Re: [PATCH v8 25/29] drm/msm/dp: mark the SST bridge disconnected when an MST-capable sink is present

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential
issue to consider:
- [High] drm/msm/dp: msm_dp_bridge_detect() sets plugged and blocks boot
  MST enable
--

commit 6523f8221c3f008ff483a89b251f28757e2c5462
Author: Abhinav Kumar <quic_abhinavk@quicinc.com>

drm/msm/dp: mark the SST bridge disconnected when an MST-capable sink is present

This commit returns connector_status_disconnected from
msm_dp_bridge_detect() when the controller supports multi-stream and the
connected sink reports MST capability.

> diff --git a/drivers/gpu/drm/msm/dp/dp_display.c b/drivers/gpu/drm/msm/dp/dp_display.c
> index eeb0e04949000..8ea3d258a98f3 100644
> --- a/drivers/gpu/drm/msm/dp/dp_display.c
> +++ b/drivers/gpu/drm/msm/dp/dp_display.c
> @@ -1063,6 +1063,11 @@ enum drm_connector_status msm_dp_bridge_detect(struct drm_bridge *bridge,
>  			status = connector_status_disconnected;
>  	}
>  
> +	/* Mark SST connector as disconnected in DP boot-up case */
> +	if (msm_dp_ctrl_get_stream_cnt(priv->ctrl) > 1 &&
> +	    drm_dp_read_mst_cap(priv->aux, dpcd) == DRM_DP_MST)
> +		status = connector_status_disconnected;

[Severity: High]
Does returning connector_status_disconnected here after setting
priv->plugged = true prevent msm_dp_hpd_plug_handle() from enabling MST on
boot?

When msm_dp_bridge_detect() runs before msm_dp_hpd_plug_handle() at boot,
it sets priv->plugged = true earlier in the function and then overrides
status to connector_status_disconnected without enabling MST.  When the
HPD plug notification subsequently runs, msm_dp_hpd_plug_handle() sees
dp->plugged is already true and returns early:

drivers/gpu/drm/msm/dp/dp_display.c:msm_dp_hpd_plug_handle() {
	...
	guard(mutex)(&dp->plugged_lock);

	if (dp->plugged)
		return 0;
	...
	ret = msm_dp_display_process_hpd_high(dp);
	...
}

As a result, msm_dp_display_process_hpd_high() and
msm_dp_display_mst_enable() are never called, leaving the SST connector
disconnected and the MST topology manager inactive until the cable is
replugged.  In addition, if msm_dp_display_mst_enable() fails during
msm_dp_display_process_hpd_high(), unconditionally returning
connector_status_disconnected here prevents falling back to SST.

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261110-msm-dp-mst-v8-0-d040fb33db5f@oss.qualcomm.com?part=25