Re: [PATCH v6 29/29] 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] Acquiring `mode_config.mutex` in `msm_dp_display_mst_enable()` introduces an AB-BA lock inversion with `plugged_lock`, leading to a hard deadlock.
--

commit 48ec7f7903ee88df5ef42da3519327d574e9dc29
Author: Yongxing Mou <yongxing.mou@oss.qualcomm.com>

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

This commit sets the SST connector state to disconnected immediately after
MST is initialized. This is intended to prevent the detect path from
introducing a transient connected to disconnected state change that could
result in 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 d2b2c7141db0f..d207ec3ca8bf2 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 acquiring mode_config.mutex here in msm_dp_display_mst_enable() introduce
an AB-BA lock inversion with plugged_lock?

During a userspace probe (like xrandr), the DRM core holds mode_config.mutex
and calls down to the bridge detect function in
drivers/gpu/drm/msm/dp/dp_display.c, which establishes this lock ordering:

Thread 1 (probe):
drm_helper_probe_single_connector_modes() (acquires mode_config.mutex)
  -> drm_bridge_connector_detect()
    -> msm_dp_bridge_detect() (acquires plugged_lock)

However, on a hotplug event, the IRQ thread handles the plug event and calls
into the driver with plugged_lock held, before calling this function:

Thread 2 (hotplug):
drm_bridge_connector_handle_hpd() (unlocks mode_config.mutex)
  -> drm_bridge_connector_hpd_notify()
    -> msm_dp_bridge_hpd_notify()
      -> msm_dp_hpd_plug_handle() (acquires plugged_lock)
        -> msm_dp_display_mst_enable() (acquires mode_config.mutex here)

If a DP cable is physically plugged in while a background process or
desktop environment concurrently probes display connectors, can these threads
deadlock?

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

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260831-msm-dp-mst-v6-0-c91d35d6fb9e@oss.qualcomm.com?part=29