of: add helper to lookup compatible child node

Add of_get_compatible_child() helper that can be used to lookup
compatible child nodes.

Several drivers currently use of_find_compatible_node() to lookup child
nodes while failing to notice that the of_find_ functions search the
entire tree depth-first (from a given start node) and therefore can
match unrelated nodes. The fact that these functions also drop a
reference to the node they start searching from (e.g. the parent node)
is typically also overlooked, something which can lead to use-after-free
bugs.

Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>

2 files changed, 33 insertions, 0 deletions

diff --git a/drivers/of/base.c b/drivers/of/base.c
index 466e3c8582f0..bc420d2aa5f5 100644
--- a/drivers/of/base.c
+++ b/drivers/of/base.c
@@ -720,6 +720,31 @@ struct device_node *of_get_next_available_child(const struct device_node *node,
 EXPORT_SYMBOL(of_get_next_available_child);
 
 /**
+ * of_get_compatible_child - Find compatible child node
+ * @parent:     parent node
+ * @compatible: compatible string
+ *
+ * Lookup child node whose compatible property contains the given compatible
+ * string.
+ *
+ * Returns a node pointer with refcount incremented, use of_node_put() on it
+ * when done; or NULL if not found.
+ */
+struct device_node *of_get_compatible_child(const struct device_node *parent,
+                                const char *compatible)
+{
+        struct device_node *child;
+
+        for_each_child_of_node(parent, child) {
+                if (of_device_is_compatible(child, compatible))
+                        break;
+        }
+
+        return child;
+}
+EXPORT_SYMBOL(of_get_compatible_child);
+
+/**
  *      of_get_child_by_name - Find the child node by name for a given parent
  *      @node:  parent node
  *      @name:  child name to look for.
diff --git a/include/linux/of.h b/include/linux/of.h
index 4d25e4f952d9..b99a1a8c2952 100644
--- a/include/linux/of.h
+++ b/include/linux/of.h
@@ -290,6 +290,8 @@ extern struct device_node *of_get_next_child(const struct device_node *node,
 extern struct device_node *of_get_next_available_child(
         const struct device_node *node, struct device_node *prev);
 
+extern struct device_node *of_get_compatible_child(const struct device_node *parent,
+                                        const char *compatible);
 extern struct device_node *of_get_child_by_name(const struct device_node *node,
                                         const char *name);
 
@@ -632,6 +634,12 @@ static inline bool of_have_populated_dt(void)
         return false;
 }
 
+static inline struct device_node *of_get_compatible_child(const struct device_node *parent,
+                                        const char *compatible)
+{
+        return NULL;
+}
+
 static inline struct device_node *of_get_child_by_name(
                                         const struct device_node *node,
                                         const char *name)