drm/sun4i: backend: Simplify the get_id logic

Using the new helpers introduced since we wrote that code, we can simplify
the code to retrieve the backend ID significantly.

The new code will also allow us to deal nicely with endpoints that don't
have a reg property, as expected in the case where there's a single
endpoint for a given port.
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Link: https://patchwork.freedesktop.org/patch/msgid/1a9bf911b0a40475da8025859032514131d5397b.1552594551.git-series.maxime.ripard@bootlin.com

drivers/gpu/drm/sun4i/sun4i_backend.c

@@ -720,33 +720,22 @@ static int sun4i_backend_free_sat(struct device *dev) {
  */
 static int sun4i_backend_of_get_id(struct device_node *node)
 {
-	struct device_node *port, *ep;
-	int ret = -EINVAL;
+	struct device_node *ep, *remote;
+	struct of_endpoint of_ep;
 
-	/* input is port 0 */
-	port = of_graph_get_port_by_id(node, 0);
-	if (!port)
+	/* Input port is 0, and we want the first endpoint. */
+	ep = of_graph_get_endpoint_by_regs(node, 0, -1);
+	if (!ep)
 		return -EINVAL;
 
-	/* try finding an upstream endpoint */
-	for_each_available_child_of_node(port, ep) {
-		struct device_node *remote;
-		u32 reg;
-
-		remote = of_graph_get_remote_endpoint(ep);
-		if (!remote)
-			continue;
-
-		ret = of_property_read_u32(remote, "reg", &reg);
-		if (ret)
-			continue;
-
-		ret = reg;
-	}
-
-	of_node_put(port);
+	remote = of_graph_get_remote_endpoint(ep);
+	of_node_put(ep);
+	if (!remote)
+		return -EINVAL;
 
-	return ret;
+	of_graph_parse_endpoint(remote, &of_ep);
+	of_node_put(remote);
+	return of_ep.id;
 }
 
 /* TODO: This needs to take multiple pipelines into account */