drm/doc: document how userspace should find out CRTC index

[sfrench/cifs-2.6.git] / Documentation / gpu / drm-uapi.rst

diff --git a/Documentation/gpu/drm-uapi.rst b/Documentation/gpu/drm-uapi.rst
index 04bdc7a91d5371fe7df901361d454d9be48c4777..7e51dd40bf6ea8adb68c637f71ecc2e3d7a0924b 100644 (file)
--- a/Documentation/gpu/drm-uapi.rst
+++ b/Documentation/gpu/drm-uapi.rst
@@ -457,6 +457,19 @@ Userspace API Structures
 .. kernel-doc:: include/uapi/drm/drm_mode.h
    :doc: overview
 
+.. _crtc_index:
+
+CRTC index
+----------
+
+CRTC's have both an object ID and an index, and they are not the same thing.
+The index is used in cases where a densely packed identifier for a CRTC is
+needed, for instance a bitmask of CRTC's. The member possible_crtcs of struct
+drm_mode_get_plane is an example.
+
+DRM_IOCTL_MODE_GETRESOURCES populates a structure with an array of CRTC ID's,
+and the CRTC index is its position in this array.
+
 .. kernel-doc:: include/uapi/drm/drm.h
    :internal: