[PATCH] USB Gadget string utility (5/6)

This adds utility code that gadget drivers can use to manage
string descriptors (drivers/usb/gadget/usbstring.c) in the
common case that the ISO-8859/1 character set is in use.

Both "Gadget Zero" and the Ethernet gadget code use this.

drivers/usb/gadget/usbstring.c

+/*
+ * Copyright (C) 2003 David Brownell
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU Lesser General Public License as published
+ * by the Free Software Foundation; either version 2.1 of the License, or
+ * (at your option) any later version.
+ */
+
+#include <linux/errno.h>
+#include <linux/kernel.h>
+#include <linux/list.h>
+#include <linux/string.h>
+#include <linux/version.h>
+
+#include <linux/usb_ch9.h>
+#include <linux/usb_gadget.h>
+
+
+/**
+ * usb_gadget_get_string - fill out a string descriptor 
+ * @table: of c strings using iso latin/1 characters
+ * @id: string id, from low byte of wValue in get string descriptor
+ * @buf: at least 256 bytes
+ *
+ * Finds the iso latin/1 string matching the ID, and converts it into a
+ * string descriptor in utf16-le.
+ * Returns length of descriptor (always even) or negative errno
+ *
+ * If your driver needs stings in multiple languages, you'll need to
+ * to use some alternate solution for languages where the ISO 8859/1
+ * (latin/1) character set can't be used.  For example, they can't be
+ * used with Chinese (Big5, GB2312, etc), Japanese, Korean, or many other
+ * languages.  You'd likely "switch (wIndex) { ... }"  in your ep0
+ * string descriptor logic, using this routine in cases where "western
+ * european" characters suffice for the strings being returned.
+ */
+int
+usb_gadget_get_string (struct usb_gadget_strings *table, int id, u8 *buf)
+{
+	struct usb_string	*s;
+	int			len;
+
+	/* descriptor 0 has the language id */
+	if (id == 0) {
+		buf [0] = 4;
+		buf [1] = USB_DT_STRING;
+		buf [2] = (u8) table->language;
+		buf [3] = (u8) (table->language >> 8);
+		return 4;
+	}
+	for (s = table->strings; s && s->s; s++)
+		if (s->id == id)
+			break;
+
+	/* unrecognized: stall. */
+	if (!s || !s->s)
+		return -EINVAL;
+
+	/* string descriptors have length, tag, then UTF16-LE text */
+	len = min ((size_t) 126, strlen (s->s));
+	buf [0] = (len + 1) * 2;
+	buf [1] = USB_DT_STRING;
+	memset (buf + 2, 0, 2 * len);	/* zero all the high bytes */
+	while (len) {
+		buf [2 * len] = s->s [len - 1];
+		len--;
+	}
+	return buf [0];
+}
+