drm/atomic-helper: Reject attempts at re-stealing encoders

This can happen when we run out of encoders for a multi-crtc modeset,
or also when userspace is silly and tries to clone multiple connectors
that need the same encoder on the same crtc.

Reported-and-Tested-and-Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1449136154-11588-1-git-send-email-daniel.vetter@ffwll.ch

drivers/gpu/drm/drm_atomic_helper.c

@@ -80,6 +80,27 @@ drm_atomic_helper_plane_changed(struct drm_atomic_state *state,
 	}
 }
 
+static bool
+check_pending_encoder_assignment(struct drm_atomic_state *state,
+				 struct drm_encoder *new_encoder,
+				 struct drm_connector *new_connector)
+{
+	struct drm_connector *connector;
+	struct drm_connector_state *conn_state;
+	int i;
+
+	for_each_connector_in_state(state, connector, conn_state, i) {
+		if (conn_state->best_encoder != new_encoder)
+			continue;
+
+		/* encoder already assigned and we're trying to re-steal it! */
+		if (connector->state->best_encoder != conn_state->best_encoder)
+			return false;
+	}
+
+	return true;
+}
+
 static struct drm_crtc *
 get_current_crtc_for_encoder(struct drm_device *dev,
 			     struct drm_encoder *encoder)
@@ -229,6 +250,13 @@ update_connector_routing(struct drm_atomic_state *state, int conn_idx)
 		return 0;
 	}
 
+	if (!check_pending_encoder_assignment(state, new_encoder, connector)) {
+		DRM_DEBUG_ATOMIC("Encoder for [CONNECTOR:%d:%s] already assigned\n",
+				 connector->base.id,
+				 connector->name);
+		return -EINVAL;
+	}
+
 	encoder_crtc = get_current_crtc_for_encoder(state->dev,
 						    new_encoder);