Re: [PATCH 2/7] drm/uAPI: Add "active color format" drm property as feedback for userspace

[Werner Sembach <wse@tuxedocomputers.com>]

Hi,

Am 09.01.24 um 19:10 schrieb Andri Yngvason:
> From: Werner Sembach <wse@tuxedocomputers.com>
>
> Add a new general drm property "active color format" which can be used by
> graphic drivers to report the used color format back to userspace.
>
> There was no way to check which color format got actually used on a given
> monitor. To surely predict this, one must know the exact capabilities of
> the monitor, the GPU, and the connection used and what the default
> behaviour of the used driver is (e.g. amdgpu prefers YCbCr 4:4:4 while i915
> prefers RGB). This property helps eliminating the guessing on this point.
>
> In the future, automatic color calibration for screens might also depend on
> this information being available.
>
> Signed-off-by: Werner Sembach <wse@tuxedocomputers.com>
> Signed-off-by: Andri Yngvason <andri@yngvason.is>
> Tested-by: Andri Yngvason <andri@yngvason.is>

One suggestion from back then was, instead picking out singular properties like 
"active color format", to just expose whatever the last HDMI or DP metadata 
block(s)/frame(s) that was sent over the display wire was to userspace and 
accompanying it with a parsing script.

Question: Does the driver really actually know what the GPU is ultimatively 
sending out the wire, or is that decided by a final firmware blob we have no 
info about?

Greetings

Werner

> ---
>   drivers/gpu/drm/drm_connector.c | 63 +++++++++++++++++++++++++++++++++
>   include/drm/drm_connector.h     | 10 ++++++
>   2 files changed, 73 insertions(+)
>
> diff --git a/drivers/gpu/drm/drm_connector.c b/drivers/gpu/drm/drm_connector.c
> index c3725086f4132..30d62e505d188 100644
> --- a/drivers/gpu/drm/drm_connector.c
> +++ b/drivers/gpu/drm/drm_connector.c
> @@ -1061,6 +1061,14 @@ static const struct drm_prop_enum_list drm_dp_subconnector_enum_list[] = {
>   	{ DRM_MODE_SUBCONNECTOR_Native,	     "Native"    }, /* DP */
>   };
>   
> +static const struct drm_prop_enum_list drm_active_color_format_enum_list[] = {
> +	{ 0, "not applicable" },
> +	{ DRM_COLOR_FORMAT_RGB444, "rgb" },
> +	{ DRM_COLOR_FORMAT_YCBCR444, "ycbcr444" },
> +	{ DRM_COLOR_FORMAT_YCBCR422, "ycbcr422" },
> +	{ DRM_COLOR_FORMAT_YCBCR420, "ycbcr420" },
> +};
> +
>   DRM_ENUM_NAME_FN(drm_get_dp_subconnector_name,
>   		 drm_dp_subconnector_enum_list)
>   
> @@ -1390,6 +1398,15 @@ static const u32 dp_colorspaces =
>    *	drm_connector_attach_max_bpc_property() to create and attach the
>    *	property to the connector during initialization.
>    *
> + * active color format:
> + *	This read-only property tells userspace the color format actually used
> + *	by the hardware display engine "on the cable" on a connector. The chosen
> + *	value depends on hardware capabilities, both display engine and
> + *	connected monitor. Drivers shall use
> + *	drm_connector_attach_active_color_format_property() to install this
> + *	property. Possible values are "not applicable", "rgb", "ycbcr444",
> + *	"ycbcr422", and "ycbcr420".
> + *
>    * Connectors also have one standardized atomic property:
>    *
>    * CRTC_ID:
> @@ -2451,6 +2468,52 @@ int drm_connector_attach_max_bpc_property(struct drm_connector *connector,
>   }
>   EXPORT_SYMBOL(drm_connector_attach_max_bpc_property);
>   
> +/**
> + * drm_connector_attach_active_color_format_property - attach "active color format" property
> + * @connector: connector to attach active color format property on.
> + *
> + * This is used to check the applied color format on a connector.
> + *
> + * Returns:
> + * Zero on success, negative errno on failure.
> + */
> +int drm_connector_attach_active_color_format_property(struct drm_connector *connector)
> +{
> +	struct drm_device *dev = connector->dev;
> +	struct drm_property *prop;
> +
> +	if (!connector->active_color_format_property) {
> +		prop = drm_property_create_enum(dev, DRM_MODE_PROP_IMMUTABLE, "active color format",
> +						drm_active_color_format_enum_list,
> +						ARRAY_SIZE(drm_active_color_format_enum_list));
> +		if (!prop)
> +			return -ENOMEM;
> +
> +		connector->active_color_format_property = prop;
> +	}
> +
> +	drm_object_attach_property(&connector->base, prop, 0);
> +
> +	return 0;
> +}
> +EXPORT_SYMBOL(drm_connector_attach_active_color_format_property);
> +
> +/**
> + * drm_connector_set_active_color_format_property - sets the active color format property for a
> + * connector
> + * @connector: drm connector
> + * @active_color_format: color format for the connector currently active "on the cable"
> + *
> + * Should be used by atomic drivers to update the active color format over a connector.
> + */
> +void drm_connector_set_active_color_format_property(struct drm_connector *connector,
> +						    u32 active_color_format)
> +{
> +	drm_object_property_set_value(&connector->base, connector->active_color_format_property,
> +				      active_color_format);
> +}
> +EXPORT_SYMBOL(drm_connector_set_active_color_format_property);
> +
>   /**
>    * drm_connector_attach_hdr_output_metadata_property - attach "HDR_OUTPUT_METADA" property
>    * @connector: connector to attach the property on.
> diff --git a/include/drm/drm_connector.h b/include/drm/drm_connector.h
> index fe88d7fc6b8f4..9ae73cfdceeb1 100644
> --- a/include/drm/drm_connector.h
> +++ b/include/drm/drm_connector.h
> @@ -1699,6 +1699,12 @@ struct drm_connector {
>   	 */
>   	struct drm_property *privacy_screen_hw_state_property;
>   
> +	/**
> +	 * @active_color_format_property: Default connector property for the
> +	 * active color format to be driven out of the connector.
> +	 */
> +	struct drm_property *active_color_format_property;
> +
>   #define DRM_CONNECTOR_POLL_HPD (1 << 0)
>   #define DRM_CONNECTOR_POLL_CONNECT (1 << 1)
>   #define DRM_CONNECTOR_POLL_DISCONNECT (1 << 2)
> @@ -2053,6 +2059,10 @@ void drm_connector_attach_privacy_screen_provider(
>   	struct drm_connector *connector, struct drm_privacy_screen *priv);
>   void drm_connector_update_privacy_screen(const struct drm_connector_state *connector_state);
>   
> +int drm_connector_attach_active_color_format_property(struct drm_connector *connector);
> +void drm_connector_set_active_color_format_property(struct drm_connector *connector,
> +						    u32 active_color_format);
> +
>   /**
>    * struct drm_tile_group - Tile group metadata
>    * @refcount: reference count