanup);
  * @idx: index of registered plane to find for
  *
  * Given a plane index, return the registered plane from DRM device's
- * list of planes with matching index.
+ * list of planes with matching index. This is the inverse of drm_plane_index().
  */
 struct drm_plane *
 drm_plane_from_index(struct drm_device *dev, int idx)