[PATCH v2 3/3] drm/rockchip: dw_dp: Attach "max bpc" connector property

[Chaoyi Chen]

Hello Igor,

On 9/10/2026 1:35 AM, Igor Paunovic wrote:
> The dw-dp bridge already filters candidate output formats on
> conn_state->max_bpc in dw_dp_bridge_atomic_get_output_bus_fmts() and
> validates the result against the link bandwidth, but the connector
> never exposes the "max bpc" property, so userspace cannot limit the
> bit depth. Attach the property with a range of 6 to 10, matching
> dw_dp_output_formats[]: the smallest entry is 6 bpc RGB and the
> deepest entries are 10 bpc.
> 
> drm_connector_attach_max_bpc_property() initializes
> connector->state->max_requested_bpc and therefore requires the
> connector to have a state. Rockchip binds its components before
> drm_mode_config_reset() runs, so the freshly created bridge connector
> does not have one yet - create the connector state before attaching
> the property, exactly like drmm_connector_hdmi_init() does for HDMI
> bridge connectors. The later drm_mode_config_reset() then re-creates
> the connector state, and the preceding patch makes
> drm_bridge_connector_create_state() restore max_requested_bpc from
> connector->max_bpc, so clients that never set the property (fbcon in
> particular) keep the full range instead of ending up with 0.
> 
> With the property attached, compositors such as KWin write "max bpc"
> on every atomic modeset; any in-range value is handled by the existing
> format filter and bandwidth checks.
> 
> Assisted-by: LLM checkpatch
> Signed-off-by: Igor Paunovic <royalnet026 at gmail.com>
> ---
>  drivers/gpu/drm/rockchip/dw_dp-rockchip.c | 26 +++++++++++++++++++++++
>  1 file changed, 26 insertions(+)
> 
> diff --git a/drivers/gpu/drm/rockchip/dw_dp-rockchip.c b/drivers/gpu/drm/rockchip/dw_dp-rockchip.c
> index b23efb153c9e6..8ff27353b1417 100644
> --- a/drivers/gpu/drm/rockchip/dw_dp-rockchip.c
> +++ b/drivers/gpu/drm/rockchip/dw_dp-rockchip.c
> @@ -112,6 +112,32 @@ static int dw_dp_rockchip_bind(struct device *dev, struct device *master, void *
>  				     "Failed to init bridge connector\n");
>  	}
>  
> +	/*
> +	 * The components are bound before drm_mode_config_reset() runs, so
> +	 * the connector has no state yet, but
> +	 * drm_connector_attach_max_bpc_property() requires one. Create the
> +	 * connector state first, like drmm_connector_hdmi_init() does.
> +	 */
> +	if (connector->funcs->atomic_create_state) {
> +		struct drm_connector_state *state;
> +
> +		state = connector->funcs->atomic_create_state(connector);
> +		if (IS_ERR(state)) {
> +			dw_dp_unbind(dp->base);
> +			return PTR_ERR(state);
> +		}
> +
> +		connector->state = state;
> +	} else if (connector->funcs->reset) {
> +		connector->funcs->reset(connector);
> +	}
> +
> +	ret = drm_connector_attach_max_bpc_property(connector, 6, 10);
> +	if (ret) {
> +		dw_dp_unbind(dp->base);
> +		return ret;
> +	}
> +
>  	return 0;
>  }
> 

I don't think this is the correct way to go.

I believe max_bpc should be handled in drm_bridge_connector_init(). 
Take a look at its implementation. HDMI already does max_bpc there, 
so DP should probably do something similar.

-- 
Best, 
Chaoyi