On 2026-06-12 11:34, Nicolas Frattaroli wrote: > Add a new KMS connector property called "min bpc". Its purpose is to > allow userspace to mandate that the output bit depth ends up greater > than or equal to a specified depth, or else have the commit fail. This > is useful for checking for output depth degradation in combination with > atomic test commits. > > Signed-off-by: Nicolas Frattaroli <[email protected]>
Reviewed-by: Harry Wentland <[email protected]> Despite your statement in the cover letter, this does still need a userspace implementation before merging, as well as an IGT test. But I like this definition and your documentation and think we have something quite workable, even long-term. Harry > --- > drivers/gpu/drm/drm_atomic_helper.c | 4 ++ > drivers/gpu/drm/drm_atomic_uapi.c | 4 ++ > drivers/gpu/drm/drm_connector.c | 77 > +++++++++++++++++++++++++++++++++++++ > include/drm/drm_connector.h | 13 +++++++ > 4 files changed, 98 insertions(+) > > diff --git a/drivers/gpu/drm/drm_atomic_helper.c > b/drivers/gpu/drm/drm_atomic_helper.c > index 285aac3554df..613a674ace87 100644 > --- a/drivers/gpu/drm/drm_atomic_helper.c > +++ b/drivers/gpu/drm/drm_atomic_helper.c > @@ -739,6 +739,10 @@ drm_atomic_helper_check_modeset(struct drm_device *dev, > new_connector_state->max_requested_bpc) > new_crtc_state->connectors_changed = true; > > + if (old_connector_state->min_requested_bpc != > + new_connector_state->min_requested_bpc) > + new_crtc_state->connectors_changed = true; > + > if (old_connector_state->color_format != > new_connector_state->color_format) > new_crtc_state->connectors_changed = true; > diff --git a/drivers/gpu/drm/drm_atomic_uapi.c > b/drivers/gpu/drm/drm_atomic_uapi.c > index 1050dddadb17..0440cb63f786 100644 > --- a/drivers/gpu/drm/drm_atomic_uapi.c > +++ b/drivers/gpu/drm/drm_atomic_uapi.c > @@ -956,6 +956,8 @@ static int drm_atomic_connector_set_property(struct > drm_connector *connector, > fence_ptr); > } else if (property == connector->max_bpc_property) { > state->max_requested_bpc = val; > + } else if (property == connector->min_bpc_property) { > + state->min_requested_bpc = val; > } else if (property == connector->privacy_screen_sw_state_property) { > state->privacy_screen_sw_state = val; > } else if (property == connector->broadcast_rgb_property) { > @@ -1043,6 +1045,8 @@ drm_atomic_connector_get_property(struct drm_connector > *connector, > *val = 0; > } else if (property == connector->max_bpc_property) { > *val = state->max_requested_bpc; > + } else if (property == connector->min_bpc_property) { > + *val = state->min_requested_bpc; > } else if (property == connector->privacy_screen_sw_state_property) { > *val = state->privacy_screen_sw_state; > } else if (property == connector->broadcast_rgb_property) { > diff --git a/drivers/gpu/drm/drm_connector.c b/drivers/gpu/drm/drm_connector.c > index cbb067d02cb9..dcf53cc113b0 100644 > --- a/drivers/gpu/drm/drm_connector.c > +++ b/drivers/gpu/drm/drm_connector.c > @@ -1738,6 +1738,46 @@ > EXPORT_SYMBOL(drm_hdmi_connector_get_output_format_name); > * drm_connector_attach_max_bpc_property() to create and attach the > * property to the connector during initialization. > * > + * min bpc: > + * This range property is used by userspace to set a minimum bit depth that > + * the driver must reach on the connector for a given atomic state. If it > + * cannot do so, the atomic commit fails. > + * > + * The precise definition of minimum bit depth used for the "min bpc" > + * property must align with the definition a given driver uses for the > + * "max bpc" property, such that when "min bpc" and "max bpc" are set to > + * the same value, the allowable range of bit depths is a single set point. > + * > + * Drivers should ideally use a definition of bit depth that refers to a > + * perceptual optical bit depth, meaning it is equivalent to an > + * uncompressed optical bit depth for a variety of content to the median > + * human eyeball in a variety of viewing environments. The display link's > + * actual optical bit depth to represent the data on the wire may be lower > + * due to compression techniques, or even higher due to padding. In > + * concrete terms, userspace should not need to know how to implement DSC > + * compression in order to set a minimum bit depth to the appropriate > + * value, and should not need to perform display-protocol-specific > + * calculations to set the right on-the-wire format. > + * > + * Userspace may use the "min bpc" property in combination with performing > + * atomic test commits in order to enumerate the possible output bit depths > + * for a given configuration. This way, userspace compositors can make > + * use-case and content-specific tradeoffs to reach a desired output bit > + * depth, such as by using chroma subsampling, or by choosing a lower > + * refresh rate mode. > + * > + * Userspace may use the "min bpc" property in combination with the > + * "max bpc" property to know what output bit depth a successful atomic > + * commit is using. By setting "min bpc" and "max bpc" to the same value, > + * a successful commit will be using an output bit depth of said value. > + * > + * When "min bpc" is set to a non-zero value, any bit depth degradation > + * logic below "min bpc" must be implemented by userspace (through lowering > + * "min bpc"), rather than by kernel drivers. For a driver to degrade to an > + * uncompressed optical output bit depth below "min bpc" is always a bug no > + * matter how good the intentions are, since it renders the property > + * useless as an oracle. > + * > * Connectors also have one standardized atomic property: > * > * CRTC_ID: > @@ -2892,6 +2932,43 @@ int drm_connector_attach_max_bpc_property(struct > drm_connector *connector, > } > EXPORT_SYMBOL(drm_connector_attach_max_bpc_property); > > +/** > + * drm_connector_attach_min_bpc_property - attach "min bpc" property > + * @connector: connector to attach min bpc property on. > + * @max: The maximum bit depth supported by the connector. > + * > + * Create and attach the "min bpc" connector property, which is used by > + * userspace to require that a certain minimum bit depth is reached by the > + * connector on atomic commit. > + * > + * Returns: > + * Zero on success, negative errno on failure. > + */ > +int drm_connector_attach_min_bpc_property(struct drm_connector *connector, > + unsigned int max) > +{ > + struct drm_device *dev = connector->dev; > + struct drm_property *prop; > + > + if (max > U8_MAX) > + return -ERANGE; > + > + prop = connector->min_bpc_property; > + if (!prop) { > + prop = drm_property_create_range(dev, 0, "min bpc", 0, max); > + if (!prop) > + return -ENOMEM; > + > + connector->min_bpc_property = prop; > + } > + > + drm_object_attach_property(&connector->base, prop, 0); > + connector->state->min_requested_bpc = 0; > + > + return 0; > +} > +EXPORT_SYMBOL(drm_connector_attach_min_bpc_property); > + > /** > * drm_connector_attach_hdr_output_metadata_property - attach > "HDR_OUTPUT_METADATA" property > * @connector: connector to attach the property on. > diff --git a/include/drm/drm_connector.h b/include/drm/drm_connector.h > index 4317166562cf..63d6ac85b3ec 100644 > --- a/include/drm/drm_connector.h > +++ b/include/drm/drm_connector.h > @@ -1261,6 +1261,11 @@ struct drm_connector_state { > */ > u8 max_requested_bpc; > > + /** > + * @min_requested_bpc: Minimum output bit depth requested by userspace > + */ > + u8 min_requested_bpc; > + > /** > * @max_bpc: Connector max_bpc based on the requested max_bpc property > * and the connector bpc limitations obtained from edid. > @@ -2291,6 +2296,12 @@ struct drm_connector { > */ > struct drm_property *max_bpc_property; > > + /** > + * @min_bpc_property: Default connector property for the minimum bit > + * depth for the output to reach, or the commit to fail otherwise. > + */ > + struct drm_property *min_bpc_property; > + > /** @privacy_screen: drm_privacy_screen for this connector, or NULL. */ > struct drm_privacy_screen *privacy_screen; > > @@ -2697,6 +2708,8 @@ int drm_connector_set_orientation_from_panel( > struct drm_panel *panel); > int drm_connector_attach_max_bpc_property(struct drm_connector *connector, > int min, int max); > +int drm_connector_attach_min_bpc_property(struct drm_connector *connector, > + unsigned int max); > void drm_connector_create_privacy_screen_properties(struct drm_connector > *conn); > void drm_connector_attach_privacy_screen_properties(struct drm_connector > *conn); > void drm_connector_attach_privacy_screen_provider( >
