drm/doc: Interlink color manager docs better

Motivated by a request from Eric.

v2: Take in suggestions from Lionel

Cc: Eric Anholt <eric@anholt.net>
Cc: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170412152006.12233-1-daniel.vetter@ffwll.ch

drivers/gpu/drm/drm_atomic_helper.c

@@ -3517,7 +3517,8 @@ EXPORT_SYMBOL(drm_atomic_helper_connector_destroy_state);
  *
  * Implements support for legacy gamma correction table for drivers
  * that support color management through the DEGAMMA_LUT/GAMMA_LUT
- * properties.
+ * properties. See drm_crtc_enable_color_mgmt() and the containing chapter for
+ * how the atomic color management and gamma tables work.
  */
 int drm_atomic_helper_legacy_gamma_set(struct drm_crtc *crtc,
 				       u16 *red, u16 *green, u16 *blue,

drivers/gpu/drm/drm_color_mgmt.c

@@ -43,7 +43,8 @@
  *
  *	Setting this to NULL (blob property value set to 0) means a
  *	linear/pass-thru gamma table should be used. This is generally the
- *	driver boot-up state too.
+ *	driver boot-up state too. Drivers can access this blob through
+ *	&drm_crtc_state.degamma_lut.
  *
  * “DEGAMMA_LUT_SIZE”:
  *	Unsinged range property to give the size of the lookup table to be set
@@ -60,7 +61,8 @@
  *
  *	Setting this to NULL (blob property value set to 0) means a
  *	unit/pass-thru matrix should be used. This is generally the driver
- *	boot-up state too.
+ *	boot-up state too. Drivers can access the blob for the color conversion
+ *	matrix through &drm_crtc_state.ctm.
  *
  * “GAMMA_LUT”:
  *	Blob property to set the gamma lookup table (LUT) mapping pixel data
@@ -72,7 +74,8 @@
  *
  *	Setting this to NULL (blob property value set to 0) means a
  *	linear/pass-thru gamma table should be used. This is generally the
- *	driver boot-up state too.
+ *	driver boot-up state too. Drivers can access this blob through
+ *	&drm_crtc_state.gamma_lut.
  *
  * “GAMMA_LUT_SIZE”:
  *	Unsigned range property to give the size of the lookup table to be set

include/drm/drm_crtc.h

@@ -93,11 +93,6 @@ struct drm_plane_helper_funcs;
  * @adjusted_mode: for use by helpers and drivers to compute adjusted mode timings
  * @mode: current mode timings
  * @mode_blob: &drm_property_blob for @mode
- * @degamma_lut: Lookup table for converting framebuffer pixel data
- *	before apply the conversion matrix
- * @ctm: Transformation matrix
- * @gamma_lut: Lookup table for converting pixel data after the
- *	conversion matrix
  * @state: backpointer to global drm_atomic_state
  *
  * Note that the distinction between @enable and @active is rather subtile:
@@ -144,9 +139,30 @@ struct drm_crtc_state {
 	/* blob property to expose current mode to atomic userspace */
 	struct drm_property_blob *mode_blob;
 
-	/* blob property to expose color management to userspace */
+	/**
+	 * @degamma_lut:
+	 *
+	 * Lookup table for converting framebuffer pixel data before apply the
+	 * color conversion matrix @ctm. See drm_crtc_enable_color_mgmt(). The
+	 * blob (if not NULL) is an array of &struct drm_color_lut.
+	 */
 	struct drm_property_blob *degamma_lut;
+
+	/**
+	 * @ctm:
+	 *
+	 * Color transformation matrix. See drm_crtc_enable_color_mgmt(). The
+	 * blob (if not NULL) is a &struct drm_color_ctm.
+	 */
 	struct drm_property_blob *ctm;
+
+	/**
+	 * @gamma_lut:
+	 *
+	 * Lookup table for converting pixel data after the color conversion
+	 * matrix @ctm.  See drm_crtc_enable_color_mgmt(). The blob (if not
+	 * NULL) is an array of &struct drm_color_lut.
+	 */
 	struct drm_property_blob *gamma_lut;
 
 	/**
@@ -313,6 +329,12 @@ struct drm_crtc_funcs {
 	 *
 	 * This callback is optional.
 	 *
+	 * Atomic drivers who want to support gamma tables should implement the
+	 * atomic color management support, enabled by calling
+	 * drm_crtc_enable_color_mgmt(), which then supports the legacy gamma
+	 * interface through the drm_atomic_helper_legacy_gamma_set()
+	 * compatibility implementation.
+	 *
 	 * NOTE:
 	 *
 	 * Drivers that support gamma tables and also fbdev emulation through