本系列解析usbseiral ko的源码，记录主要函数，调用流程，USB一端和串口一端的注册流程，本节简介主要函数以及替换规则。

首先，usbserial是USB转串口驱动的一个基础模板，其中有许多默认函数，他们的定义方式如下：

const struct bus_type usb_serial_bus_type = {
	.name =		"usb-serial",
	.match =	usb_serial_device_match,
	.probe =	usb_serial_device_probe,
	.remove =	usb_serial_device_remove,
	.drv_groups = 	usb_serial_drv_groups,
};

static struct usb_serial_driver usb_serial_generic_device = {
	.driver = {
		.owner =	THIS_MODULE,
		.name =		"generic",
	},
	.id_table =		generic_device_ids,
	.probe =		usb_serial_generic_probe,
	.calc_num_ports =	usb_serial_generic_calc_num_ports,
	.throttle =		usb_serial_generic_throttle,
	.unthrottle =		usb_serial_generic_unthrottle,
	.resume =		usb_serial_generic_resume,
};

static const struct tty_port_operations serial_port_ops = {
	.carrier_raised		= serial_port_carrier_raised,
	.dtr_rts		= serial_port_dtr_rts,
	.activate		= serial_port_activate,
	.shutdown		= serial_port_shutdown,
};

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,
};

其中，tty operation中有部分函数可以被替换掉，替换的过程如下：

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);
}

可见open、write、close、write_room、chars_in_buffer、read_bulk_callback、write_bulk_callback、process_read_urb、prepare_write_buffer函数可以被替换掉，不过有些函数采取的是插入的方式，目的是在generic的基础上，增加一些属vendor的其他操作，比如probe

以generic 的probe为例子，usbseiral设备在被probe的时候，一定会进行search_serial_device、create_serial等等操作，然后再简单的打印一些信息——如果有一些vendor specific的probe操作，可以单独添加——endpoint的查找在vendor的probe的后面，也是一定会进行的。