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

diff --git a/drivers/gpu/drm/drm_color_mgmt.c b/drivers/gpu/drm/drm_color_mgmt.c
index bb14f48..9079fbe 100644 (file)
--- a/drivers/gpu/drm/drm_color_mgmt.c
+++ b/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).

diff --git a/include/drm/drm_crtc.h b/include/drm/drm_crtc.h
index 13eeba2..f9f9af8 100644 (file)
--- a/include/drm/drm_crtc.h
+++ b/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;