Reviewed-by: Lyude Paul <[email protected]>
Will push upstream in just a moment
On Sun, 2026-05-03 at 05:45 +0200, Jonas Emilsson wrote:
> 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]>
> ---
> drivers/gpu/drm/display/drm_dp_mst_topology.c | 4 +++-
> 1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/gpu/drm/display/drm_dp_mst_topology.c
> b/drivers/gpu/drm/display/drm_dp_mst_topology.c
> index 8757972e8..0cb341ce1 100644
> --- a/drivers/gpu/drm/display/drm_dp_mst_topology.c
> +++ b/drivers/gpu/drm/display/drm_dp_mst_topology.c
> @@ -3738,8 +3738,10 @@ void drm_dp_mst_topology_queue_probe(struct
> drm_dp_mst_topology_mgr *mgr)
> {
> 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_mst(mgr->dev, "queue_probe skipped: topology
> torn down\n");
> goto out_unlock;
> + }
>
> drm_dp_mst_topology_mgr_invalidate_mstb(mgr->mst_primary);
