libfdt: Add function to explicitly expand aliases

Kumar has already added alias expansion to fdt_path_offset().
However, in some circumstances it may be convenient for the user of
libfdt to explicitly get the string expansion of an alias.  This patch
adds a function to do this, fdt_get_alias(), and uses it to implement
fdt_path_offset().

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

include/libfdt.h

@@ -458,6 +458,32 @@ static inline void *fdt_getprop_w(void *fdt, int nodeoffset,
  */
 uint32_t fdt_get_phandle(const void *fdt, int nodeoffset);
 
+/**
+ * fdt_get_namelen - get alias based on substring
+ * @fdt: pointer to the device tree blob
+ * @name: name of the alias th look up
+ * @namelen: number of characters of name to consider
+ *
+ * Identical to fdt_get_alias(), but only examine the first namelen
+ * characters of name for matching the alias name.
+ */
+const char *fdt_get_alias_namelen(const void *fdt,
+				  const char *name, int namelen);
+
+/**
+ * fdt_get_alias - retreive the path referenced by a given alias
+ * @fdt: pointer to the device tree blob
+ * @name: name of the alias th look up
+ *
+ * fdt_get_alias() retrieves the value of a given alias.  That is, the
+ * value of the property named 'name' in the node /aliases.
+ *
+ * returns:
+ *	a pointer to the expansion of the alias named 'name', of it exists
+ *	NULL, if the given alias or the /aliases node does not exist
+ */
+const char *fdt_get_alias(const void *fdt, const char *name);
+
 /**
  * fdt_get_path - determine the full path of a node
  * @fdt: pointer to the device tree blob

libfdt/fdt_ro.c

@@ -145,17 +145,12 @@ int fdt_path_offset(const void *fdt, const char *path)
 
 	/* see if we have an alias */
 	if (*path != '/') {
-		const char *q;
+		const char *q = strchr(path, '/');
-		int aliasoffset = fdt_path_offset(fdt, "/aliases");
 
-		if (aliasoffset < 0)
-			return -FDT_ERR_BADPATH;
-
-		q = strchr(path, '/');
 		if (!q)
 			q = end;
 
-		p = fdt_getprop_namelen(fdt, aliasoffset, path, q - p, NULL);
+		p = fdt_get_alias_namelen(fdt, p, q - p);
 		if (!p)
 			return -FDT_ERR_BADPATH;
 		offset = fdt_path_offset(fdt, p);
@@ -306,6 +301,23 @@ uint32_t fdt_get_phandle(const void *fdt, int nodeoffset)
 	return fdt32_to_cpu(*php);
 }
 
+const char *fdt_get_alias_namelen(const void *fdt,
+				  const char *name, int namelen)
+{
+	int aliasoffset;
+
+	aliasoffset = fdt_path_offset(fdt, "/aliases");
+	if (aliasoffset < 0)
+		return NULL;
+
+	return fdt_getprop_namelen(fdt, aliasoffset, name, namelen, NULL);
+}
+
+const char *fdt_get_alias(const void *fdt, const char *name)
+{
+	return fdt_get_alias_namelen(fdt, name, strlen(name));
+}
+
 int fdt_get_path(const void *fdt, int nodeoffset, char *buf, int buflen)
 {
 	int pdepth = 0, p = 0;