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-rw-r--r--Documentation/gpu/drm-uapi.rst13
1 files changed, 13 insertions, 0 deletions
diff --git a/Documentation/gpu/drm-uapi.rst b/Documentation/gpu/drm-uapi.rst
index 04bdc7a91d53..7e51dd40bf6e 100644
--- 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: