Re: [PATCH v4 03/17] drm/uAPI: Add "active bpc" as feedback channel for "max bpc" drm property

[Werner Sembach <wse@tuxedocomputers.com>]

Am 18.06.21 um 11:11 schrieb Werner Sembach:
> Add a new general drm property "active bpc" which can be used by graphic
> drivers to report the applied bit depth per pixel back to userspace.
>
> While "max bpc" can be used to change the color depth, there was no way to
> check which one actually got used. While in theory the driver chooses the
> best/highest color depth within the max bpc setting a user might not be
> fully aware what his hardware is or isn't capable off. This is meant as a
> quick way to double check the setup.
>
> In the future, automatic color calibration for screens might also depend on
> this information being available.
>
> Signed-off-by: Werner Sembach <wse@tuxedocomputers.com>
> ---
>  drivers/gpu/drm/drm_connector.c | 51 +++++++++++++++++++++++++++++++++
>  include/drm/drm_connector.h     |  8 ++++++
>  2 files changed, 59 insertions(+)
>
> diff --git a/drivers/gpu/drm/drm_connector.c b/drivers/gpu/drm/drm_connector.c
> index da39e7ff6965..943f6b61053b 100644
> --- a/drivers/gpu/drm/drm_connector.c
> +++ b/drivers/gpu/drm/drm_connector.c
> @@ -1197,6 +1197,14 @@ static const struct drm_prop_enum_list dp_colorspaces[] = {
>   *	drm_connector_attach_max_bpc_property() to create and attach the
>   *	property to the connector during initialization.
>   *
> + * active bpc:
> + *	This read-only range property tells userspace the pixel color bit depth
> + *	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, and the "max bpc" property.
> + *	Drivers shall use drm_connector_attach_active_bpc_property() to install
> + *	this property.
> + *

Regarding "on the cable" and dithering: As far as I can tell, what the dithering option does, is setting a hardware
register here:

- https://elixir.bootlin.com/linux/v5.13/source/drivers/gpu/drm/i915/display/intel_display.c#L4534

- https://elixir.bootlin.com/linux/v5.13/source/drivers/gpu/drm/i915/display/intel_display.c#L4571

So dithering seems to be calculated by fixed purpose hardware/firmware outside of the driver?

The Intel driver does not seem to set a target bpc/bpp for this hardware so I guess it defaults to 6 or 8 bpc?

Similar things happen on amd. Here the output dither depth seems to be written to a fixed value however:

- https://elixir.bootlin.com/linux/v5.13/source/drivers/gpu/drm/amd/display/dc/dce/dce_transform.c#L828

- https://elixir.bootlin.com/linux/v5.13/source/drivers/gpu/drm/amd/display/dc/dce/dce_transform.c#L769

Does anyone know about a resource where I can read up on the used registers and what this hardware actually does?

My proposal for now: "max bpc" affects what happens before dither, so I would keep "active bpc" the same and add another
drm property "dither active: true/false". No additional property to control dither, as amdgpu does have one already
(which isn't always active?) and Intel driver does only seem prepared for dithering at 6bpc (albeit I don't know why to
dither at 6bpc and what depth to dither to?).

>   * Connectors also have one standardized atomic property:
>   *
>   * CRTC_ID:
> @@ -2152,6 +2160,49 @@ int drm_connector_attach_max_bpc_property(struct drm_connector *connector,
>  }
>  EXPORT_SYMBOL(drm_connector_attach_max_bpc_property);
>  
> +/**
> + * drm_connector_attach_active_bpc_property - attach "active bpc" property
> + * @connector: connector to attach active bpc property on.
> + * @min: The minimum bit depth supported by the connector.
> + * @max: The maximum bit depth supported by the connector.
> + *
> + * This is used to check the applied bit depth on a connector.
> + *
> + * Returns:
> + * Zero on success, negative errno on failure.
> + */
> +int drm_connector_attach_active_bpc_property(struct drm_connector *connector, int min, int max)
> +{
> +	struct drm_device *dev = connector->dev;
> +	struct drm_property *prop;
> +
> +	if (!connector->active_bpc_property) {
> +		prop = drm_property_create_range(dev, DRM_MODE_PROP_IMMUTABLE, "active bpc",
> +						 min, max);
> +		if (!prop)
> +			return -ENOMEM;
> +
> +		connector->active_bpc_property = prop;
> +		drm_object_attach_property(&connector->base, prop, 0);
> +	}
> +
> +	return 0;
> +}
> +EXPORT_SYMBOL(drm_connector_attach_active_bpc_property);
> +
> +/**
> + * drm_connector_set_active_bpc_property - sets the active bits per color property for a connector
> + * @connector: drm connector
> + * @active_bpc: bits per color for the connector currently active on "the cable"
> + *
> + * Should be used by atomic drivers to update the active bits per color over a connector.
> + */
> +void drm_connector_set_active_bpc_property(struct drm_connector *connector, int active_bpc)
> +{
> +	drm_object_property_set_value(&connector->base, connector->active_bpc_property, active_bpc);
> +}
> +EXPORT_SYMBOL(drm_connector_set_active_bpc_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 714d1a01c065..eee86de62a5f 100644
> --- a/include/drm/drm_connector.h
> +++ b/include/drm/drm_connector.h
> @@ -1380,6 +1380,12 @@ struct drm_connector {
>  	 */
>  	struct drm_property *max_bpc_property;
>  
> +	/**
> +	 * @active_bpc_property: Default connector property for the active bpc
> +	 * to be driven out of the connector.
> +	 */
> +	struct drm_property *active_bpc_property;
> +
>  #define DRM_CONNECTOR_POLL_HPD (1 << 0)
>  #define DRM_CONNECTOR_POLL_CONNECT (1 << 1)
>  #define DRM_CONNECTOR_POLL_DISCONNECT (1 << 2)
> @@ -1702,6 +1708,8 @@ int drm_connector_set_panel_orientation_with_quirk(
>  	int width, int height);
>  int drm_connector_attach_max_bpc_property(struct drm_connector *connector,
>  					  int min, int max);
> +int drm_connector_attach_active_bpc_property(struct drm_connector *connector, int min, int max);
> +void drm_connector_set_active_bpc_property(struct drm_connector *connector, int active_bpc);
>  
>  /**
>   * struct drm_tile_group - Tile group metadata