diff --git a/drivers/gpu/drm/drm_blend.c b/drivers/gpu/drm/drm_blend.c
index 37ac168fcb60ff46faadf4b59ce9bc53c7d1bf9a..d02709dd2d4a38cb89f08f25912146cc9a7fca42 100644
--- a/drivers/gpu/drm/drm_blend.c
+++ b/drivers/gpu/drm/drm_blend.c
@@ -130,7 +130,12 @@
  *	Z position is set up with drm_plane_create_zpos_immutable_property() and
  *	drm_plane_create_zpos_property(). It controls the visibility of overlapping
  *	planes. Without this property the primary plane is always below the cursor
- *	plane, and ordering between all other planes is undefined.
+ *	plane, and ordering between all other planes is undefined. The positive
+ *	Z axis points towards the user, i.e. planes with lower Z position values
+ *	are underneath planes with higher Z position values. Note that the Z
+ *	position value can also be immutable, to inform userspace about the
+ *	hard-coded stacking of overlay planes, see
+ *	drm_plane_create_zpos_immutable_property().
  *
  * pixel blend mode:
  *	Pixel blend mode is set up with drm_plane_create_blend_mode_property().