drm/atomic-helper: relax unregistered connector check

The driver might pull connectors which weren't submitted by
user-space into the atomic state. For instance,
intel_dp_mst_atomic_master_trans_check() pulls in connectors
sharing the same DP-MST stream. However, if the connector is
unregistered, this later fails with:

    [  559.425658] i915 0000:00:02.0: [drm:drm_atomic_helper_check_modeset] [CONNECTOR:378:DP-7] is not registered

Skip the unregistered connector check to allow user-space to turn
off connectors one-by-one.

See this wlroots issue:
https://gitlab.freedesktop.org/wlroots/wlroots/-/issues/3407

Previous discussion:
https://lore.kernel.org/intel-gfx/Y6GX7z17WmDSKwta@ideak-desk.fi.intel.com/Signed-off-by: Simon Ser <contact@emersion.fr>
Cc: stable@vger.kernel.org
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231005131623.114379-1-contact@emersion.fr

drivers/gpu/drm/drm_atomic_helper.c

@@ -290,7 +290,8 @@ static int
 update_connector_routing(struct drm_atomic_state *state,
 			 struct drm_connector *connector,
 			 struct drm_connector_state *old_connector_state,
-			 struct drm_connector_state *new_connector_state)
+			 struct drm_connector_state *new_connector_state,
+			 bool added_by_user)
 {
 	const struct drm_connector_helper_funcs *funcs;
 	struct drm_encoder *new_encoder;
@@ -339,9 +340,13 @@ update_connector_routing(struct drm_atomic_state *state,
 	 * there's a chance the connector may have been destroyed during the
 	 * process, but it's better to ignore that then cause
 	 * drm_atomic_helper_resume() to fail.
+	 *
+	 * Last, we want to ignore connector registration when the connector
+	 * was not pulled in the atomic state by user-space (ie, was pulled
+	 * in by the driver, e.g. when updating a DP-MST stream).
 	 */
 	if (!state->duplicated && drm_connector_is_unregistered(connector) &&
-	    crtc_state->active) {
+	    added_by_user && crtc_state->active) {
 		drm_dbg_atomic(connector->dev,
 			       "[CONNECTOR:%d:%s] is not registered\n",
 			       connector->base.id, connector->name);
@@ -620,7 +625,10 @@ drm_atomic_helper_check_modeset(struct drm_device *dev,
 	struct drm_connector *connector;
 	struct drm_connector_state *old_connector_state, *new_connector_state;
 	int i, ret;
-	unsigned int connectors_mask = 0;
+	unsigned int connectors_mask = 0, user_connectors_mask = 0;
+
+	for_each_oldnew_connector_in_state(state, connector, old_connector_state, new_connector_state, i)
+		user_connectors_mask |= BIT(i);
 
 	for_each_oldnew_crtc_in_state(state, crtc, old_crtc_state, new_crtc_state, i) {
 		bool has_connectors =
@@ -685,7 +693,8 @@ drm_atomic_helper_check_modeset(struct drm_device *dev,
 		 */
 		ret = update_connector_routing(state, connector,
 					       old_connector_state,
-					       new_connector_state);
+					       new_connector_state,
+					       BIT(i) & user_connectors_mask);
 		if (ret)
 			return ret;
 		if (old_connector_state->crtc) {