drm/display/dp_mst: Add helpers for serializing SST <-> MST transitions

There's another kind of situation where we could potentially race with
nonblocking modesets and MST, especially if we were to only use the locking
provided by atomic modesetting:

* Display 1 begins as enabled on DP-1 in SST mode
* Display 1 switches to MST mode, exposes one sink in MST mode
* Userspace does non-blocking modeset to disable the SST display
* Userspace does non-blocking modeset to enable the MST display with a
  different CRTC, but the SST display hasn't been fully taken down yet
* Execution order between the last two commits isn't guaranteed since they
  share no drm resources

We can fix this however, by ensuring that we always pull in the atomic
topology state whenever a connector capable of driving an MST display
performs its atomic check - and then tracking CRTC commits happening on the
SST connector in the MST topology state. So, let's add some simple helpers
for doing that and hook them up in various drivers.

v2:
* Use intel_dp_mst_source_support() to check for MST support in i915, fixes
  CI failures

Signed-off-by: Lyude Paul <lyude@redhat.com>
Cc: Wayne Lin <Wayne.Lin@amd.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Fangzhi Zuo <Jerry.Zuo@amd.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Imre Deak <imre.deak@intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Sean Paul <sean@poorly.run>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220817193847.557945-14-lyude@redhat.com

1 files changed, 59 insertions, 0 deletions

diff --git a/drivers/gpu/drm/display/drm_dp_mst_topology.c b/drivers/gpu/drm/display/drm_dp_mst_topology.c
index 2f7c43f88d74..97e8f8a83ed4 100644
--- a/drivers/gpu/drm/display/drm_dp_mst_topology.c
+++ b/drivers/gpu/drm/display/drm_dp_mst_topology.c
@@ -4598,6 +4598,65 @@ void drm_dp_mst_atomic_wait_for_dependencies(struct drm_atomic_state *state)
 EXPORT_SYMBOL(drm_dp_mst_atomic_wait_for_dependencies);
 
 /**
+ * drm_dp_mst_root_conn_atomic_check() - Serialize CRTC commits on MST-capable connectors operating
+ * in SST mode
+ * @new_conn_state: The new connector state of the &drm_connector
+ * @mgr: The MST topology manager for the &drm_connector
+ *
+ * Since MST uses fake &drm_encoder structs, the generic atomic modesetting code isn't able to
+ * serialize non-blocking commits happening on the real DP connector of an MST topology switching
+ * into/away from MST mode - as the CRTC on the real DP connector and the CRTCs on the connector's
+ * MST topology will never share the same &drm_encoder.
+ *
+ * This function takes care of this serialization issue, by checking a root MST connector's atomic
+ * state to determine if it is about to have a modeset - and then pulling in the MST topology state
+ * if so, along with adding any relevant CRTCs to &drm_dp_mst_topology_state.pending_crtc_mask.
+ *
+ * Drivers implementing MST must call this function from the
+ * &drm_connector_helper_funcs.atomic_check hook of any physical DP &drm_connector capable of
+ * driving MST sinks.
+ *
+ * Returns:
+ * 0 on success, negative error code otherwise
+ */
+int drm_dp_mst_root_conn_atomic_check(struct drm_connector_state *new_conn_state,
+				      struct drm_dp_mst_topology_mgr *mgr)
+{
+	struct drm_atomic_state *state = new_conn_state->state;
+	struct drm_connector_state *old_conn_state =
+		drm_atomic_get_old_connector_state(state, new_conn_state->connector);
+	struct drm_crtc_state *crtc_state;
+	struct drm_dp_mst_topology_state *mst_state = NULL;
+
+	if (new_conn_state->crtc) {
+		crtc_state = drm_atomic_get_new_crtc_state(state, new_conn_state->crtc);
+		if (crtc_state && drm_atomic_crtc_needs_modeset(crtc_state)) {
+			mst_state = drm_atomic_get_mst_topology_state(state, mgr);
+			if (IS_ERR(mst_state))
+				return PTR_ERR(mst_state);
+
+			mst_state->pending_crtc_mask |= drm_crtc_mask(new_conn_state->crtc);
+		}
+	}
+
+	if (old_conn_state->crtc) {
+		crtc_state = drm_atomic_get_new_crtc_state(state, old_conn_state->crtc);
+		if (crtc_state && drm_atomic_crtc_needs_modeset(crtc_state)) {
+			if (!mst_state) {
+				mst_state = drm_atomic_get_mst_topology_state(state, mgr);
+				if (IS_ERR(mst_state))
+					return PTR_ERR(mst_state);
+			}
+
+			mst_state->pending_crtc_mask |= drm_crtc_mask(old_conn_state->crtc);
+		}
+	}
+
+	return 0;
+}
+EXPORT_SYMBOL(drm_dp_mst_root_conn_atomic_check);
+
+/**
  * drm_dp_mst_update_slots() - updates the slot info depending on the DP ecoding format
  * @mst_state: mst_state to update
  * @link_encoding_cap: the ecoding format on the link