of: property: add of_graph_get_next_port_endpoint()

We already have of_graph_get_next_endpoint(), but it is not
intuitive to use in some case.

(X)	node {
(Y)		ports {
(P0)			port@0 { endpoint { remote-endpoint = ...; };};
(P10)			port@1 { endpoint { remote-endpoint = ...; };
(P11)				 endpoint { remote-endpoint = ...; };};
(P2)			port@2 { endpoint { remote-endpoint = ...; };};
		};
	};

For example, if I want to handle port@1's 2 endpoints (= P10, P11),
I want to use like below

	P10 = of_graph_get_next_endpoint(port1, NULL);
	P11 = of_graph_get_next_endpoint(port1, P10);

But 1st one will be error, because of_graph_get_next_endpoint()
requested 1st parameter is "node" (X) or "ports" (Y), not but "port".
Below works well, but it will get P0

	P0 = of_graph_get_next_endpoint(node,  NULL);
	P0 = of_graph_get_next_endpoint(ports, NULL);

In other words, we can't handle P10/P11 directly via
of_graph_get_next_endpoint().

There is another non intuitive behavior on of_graph_get_next_endpoint().
In case of if I could get P10 pointer for some way, and if I want to
handle port@1 things by loop, I would like use it like below

	/*
	 * "ep" is now P10, and handle port1 things here,
	 * but we don't know how many endpoints port1 have.
	 *
	 * Because "ep" is non NULL now, we can use port1
	 * as of_graph_get_next_endpoint(port1, xxx)
	 */
	do {
		/* do something for port1 specific things here */
	} while (ep = of_graph_get_next_endpoint(port1, ep))

But it also not worked as I expected.
I expect it will be P10 -> P11 -> NULL,
but      it will be P10 -> P11 -> P2,    because
of_graph_get_next_endpoint() will fetch "endpoint" beyond the "port".
It is not useful for generic driver.

To handle endpoint more intuitive, create of_graph_get_next_port_endpoint()

	of_graph_get_next_port_endpoint(port1, NULL); // P10
	of_graph_get_next_port_endpoint(port1, P10);  // P11
	of_graph_get_next_port_endpoint(port1, P11);  // NULL

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Link: https://lore.kernel.org/r/87jzdyb5t5.wl-kuninori.morimoto.gx@renesas.com
Signed-off-by: Rob Herring (Arm) <robh@kernel.org>

drivers/of/property.c

@@ -667,6 +667,33 @@ struct device_node *of_graph_get_next_port(const struct device_node *parent,
 }
 EXPORT_SYMBOL(of_graph_get_next_port);
 
+/**
+ * of_graph_get_next_port_endpoint() - get next endpoint node in port.
+ * If it reached to end of the port, it will return NULL.
+ * @port: pointer to the target port node
+ * @prev: previous endpoint node, or NULL to get first
+ *
+ * Return: An 'endpoint' node pointer with refcount incremented. Refcount
+ * of the passed @prev node is decremented.
+ */
+struct device_node *of_graph_get_next_port_endpoint(const struct device_node *port,
+						    struct device_node *prev)
+{
+	while (1) {
+		prev = of_get_next_child(port, prev);
+		if (!prev)
+			break;
+		if (WARN(!of_node_name_eq(prev, "endpoint"),
+			 "non endpoint node is used (%pOF)", prev))
+			continue;
+
+		break;
+	}
+
+	return prev;
+}
+EXPORT_SYMBOL(of_graph_get_next_port_endpoint);
+
 /**
  * of_graph_get_next_endpoint() - get next endpoint node
  * @parent: pointer to the parent device node

include/linux/of_graph.h

@@ -50,6 +50,18 @@ struct of_endpoint {
 	for (struct device_node *child __free(device_node) = of_graph_get_next_port(parent, NULL);\
 	     child != NULL; child = of_graph_get_next_port(parent, child))
 
+/**
+ * for_each_of_graph_port_endpoint - iterate over every endpoint in a port node
+ * @parent: parent port node
+ * @child: loop variable pointing to the current endpoint node
+ *
+ * When breaking out of the loop, and continue to use the @child, you need to
+ * use return_ptr(@child) or no_free_ptr(@child) not to call __free() for it.
+ */
+#define for_each_of_graph_port_endpoint(parent, child)			\
+	for (struct device_node *child __free(device_node) = of_graph_get_next_port_endpoint(parent, NULL);\
+	     child != NULL; child = of_graph_get_next_port_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,
@@ -61,6 +73,8 @@ struct device_node *of_graph_get_next_endpoint(const struct device_node *parent,
 					struct device_node *previous);
 struct device_node *of_graph_get_next_port(const struct device_node *parent,
 					   struct device_node *port);
+struct device_node *of_graph_get_next_port_endpoint(const struct device_node *port,
+						    struct device_node *prev);
 struct device_node *of_graph_get_endpoint_by_regs(
 		const struct device_node *parent, int port_reg, int reg);
 struct device_node *of_graph_get_remote_endpoint(
@@ -114,6 +128,13 @@ static inline struct device_node *of_graph_get_next_port(
 	return NULL;
 }
 
+static inline struct device_node *of_graph_get_next_port_endpoint(
+					const struct device_node *parent,
+					struct device_node *previous)
+{
+	return NULL;
+}
+
 static inline struct device_node *of_graph_get_endpoint_by_regs(
 		const struct device_node *parent, int port_reg, int reg)
 {