Re: [PATCH v7 32/35] drm/msm/dp: mark the SST connector disconnected when MST is enabled

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] drm/msm: AB-BA deadlock between mode_config.mutex and plugged_lock
--

commit 44822203f1a87c392f32673917903210fbaa2c96
Author: Yongxing Mou <yongxing.mou@oss.qualcomm.com>

drm/msm/dp: mark the SST connector disconnected when MST is enabled

This patch updates the SST connector state to disconnected immediately
after MST is initialized. This prevents the polling logic from observing
a transient connected-to-disconnected transition and generating an
unnecessary hotplug event.

> diff --git a/drivers/gpu/drm/msm/dp/dp_display.c b/drivers/gpu/drm/msm/dp/dp_display.c
> index 0e8181932c27d..327777f5c95e6 100644
> --- a/drivers/gpu/drm/msm/dp/dp_display.c
> +++ b/drivers/gpu/drm/msm/dp/dp_display.c
> @@ -290,6 +290,10 @@ static void msm_dp_display_mst_enable(struct msm_dp_display_private *dp)
>  	}
>  
>  	msm_dp->mst_active = true;
> +
> +	scoped_guard(mutex, &msm_dp->connector->dev->mode_config.mutex)
> +		msm_dp->connector->status = connector_status_disconnected;

[Severity: High]
Does this introduce an AB-BA deadlock between mode_config.mutex and
plugged_lock?

When a DP MST monitor is hotplugged, the hotplug handler acquires
plugged_lock first and eventually calls down to msm_dp_display_mst_enable(),
which acquires mode_config.mutex here:

msm_dp_hpd_plug_handle()
  guard(mutex)(&dp->plugged_lock);
  ...
  msm_dp_display_process_hpd_high(dp);
    msm_dp_display_mst_enable()
      scoped_guard(mutex, &msm_dp->connector->dev->mode_config.mutex)

However, concurrently, the DRM subsystem probes display modes and holds
mode_config.mutex prior to calling the connector's detect callback. The
detect callback then attempts to acquire plugged_lock:

drm_helper_probe_single_connector_modes()
  ... (holds mode_config.mutex)
  msm_dp_bridge_detect()
    guard(mutex)(&priv->plugged_lock);

Can this lock inversion deadlock the system when DP MST monitors are
plugged in?

> +
>  	msm_dp_mst_display_set_mgr_state(msm_dp, true);
>  }
>

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260929-msm-dp-mst-v7-0-4b81473185e7@oss.qualcomm.com?part=32