drm/docs: Document where the C8 color lut is stored

Also add notes that for atomic drivers it's really somewhere else and
no longer in struct drm_crtc.

Maybe we should put a bigger warning here that this is confusing,
since the pixel format is a plane property, but the GAMMA_LUT property
is on the crtc. But I think we can fix this if/when someone finds a
need for a per-plane CLUT, since I'm not sure such hw even exists. I'm
also not sure whether even hardware with a CLUT and a full color
correction pipeline with degamm/cgm/gamma exists.

Motivated by comments from Geert that we have a gap here.

v2: More names for color luts (Laurent).
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220124221633.952374-1-daniel.vetter@ffwll.ch

drivers/gpu/drm/drm_color_mgmt.c

@@ -82,6 +82,10 @@
  *	driver boot-up state too. Drivers can access this blob through
  *	&drm_crtc_state.gamma_lut.
  *
+ *	Note that for mostly historical reasons stemming from Xorg heritage,
+ *	this is also used to store the color map (also sometimes color lut, CLUT
+ *	or color palette) for indexed formats like DRM_FORMAT_C8.
+ *
  * “GAMMA_LUT_SIZE”:
  *	Unsigned range property to give the size of the lookup table to be set
  *	on the GAMMA_LUT property (the size depends on the underlying hardware).

include/drm/drm_crtc.h

@@ -285,6 +285,10 @@ struct drm_crtc_state {
 	 * 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.
+	 *
+	 * Note that for mostly historical reasons stemming from Xorg heritage,
+	 * this is also used to store the color map (also sometimes color lut,
+	 * CLUT or color palette) for indexed formats like DRM_FORMAT_C8.
 	 */
 	struct drm_property_blob *gamma_lut;
 
@@ -1075,12 +1079,18 @@ struct drm_crtc {
 	/**
 	 * @gamma_size: Size of legacy gamma ramp reported to userspace. Set up
 	 * by calling drm_mode_crtc_set_gamma_size().
+	 *
+	 * Note that atomic drivers need to instead use
+	 * &drm_crtc_state.gamma_lut. See drm_crtc_enable_color_mgmt().
 	 */
 	uint32_t gamma_size;
 
 	/**
 	 * @gamma_store: Gamma ramp values used by the legacy SETGAMMA and
 	 * GETGAMMA IOCTls. Set up by calling drm_mode_crtc_set_gamma_size().
+	 *
+	 * Note that atomic drivers need to instead use
+	 * &drm_crtc_state.gamma_lut. See drm_crtc_enable_color_mgmt().
 	 */
 	uint16_t *gamma_store;