static int cypress_chars_in_buffer(struct tty_struct *tty)
{
	struct usb_serial_port *port = tty->driver_data;
	struct cypress_private *priv = usb_get_serial_port_data(port);
	int chars = 0;
	unsigned long flags;

	spin_lock_irqsave(&priv->lock, flags);
	chars = kfifo_len(&priv->write_fifo);
	spin_unlock_irqrestore(&priv->lock, flags);

	dev_dbg(&port->dev, "%s - returns %d\n", __func__, chars);
	return chars;
}

Unknown