usb转串口设备需要进行收发，主机接到uart rx线上的数据，把主机数据发送到 uart  tx线上，对应的就是read函数和write函数，以上两个Part提到了tty有关的函数，在以下结构体被定义，write被指定为serial_write，read在open里面被调用

static const struct tty_operations serial_ops = {
	.open =			serial_open,
	.close =		serial_close,
	.write =		serial_write,
	.hangup =		serial_hangup,
	.write_room =		serial_write_room,
	.ioctl =		serial_ioctl,
	.set_termios =		serial_set_termios,
	.throttle =		serial_throttle,
	.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,
	.set_serial =		serial_set_serial,
	.get_serial =		serial_get_serial,
	.cleanup =		serial_cleanup,
	.install =		serial_install,
	.proc_show =		serial_proc_show,
};

write：

write的调用流程是这样的：

此write被指定为generic write：

static void usb_serial_operations_init(struct usb_serial_driver *device)
{
	set_to_generic_if_null(device, open);
	set_to_generic_if_null(device, write);
	set_to_generic_if_null(device, close);
	set_to_generic_if_null(device, write_room);
	set_to_generic_if_null(device, chars_in_buffer);
	if (device->tx_empty)
		set_to_generic_if_null(device, wait_until_sent);
	set_to_generic_if_null(device, read_bulk_callback);
	set_to_generic_if_null(device, write_bulk_callback);
	set_to_generic_if_null(device, process_read_urb);
	set_to_generic_if_null(device, prepare_write_buffer);
}

在generic write中，buf也就是tty要下发的值，存在了write fifo里面

在write start里面，调用prepare_write_buffer，就像上面所说，也被指定为了generic

int usb_serial_generic_prepare_write_buffer(struct usb_serial_port *port,
						void *dest, size_t size)
{
	return kfifo_out_locked(&port->write_fifo, dest, size, &port->lock);
}

这个操作把write fifo里面的待发送数据放到urb里面，然后submit即可：

read：

可以看到，open usb转串口设备的时候就会开始read

最后会调用到read urb，然后submit urb即可