Re: [PATCH v2 03/20] drm/bridge-connector: Attach max bpc for non-HDMI bridges

[Chaoyi Chen <chaoyi.chen@rock-chips.com>]

Hi Xilin,

On 10/9/2026 11:15 AM, Xilin Wu wrote:
> Allow non-HDMI bridges to advertise their output bit-depth limit through
> max_bpc. Attach the max bpc property when the lowest reported limit in the
> chain exceeds 8, ignoring bridges that do not report a limit. This lets
> DP drivers expose deep-color support without attaching the property in
> their connector initialization code.
> 
> Use a lower bound of 6 for DP and eDP, which support 6 bpc RGB output, and
> 8 for other connector types. Keep the existing HDMI registration path.
> The property can be attached before connector state allocation; the
> atomic state helper initializes its default when state is created.
> 
> Assisted-by: LLM
> Signed-off-by: Xilin Wu <sophon@radxa.com>
> ---
>  drivers/gpu/drm/display/drm_bridge_connector.c | 18 ++++++++++++++++++
>  include/drm/drm_bridge.h                       | 11 ++++++++---
>  2 files changed, 26 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/gpu/drm/display/drm_bridge_connector.c b/drivers/gpu/drm/display/drm_bridge_connector.c
> index 8b54069fa53a..be2e08a3694a 100644
> --- a/drivers/gpu/drm/display/drm_bridge_connector.c
> +++ b/drivers/gpu/drm/display/drm_bridge_connector.c
> @@ -825,6 +825,7 @@ struct drm_connector *drm_bridge_connector_init(struct drm_device *drm,
>  	struct drm_bridge *panel_bridge __free(drm_bridge_put) = NULL;
>  	unsigned int supported_formats = BIT(DRM_OUTPUT_COLOR_FORMAT_RGB444);
>  	unsigned int max_bpc = 8;
> +	unsigned int chain_max_bpc = 0;
>  	bool support_hdcp = false;
>  	int connector_type;
>  	int ret;
> @@ -985,6 +986,10 @@ struct drm_connector *drm_bridge_connector_init(struct drm_device *drm,
>  			panel_bridge = drm_bridge_get(bridge);
>  		}
>  
> +		if (bridge->max_bpc)
> +			chain_max_bpc = chain_max_bpc ?
> +				min(chain_max_bpc, bridge->max_bpc) : bridge->max_bpc;
> +

The check on chain_max_bpc here doesn't seem to make any sense.


>  		if (bridge->support_hdcp)
>  			support_hdcp = true;
>  	}
> @@ -1026,6 +1031,19 @@ struct drm_connector *drm_bridge_connector_init(struct drm_device *drm,
>  					  connector_type, ddc);
>  		if (ret)
>  			return ERR_PTR(ret);
> +
> +		if (chain_max_bpc > 8) {
> +			unsigned int min_bpc = 8;
> +
> +			if (connector_type == DRM_MODE_CONNECTOR_DisplayPort ||
> +			    connector_type == DRM_MODE_CONNECTOR_eDP)
> +				min_bpc = 6;
> +
> +			ret = drm_connector_attach_max_bpc_property(connector, min_bpc,
> +								    chain_max_bpc);
> +			if (ret)
> +				return ERR_PTR(ret);
> +		}
>  	}
>  
>  	if (bridge_connector->bridge_hdmi_audio ||
> diff --git a/include/drm/drm_bridge.h b/include/drm/drm_bridge.h
> index 58fff047f43b..01ad1878149e 100644
> --- a/include/drm/drm_bridge.h
> +++ b/include/drm/drm_bridge.h
> @@ -1089,9 +1089,14 @@ struct drm_bridge {
>  	unsigned int supported_formats;
>  
>  	/**
> -	 * @max_bpc: Maximum bits per char the HDMI bridge supports. Allowed
> -	 * values are 8, 10 and 12. This is only relevant if
> -	 * @DRM_BRIDGE_OP_HDMI is set.
> +	 * @max_bpc: Maximum bits per color component the bridge supports.
> +	 * Zero means that the bridge does not report a limit. For HDMI bridges
> +	 * with @DRM_BRIDGE_OP_HDMI set, allowed values are 8, 10 and 12.
> +	 *
> +	 * For non-HDMI connectors, drm_bridge_connector_init() attaches the
> +	 * max bpc property when the lowest nonzero limit in the bridge chain
> +	 * exceeds 8. Bridges remain responsible for validating configurations
> +	 * against any limits not reported here.
>  	 */
>  	unsigned int max_bpc;
>  
> 

-- 
Best, 
Chaoyi