USB: sierra: fix characters being dropped at close

Fix characters potentially being dropped at close due to missing
chars_in_buffer implementation.

Note that currently the write urbs are not even killed at close (will be
fixed separately), but this could still lead to dropped data since we
have lowered DTR/RTS.
Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/serial/sierra.c

@@ -675,6 +675,23 @@ static int sierra_write_room(struct tty_struct *tty)
 	return 2048;
 }
 
+static int sierra_chars_in_buffer(struct tty_struct *tty)
+{
+	struct usb_serial_port *port = tty->driver_data;
+	struct sierra_port_private *portdata = usb_get_serial_port_data(port);
+	unsigned long flags;
+	int chars;
+
+	/* NOTE: This overcounts somewhat. */
+	spin_lock_irqsave(&portdata->lock, flags);
+	chars = portdata->outstanding_urbs * MAX_TRANSFER;
+	spin_unlock_irqrestore(&portdata->lock, flags);
+
+	dev_dbg(&port->dev, "%s - %d\n", __func__, chars);
+
+	return chars;
+}
+
 static void sierra_stop_rx_urbs(struct usb_serial_port *port)
 {
 	int i;
@@ -1060,6 +1077,7 @@ static struct usb_serial_driver sierra_device = {
 	.dtr_rts	   = sierra_dtr_rts,
 	.write             = sierra_write,
 	.write_room        = sierra_write_room,
+	.chars_in_buffer   = sierra_chars_in_buffer,
 	.set_termios       = sierra_set_termios,
 	.tiocmget          = sierra_tiocmget,
 	.tiocmset          = sierra_tiocmset,