[PATCH 6.12 446/602] drm/dp_mst: Handle torn-down topology gracefully in drm_dp_mst_topology_queue_probe()

6.12-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Luca Coelho <[email protected]>

commit 613059875958e7b217b250ed14c3b189f9488421 upstream.

A hotplug or link-loss event can tear down the MST topology
(setting mgr->mst_state = false and mgr->mst_primary = NULL) concurrently
with a caller invoking drm_dp_mst_topology_queue_probe(). Since the check
is already performed under mgr->lock, the condition is not a programming
error but a valid race -- the topology was valid when the caller decided
to call this function, but was torn down before the lock was acquired.

Replace the drm_WARN_ON() with a graceful early return. This eliminates
spurious kernel warnings and the resulting compositor crashes observed
when connecting/disconnecting DP MST monitors, while keeping the correct
behavior of doing nothing when MST is not active. A drm_dbg_mst() trace
is added so the skipped probe remains observable under MST debug logging.

The existing WARN_ON(mgr->mst_primary) in drm_dp_mst_topology_mgr_set_mst()
already catches the case where the topology is initialized twice, so no
diagnostic coverage is lost.

Fixes: dbaeef363ea5 ("drm/dp_mst: Add a helper to queue a topology probe")
Cc: Imre Deak <[email protected]>
Cc: Lyude Paul <[email protected]>
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Signed-off-by: Jonas Emilsson <[email protected]>
Signed-off-by: Luca Coelho <[email protected]>
Link: https://lore.kernel.org/all/[email protected]
Acked-by: Imre Deak <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Maarten Lankhorst <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
---
 drivers/gpu/drm/display/drm_dp_mst_topology.c |    4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

--- a/drivers/gpu/drm/display/drm_dp_mst_topology.c
+++ b/drivers/gpu/drm/display/drm_dp_mst_topology.c
@@ -3743,8 +3743,10 @@ void drm_dp_mst_topology_queue_probe(str
 {
 	mutex_lock(&mgr->lock);
 
-	if (drm_WARN_ON(mgr->dev, !mgr->mst_state || !mgr->mst_primary))
+	if (!mgr->mst_state || !mgr->mst_primary) {
+		drm_dbg_kms(mgr->dev, "queue_probe skipped: topology torn down\n");
 		goto out_unlock;
+	}
 
 	drm_dp_mst_topology_mgr_invalidate_mstb(mgr->mst_primary);
 	drm_dp_mst_queue_probe_work(mgr);