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

[Alex Deucher]

On Mon, Jan 24, 2022 at 5:16 PM Daniel Vetter <daniel.vetter at ffwll.ch> wrote:
>
> 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).
>
> Cc: Laurent Pinchart <laurent.pinchart at ideasonboard.com>
> Reviewed-by: Laurent Pinchart <laurent.pinchart at ideasonboard.com>
> Cc: Geert Uytterhoeven <geert at linux-m68k.org>
> Cc: Ville Syrjälä <ville.syrjala at linux.intel.com>
> Cc: Maarten Lankhorst <maarten.lankhorst at linux.intel.com>
> Cc: Maxime Ripard <mripard at kernel.org>
> Cc: Thomas Zimmermann <tzimmermann at suse.de>
> Cc: David Airlie <airlied at linux.ie>
> Cc: Daniel Vetter <daniel at ffwll.ch>
> Signed-off-by: Daniel Vetter <daniel.vetter at intel.com>

Reviewed-by: Alex Deucher <alexander.deucher at amd.com>

> ---
>  drivers/gpu/drm/drm_color_mgmt.c |  4 ++++
>  include/drm/drm_crtc.h           | 10 ++++++++++
>  2 files changed, 14 insertions(+)
>
> diff --git a/drivers/gpu/drm/drm_color_mgmt.c b/drivers/gpu/drm/drm_color_mgmt.c
> index bb14f488c8f6..9079fbe21d2f 100644
> --- 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 4d01b4d89775..a70baea0636c 100644
> --- 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;
>
> --
> 2.33.0
>