USB: serial: add wait_until_sent operation

Add wait_until_sent operation which can be used to wait for hardware
buffers to drain.

Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/serial/usb-serial.c

@@ -375,6 +375,22 @@ static int serial_chars_in_buffer(struct tty_struct *tty)
 	return count;
 }
 
+static void serial_wait_until_sent(struct tty_struct *tty, int timeout)
+{
+	struct usb_serial_port *port = tty->driver_data;
+	struct usb_serial *serial = port->serial;
+
+	dev_dbg(tty->dev, "%s\n", __func__);
+
+	if (!port->serial->type->wait_until_sent)
+		return;
+
+	mutex_lock(&serial->disc_mutex);
+	if (!serial->disconnected)
+		port->serial->type->wait_until_sent(tty, timeout);
+	mutex_unlock(&serial->disc_mutex);
+}
+
 static void serial_throttle(struct tty_struct *tty)
 {
 	struct usb_serial_port *port = tty->driver_data;
@@ -1191,6 +1207,7 @@ static const struct tty_operations serial_ops = {
 	.unthrottle =		serial_unthrottle,
 	.break_ctl =		serial_break,
 	.chars_in_buffer =	serial_chars_in_buffer,
+	.wait_until_sent =	serial_wait_until_sent,
 	.tiocmget =		serial_tiocmget,
 	.tiocmset =		serial_tiocmset,
 	.get_icount =		serial_get_icount,

include/linux/usb/serial.h

@@ -268,6 +268,7 @@ struct usb_serial_driver {
 			struct usb_serial_port *port, struct ktermios *old);
 	void (*break_ctl)(struct tty_struct *tty, int break_state);
 	int  (*chars_in_buffer)(struct tty_struct *tty);
+	void (*wait_until_sent)(struct tty_struct *tty, long timeout);
 	void (*throttle)(struct tty_struct *tty);
 	void (*unthrottle)(struct tty_struct *tty);
 	int  (*tiocmget)(struct tty_struct *tty);