Re: [PATCH] drm/dp/mst: skip connector creation for unplugged downstream ports

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] drm/dp/mst: missing connector creation breaks hotplug for empty downstream ports
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commit b7642d2c459f1deba3af19a444979a2c15cbf1e0
Author: Xiao Lu <xiaolu.xie@intel.com>

drm/dp/mst: skip connector creation for unplugged downstream ports

This patch modifies the MST link enumeration path to skip creating DRM
connectors for downstream ports that do not have a device connected.
It aims to reduce error log noise and spurious sideband messages from
DPCD reads on empty ports.

> diff --git a/drivers/gpu/drm/display/drm_dp_mst_topology.c b/drivers/gpu/drm/display/drm_dp_mst_topology.c
> --- a/drivers/gpu/drm/display/drm_dp_mst_topology.c
> +++ b/drivers/gpu/drm/display/drm_dp_mst_topology.c
> @@ -2448,7 +2448,7 @@ drm_dp_mst_handle_link_address_port(struct drm_dp_mst_branch *mstb,
>  
>  	if (port->connector)
>  		drm_modeset_unlock(&mgr->base.lock);
> -	else if (!port->input)
> +	else if (!port->input && port->ddps)
>  		drm_dp_mst_port_add_connector(mstb, port);

[Severity: High]
If we skip connector creation here for empty ports, will the hotplug path
actually create the connector when a monitor is plugged in?

Looking at drm_dp_mst_handle_conn_stat(), create_connector is only set to
true when transitioning from an input port to an output port:

    } else if (port->input && !conn_stat->input_port) {
        create_connector = true;

For standard downstream ports, port->input is 0, so create_connector
remains false during a hotplug event.

Later in drm_dp_mst_handle_conn_stat(), it checks:

    else if (create_connector)
        drm_dp_mst_port_add_connector(mstb, port);

Since create_connector evaluated to false, the dynamic connector creation
is bypassed.

Also, drm_dp_port_set_pdt() returns 0 for regular SST monitors:

        if (drm_dp_mst_is_end_device(port->pdt, port->mcs)) {
            /* add i2c over sideband */
            ret = drm_dp_mst_register_i2c_bus(port);

This prevents the caller from setting dowork = true and triggering a
fallback link address reprobe.

Does this mean a display plugged into an MST hub port that was empty at
boot will remain permanently dark because its DRM connector is never
created?

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260917080126.1729898-1-xiaolu.xie@intel.com?part=1