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-rw-r--r--include/drm/drm_plane.h8
1 files changed, 7 insertions, 1 deletions
diff --git a/include/drm/drm_plane.h b/include/drm/drm_plane.h
index 43cf193e54d6..8b4dc62470ff 100644
--- a/include/drm/drm_plane.h
+++ b/include/drm/drm_plane.h
@@ -47,8 +47,14 @@ struct drm_crtc;
* @src_h: height of visible portion of plane (in 16.16)
* @rotation: rotation of the plane
* @zpos: priority of the given plane on crtc (optional)
+ * Note that multiple active planes on the same crtc can have an identical
+ * zpos value. The rule to solving the conflict is to compare the plane
+ * object IDs; the plane with a higher ID must be stacked on top of a
+ * plane with a lower ID.
* @normalized_zpos: normalized value of zpos: unique, range from 0 to N-1
- * where N is the number of active planes for given crtc
+ * where N is the number of active planes for given crtc. Note that
+ * the driver must call drm_atomic_normalize_zpos() to update this before
+ * it can be trusted.
* @src: clipped source coordinates of the plane (in 16.16)
* @dst: clipped destination coordinates of the plane
* @visible: visibility of the plane