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/device.h>
#include <linux/init.h>

#include <linux/usb/ch9.h>
#include <linux/usb_gadget.h>

#include <asm/unaligned.h>


static int utf8_to_utf16le(const char *s, __le16 *cp, unsigned len)
{
	int	count = 0;
	u8	c;
	u16	uchar;

	/* this insists on correct encodings, though not minimal ones.
	 * BUT it currently rejects legit 4-byte UTF-8 code points,
	 * which need surrogate pairs.  (Unicode 3.1 can use them.)
	 */
	while (len != 0 && (c = (u8) *s++) != 0) {
		if (unlikely(c & 0x80)) {
			// 2-byte sequence:
			// 00000yyyyyxxxxxx = 110yyyyy 10xxxxxx
			if ((c & 0xe0) == 0xc0) {
				uchar = (c & 0x1f) << 6;

				c = (u8) *s++;
				if ((c & 0xc0) != 0xc0)
					goto fail;
				c &= 0x3f;
				uchar |= c;

			// 3-byte sequence (most CJKV characters):
			// zzzzyyyyyyxxxxxx = 1110zzzz 10yyyyyy 10xxxxxx
			} else if ((c & 0xf0) == 0xe0) {
				uchar = (c & 0x0f) << 12;

				c = (u8) *s++;
				if ((c & 0xc0) != 0xc0)
					goto fail;
				c &= 0x3f;
				uchar |= c << 6;

				c = (u8) *s++;
				if ((c & 0xc0) != 0xc0)
					goto fail;
				c &= 0x3f;
				uchar |= c;

				/* no bogus surrogates */
				if (0xd800 <= uchar && uchar <= 0xdfff)
					goto fail;

			// 4-byte sequence (surrogate pairs, currently rare):
			// 11101110wwwwzzzzyy + 110111yyyyxxxxxx
			//     = 11110uuu 10uuzzzz 10yyyyyy 10xxxxxx
			// (uuuuu = wwww + 1)
			// FIXME accept the surrogate code points (only)

			} else
				goto fail;
		} else
			uchar = c;
		put_unaligned (cpu_to_le16 (uchar), cp++);
		count++;
		len--;
	}
	return count;
fail:
	return -1;
}


/**
 * usb_gadget_get_string - fill out a string descriptor 
 * @table: of c strings encoded using UTF-8
 * @id: string id, from low byte of wValue in get string descriptor
 * @buf: at least 256 bytes
 *
 * Finds the UTF-8 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 probably
 * "switch (wIndex) { ... }"  in your ep0 string descriptor logic,
 * using this routine after choosing which set of UTF-8 strings to use.
 * Note that US-ASCII is a strict subset of UTF-8; any string bytes with
 * the eighth bit set will be multibyte UTF-8 characters, not ISO-8859/1
 * characters (which are also widely used in C strings).
 */
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));
	memset (buf + 2, 0, 2 * len);	/* zero all the bytes */
	len = utf8_to_utf16le(s->s, (__le16 *)&buf[2], len);
	if (len < 0)
		return -EINVAL;
	buf [0] = (len + 1) * 2;
	buf [1] = USB_DT_STRING;
	return buf [0];
}