usb: gadget: add "usb_validate_langid" function

The USB LANGID validation code in "check_user_usb_string" function is
moved to "usb_validate_langid" function which can be used by other usb
gadget drivers.
Signed-off-by: Tao Ren <rentao.bupt@gmail.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>

drivers/usb/gadget/configfs.c

@@ -13,8 +13,6 @@
 int check_user_usb_string(const char *name,
 		struct usb_gadget_strings *stringtab_dev)
 {
-	unsigned primary_lang;
-	unsigned sub_lang;
 	u16 num;
 	int ret;
 
@@ -22,17 +20,7 @@ int check_user_usb_string(const char *name,
 	if (ret)
 		return ret;
 
-	primary_lang = num & 0x3ff;
-	sub_lang = num >> 10;
-
-	/* simple sanity check for valid langid */
-	switch (primary_lang) {
-	case 0:
-	case 0x62 ... 0xfe:
-	case 0x100 ... 0x3ff:
-		return -EINVAL;
-	}
-	if (!sub_lang)
+	if (!usb_validate_langid(num))
 		return -EINVAL;
 
 	stringtab_dev->language = num;

drivers/usb/gadget/usbstring.c

@@ -65,3 +65,27 @@ usb_gadget_get_string (const struct usb_gadget_strings *table, int id, u8 *buf)
 	return buf [0];
 }
 EXPORT_SYMBOL_GPL(usb_gadget_get_string);
+
+/**
+ * usb_validate_langid - validate usb language identifiers
+ * @lang: usb language identifier
+ *
+ * Returns true for valid language identifier, otherwise false.
+ */
+bool usb_validate_langid(u16 langid)
+{
+	u16 primary_lang = langid & 0x3ff;	/* bit [9:0] */
+	u16 sub_lang = langid >> 10;		/* bit [15:10] */
+
+	switch (primary_lang) {
+	case 0:
+	case 0x62 ... 0xfe:
+	case 0x100 ... 0x3ff:
+		return false;
+	}
+	if (!sub_lang)
+		return false;
+
+	return true;
+}
+EXPORT_SYMBOL_GPL(usb_validate_langid);

include/linux/usb/gadget.h

@@ -773,6 +773,9 @@ struct usb_gadget_string_container {
 /* put descriptor for string with that id into buf (buflen >= 256) */
 int usb_gadget_get_string(const struct usb_gadget_strings *table, int id, u8 *buf);
 
+/* check if the given language identifier is valid */
+bool usb_validate_langid(u16 langid);
+
 /*-------------------------------------------------------------------------*/
 
 /* utility to simplify managing config descriptors */