drm: add docs for standard CRTC properties

This patch adds docs for the ACTIVE and MODE_ID CRTC properties.

Signed-off-by: Simon Ser <contact@emersion.fr>
Cc: Daniel Vetter <daniel.vetter@intel.com>
Cc: Ville Syrjala <ville.syrjala@linux.intel.com>
Cc: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Cc: Michel Dänzer <michel@daenzer.net>
Cc: Daniel Stone <daniel@fooishbar.org>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/k52vYFBQ5ZO18TgZl3W8MgP6f4qu5Ncir7w-On8Dm0V2KTAcVkUoS7-IGPcvDJAXLsyJAUsD0QFJts3Dy0yWyHXVh85axrZkybh3MGGFhQc=@emersion.fr

Documentation/gpu/drm-kms.rst

@@ -460,6 +460,12 @@ HDMI Specific Connector Properties
 .. kernel-doc:: drivers/gpu/drm/drm_connector.c
    :doc: HDMI connector properties
 
+Standard CRTC Properties
+------------------------
+
+.. kernel-doc:: drivers/gpu/drm/drm_crtc.c
+   :doc: standard CRTC properties
+
 Plane Composition Properties
 ----------------------------
 

drivers/gpu/drm/drm_crtc.c

@@ -204,6 +204,33 @@ struct dma_fence *drm_crtc_create_fence(struct drm_crtc *crtc)
 	return fence;
 }
 
+/**
+ * DOC: standard CRTC properties
+ *
+ * DRM CRTCs have a few standardized properties:
+ *
+ * ACTIVE:
+ * 	Atomic property for setting the power state of the CRTC. When set to 1
+ * 	the CRTC will actively display content. When set to 0 the CRTC will be
+ * 	powered off. There is no expectation that user-space will reset CRTC
+ * 	resources like the mode and planes when setting ACTIVE to 0.
+ *
+ * 	User-space can rely on an ACTIVE change to 1 to never fail an atomic
+ * 	test as long as no other property has changed. If a change to ACTIVE
+ * 	fails an atomic test, this is a driver bug. For this reason setting
+ * 	ACTIVE to 0 must not release internal resources (like reserved memory
+ * 	bandwidth or clock generators).
+ *
+ * 	Note that the legacy DPMS property on connectors is internally routed
+ * 	to control this property for atomic drivers.
+ * MODE_ID:
+ * 	Atomic property for setting the CRTC display timings. The value is the
+ * 	ID of a blob containing the DRM mode info. To disable the CRTC,
+ * 	user-space must set this property to 0.
+ *
+ * 	Setting MODE_ID to 0 will release reserved resources for the CRTC.
+ */
+
 /**
  * drm_crtc_init_with_planes - Initialise a new CRTC object with
  *    specified primary and cursor planes.