drm/i915/dp_mst: Fix MST state after a sink reset

In some cases the sink can reset itself after it was configured into MST
mode, without the driver noticing the disconnected state. For instance
the reset may happen in the middle of a modeset, or the (long) HPD pulse
generated may be not long enough for the encoder detect handler to
observe the HPD's deasserted state. In this case the sink's DPCD
register programmed to enable MST will be reset, while the driver still
assumes MST is still enabled. Detect this condition, which will tear
down and recreate/re-enable the MST topology.

v2:
- Add a code comment about adjusting the expected DP_MSTM_CTRL register
  value for SST + SideBand. (Suraj, Jani)
- Print a debug message about detecting the link reset. (Jani)
- Verify the DPCD MST state only if it wasn't already determined that
  the sink is disconnected.

Cc: stable@vger.kernel.org
Cc: Jani Nikula <jani.nikula@intel.com>
Closes: https://gitlab.freedesktop.org/drm/i915/kernel/-/issues/11195
Reviewed-by: Suraj Kandpal <suraj.kandpal@intel.com> (v1)
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240823162918.1211875-1-imre.deak@intel.com

1 files changed, 12 insertions, 0 deletions

diff --git a/drivers/gpu/drm/i915/display/intel_dp.c b/drivers/gpu/drm/i915/display/intel_dp.c
index 977f149551f6..bceccaa40f32 100644
--- a/drivers/gpu/drm/i915/display/intel_dp.c
+++ b/drivers/gpu/drm/i915/display/intel_dp.c
@@ -5999,6 +5999,18 @@ intel_dp_detect(struct drm_connector *connector,
 	else
 		status = connector_status_disconnected;
 
+	if (status != connector_status_disconnected &&
+	    !intel_dp_mst_verify_dpcd_state(intel_dp))
+		/*
+		 * This requires retrying detection for instance to re-enable
+		 * the MST mode that got reset via a long HPD pulse. The retry
+		 * will happen either via the hotplug handler's retry logic,
+		 * ensured by setting the connector here to SST/disconnected,
+		 * or via a userspace connector probing in response to the
+		 * hotplug uevent sent when removing the MST connectors.
+		 */
+		status = connector_status_disconnected;
+
 	if (status == connector_status_disconnected) {
 		memset(&intel_dp->compliance, 0, sizeof(intel_dp->compliance));
 		memset(intel_connector->dp.dsc_dpcd, 0, sizeof(intel_connector->dp.dsc_dpcd));