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

[Joonhoe Kim <26rote@gmail.com>]

On Tue, 29 Sep 2026 04:54:29 +0000, sashiko-bot@kernel.org wrote:
> Does this introduce an AB-BA deadlock between mode_config.mutex and
> plugged_lock?

Yes, I hit it on the Lenovo Y700 Gen 5 (TB323FU, Kaanapali) when
booting with an MST monitor on the USB-C port (Linux v7.3 with Dmitry's
"drm: handle IRQ_HPD events correctly" v8 [1] and this series). The
lock order is the same in v7 as posted; nothing in my tree changes it.
The HPD worker holds plugged_lock and waits for mode_config.mutex
(msm_dp_display_mst_enable() is inlined into
msm_dp_display_process_hpd_high()), and the compositor's GETCONNECTOR
holds mode_config.mutex and waits for plugged_lock:

 INFO: task kworker/0:0:9 is blocked on a mutex likely owned by task <compositor>:2439.
 INFO: task <compositor>:2439 is blocked on a mutex likely owned by task kworker/0:0:9.

 Workqueue: events_freezable pmic_glink_altmode_worker
  mutex_lock+0x30/0x70
  msm_dp_display_process_hpd_high+0x23c/0x2c8
  msm_dp_hpd_plug_handle+0xf4/0x208
  msm_dp_bridge_hpd_notify+0x118/0x290
  drm_bridge_connector_hpd_notify+0x7c/0xf0
  drm_bridge_connector_handle_hpd+0x5c/0xc0
  drm_bridge_connector_hpd_cb+0x1c/0x30
  drm_bridge_hpd_notify_extra+0x4c/0x70
  drm_aux_hpd_bridge_notify_extra+0x4c/0x70
  pmic_glink_altmode_worker+0x274/0x520

 <compositor>:
  mutex_lock+0x30/0x70
  msm_dp_bridge_detect+0x54/0x338
  drm_bridge_connector_detect+0x40/0xc8
  drm_helper_probe_single_connector_modes+0x158/0x608
  drm_mode_getconnector+0x19c/0x448

32/35 is there to avoid a transient connected -> disconnected change
seen by polling. With 25/35, drm_bridge_connector_handle_hpd() stores
the result of detect() in connector->status right after hpd_notify()
returns, and detect() returns disconnected once mst_active is set
(29/35), so I wonder whether the write is still needed. If it is, it
could be done without holding plugged_lock.

To get past the deadlock I dropped 32/35 locally. Since then, booting
with the monitor attached has not hung (4 boots), and over 3 plugs of
the monitor with drm.debug=0x4 the SST connector never showed a status
change in the probe helper; it stays disconnected while MST is active.

[1] https://lore.kernel.org/all/20261024-hpd-irq-events-v8-0-6ee949e7a384@oss.qualcomm.com/

Thanks,
Joonhoe Kim