USB: ti_usb_3410_5052: query hardware-buffer status in chars_in_buffer

Query hardware-buffer status in chars_in_buffer should the write fifo be
empty.

This is needed to make the tty layer wait for hardware buffers to drain
on close.
Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/serial/ti_usb_3410_5052.c

@@ -687,6 +687,8 @@ static int ti_chars_in_buffer(struct tty_struct *tty)
 	struct ti_port *tport = usb_get_serial_port_data(port);
 	int chars = 0;
 	unsigned long flags;
+	int ret;
+	u8 lsr;
 
 	if (tport == NULL)
 		return 0;
@@ -695,6 +697,12 @@ static int ti_chars_in_buffer(struct tty_struct *tty)
 	chars = kfifo_len(&tport->write_fifo);
 	spin_unlock_irqrestore(&tport->tp_lock, flags);
 
+	if (!chars) {
+		ret = ti_get_lsr(tport, &lsr);
+		if (!ret && !(lsr & TI_LSR_TX_EMPTY))
+			chars = 1;
+	}
+
 	dev_dbg(&port->dev, "%s - returns %d\n", __func__, chars);
 	return chars;
 }