[POWERPC] Add of_get_next_parent()

Iterating through a device node's parents is simple enough, but dealing
with the refcounts properly is a little ugly, and replicating that logic
is asking for someone to get it wrong or forget it all together, eg:

while (dn != NULL) {
	/* loop body */
	tmp = of_get_parent(dn);
	of_node_put(dn);
	dn = tmp;
}

So add of_get_next_parent(), inspired by of_get_next_child().  The
contract is that it returns the parent and drops the reference on the
current node, this makes the loop look like:

while (dn != NULL) {
	/* loop body */
	dn = of_get_next_parent(dn);
}

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Paul Mackerras <paulus@samba.org>

drivers/of/base.c

@@ -137,6 +137,31 @@ struct device_node *of_get_parent(const struct device_node *node)
 }
 EXPORT_SYMBOL(of_get_parent);
 
+/**
+ *	of_get_next_parent - Iterate to a node's parent
+ *	@node:	Node to get parent of
+ *
+ * 	This is like of_get_parent() except that it drops the
+ * 	refcount on the passed node, making it suitable for iterating
+ * 	through a node's parents.
+ *
+ *	Returns a node pointer with refcount incremented, use
+ *	of_node_put() on it when done.
+ */
+struct device_node *of_get_next_parent(struct device_node *node)
+{
+	struct device_node *parent;
+
+	if (!node)
+		return NULL;
+
+	read_lock(&devtree_lock);
+	parent = of_node_get(node->parent);
+	of_node_put(node);
+	read_unlock(&devtree_lock);
+	return parent;
+}
+
 /**
  *	of_get_next_child - Iterate a node childs
  *	@node:	parent node

include/linux/of.h

@@ -50,6 +50,7 @@ extern struct device_node *of_find_matching_node(struct device_node *from,
 extern struct device_node *of_find_node_by_path(const char *path);
 extern struct device_node *of_find_node_by_phandle(phandle handle);
 extern struct device_node *of_get_parent(const struct device_node *node);
+extern struct device_node *of_get_next_parent(struct device_node *node);
 extern struct device_node *of_get_next_child(const struct device_node *node,
 					     struct device_node *prev);
 #define for_each_child_of_node(parent, child) \