fdt: Add functions to retrieve strings

Given a device tree node, a property name and an index, the new function
fdt_get_string_index() will return in an output argument a pointer to
the index'th string in the property's value.

The fdt_get_string() is a shortcut for the above with the index being 0.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Acked-by: Simon Glass <sjg@chromium.org>

include/libfdt.h

@@ -877,6 +877,33 @@ int fdt_count_strings(const void *fdt, int node, const char *property);
 int fdt_find_string(const void *fdt, int node, const char *property,
 		    const char *string);
 
+/**
+ * fdt_get_string_index() - obtain the string at a given index in a string list
+ * @fdt: pointer to the device tree blob
+ * @node: offset of the node
+ * @property: name of the property containing the string list
+ * @index: index of the string to return
+ * @output: return location for the string
+ * @return: 0 if the string was found or a negative error code otherwise
+ */
+int fdt_get_string_index(const void *fdt, int node, const char *property,
+			 int index, const char **output);
+
+/**
+ * fdt_get_string() - obtain the string at a given index in a string list
+ * @fdt: pointer to the device tree blob
+ * @node: offset of the node
+ * @property: name of the property containing the string list
+ * @output: return location for the string
+ * @return: 0 if the string was found or a negative error code otherwise
+ *
+ * This is a shortcut for:
+ *
+ *	fdt_get_string_index(fdt, node, property, 0, output).
+ */
+int fdt_get_string(const void *fdt, int node, const char *property,
+		   const char **output);
+
 /**********************************************************************/
 /* Read-only functions (addressing related)                           */
 /**********************************************************************/

lib/libfdt/fdt_ro.c

@@ -537,6 +537,36 @@ int fdt_find_string(const void *fdt, int node, const char *property,
 	return -FDT_ERR_NOTFOUND;
 }
 
+int fdt_get_string_index(const void *fdt, int node, const char *property,
+			 int index, const char **output)
+{
+	const char *list;
+	int length, i;
+
+	list = fdt_getprop(fdt, node, property, &length);
+
+	for (i = 0; i < length; i++) {
+		int len = strlen(list);
+
+		if (index == 0) {
+			*output = list;
+			return 0;
+		}
+
+		list += len + 1;
+		i += len;
+		index--;
+	}
+
+	return FDT_ERR_NOTFOUND;
+}
+
+int fdt_get_string(const void *fdt, int node, const char *property,
+		   const char **output)
+{
+	return fdt_get_string_index(fdt, node, property, 0, output);
+}
+
 int fdt_node_check_compatible(const void *fdt, int nodeoffset,
 			      const char *compatible)
 {