of_graph: add of_graph_is_present()

In some cases it's very useful to silently check whether port node exists
at all in a device-tree before proceeding with parsing the graph. The DRM
bridges code is one example of such case where absence of a graph in a
device-tree is a legit condition.

This patch adds of_graph_is_present() which returns true if given
device-tree node contains OF graph port.

Reviewed-by: Rob Herring <[e-mail]>
Signed-off-by: Dmitry Osipenko <[e-mail]>
Reviewed-by: Laurent Pinchart <[e-mail]>
Signed-off-by: Sam Ravnborg <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/of/property.c

···
 #include "of_private.h"
 
 /**
+ * of_graph_is_present() - check graph's presence
+ * @node: pointer to device_node containing graph port
+ *
+ * Return: True if @node has a port or ports (with a port) sub-node,
+ * false otherwise.
+ */
+bool of_graph_is_present(const struct device_node *node)
+{
+	struct device_node *ports, *port;
+
+	ports = of_get_child_by_name(node, "ports");
+	if (ports)
+		node = ports;
+
+	port = of_get_child_by_name(node, "port");
+	of_node_put(ports);
+	of_node_put(port);
+
+	return !!port;
+}
+EXPORT_SYMBOL(of_graph_is_present);
+
+/**
  * of_property_count_elems_of_size - Count the number of elements in a property
  *
  * @np:		device node from which the property value is to be read.

include/linux/of_graph.h

···
 	     child = of_graph_get_next_endpoint(parent, child))
 
 #ifdef CONFIG_OF
+bool of_graph_is_present(const struct device_node *node);
 int of_graph_parse_endpoint(const struct device_node *node,
 				struct of_endpoint *endpoint);
 int of_graph_get_endpoint_count(const struct device_node *np);
···
 struct device_node *of_graph_get_remote_node(const struct device_node *node,
 					     u32 port, u32 endpoint);
 #else
+
+static inline bool of_graph_is_present(const struct device_node *node)
+{
+	return false;
+}
 
 static inline int of_graph_parse_endpoint(const struct device_node *node,
 					struct of_endpoint *endpoint)