USB: ftdi_sio: fix chars_in_buffer overhead

Use the new generic usb-serial wait_until_sent implementation to wait
for hardware buffers to drain.

This removes the need to check the hardware buffers in chars_in_buffer
and thus removes the overhead introduced by commit 6f602912 ("usb:
serial: ftdi_sio: Add missing chars_in_buffer function") without
breaking tty_wait_until_sent (used by, for example, tcdrain, tcsendbreak
and close).
Reported-by: NStas Sergeev <stsp@list.ru>
Signed-off-by: NJohan Hovold <jhovold@gmail.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/serial/ftdi_sio.c

@@ -924,7 +924,7 @@ static int  ftdi_tiocmset(struct tty_struct *tty,
 static int  ftdi_ioctl(struct tty_struct *tty,
 			unsigned int cmd, unsigned long arg);
 static void ftdi_break_ctl(struct tty_struct *tty, int break_state);
-static int ftdi_chars_in_buffer(struct tty_struct *tty);
+static bool ftdi_tx_empty(struct usb_serial_port *port);
 static int ftdi_get_modem_status(struct usb_serial_port *port,
 						unsigned char status[2]);
 
@@ -961,7 +961,7 @@ static struct usb_serial_driver ftdi_sio_device = {
 	.ioctl =		ftdi_ioctl,
 	.set_termios =		ftdi_set_termios,
 	.break_ctl =		ftdi_break_ctl,
-	.chars_in_buffer =      ftdi_chars_in_buffer,
+	.tx_empty =		ftdi_tx_empty,
 };
 
 static struct usb_serial_driver * const serial_drivers[] = {
@@ -2056,27 +2056,18 @@ static void ftdi_break_ctl(struct tty_struct *tty, int break_state)
 
 }
 
-static int ftdi_chars_in_buffer(struct tty_struct *tty)
+static bool ftdi_tx_empty(struct usb_serial_port *port)
 {
-	struct usb_serial_port *port = tty->driver_data;
-	int chars;
 	unsigned char buf[2];
 	int ret;
 
-	chars = usb_serial_generic_chars_in_buffer(tty);
-	if (chars)
-		goto out;
-
-	/* Check if hardware buffer is empty. */
 	ret = ftdi_get_modem_status(port, buf);
 	if (ret == 2) {
 		if (!(buf[1] & FTDI_RS_TEMT))
-			chars = 1;
+			return false;
 	}
-out:
-	dev_dbg(&port->dev, "%s - %d\n", __func__, chars);
 
-	return chars;
+	return true;
 }
 
 /* old_termios contains the original termios settings and tty->termios contains