linux下usb转串口驱动分析

http://blog.csdn.net/alading664/article/details/7638714

最近要做一个关于LTE的项目，模块厂家提供的驱动里面有usb转串口驱动，usb网卡驱动，项目还没立项所以比较空，闲的蛋疼把usb转串口驱动研究了一遍，本文尽量用画图把事情说明白，献给各位纠结的童鞋。

首先说一下linux的风格，个人理解

1. linux大小结构体其实是面向对象的方法，（如果把struct 比作类，kmalloc就是类的实例化，结构体里面的函数指针就是方法，还有重构，多态）

2. 在linux里面，设备是对象，驱动也是对象，并且这两个是分开的

现在我们来看驱动的总体架构

linux下usb转串口驱动分析

并不用太在意这个图，对用户来说usb_serial设备就是普通的串口设备

我们可以看驱动里面几个主要的源代码文件

usb-serial.c 模块的主要实现

bus.c usb_serial总线驱动，驱动和设备都要注册到这条总线上

generic.c 通用的用户驱动，用户如果写自己的驱动只需拿自己的实现代替generic.c的函数，一般这个驱动已经能适应大部分设备了

现在我们来看usb_serial模块的初始化过程

[cpp] view plain copy print ?

static int __init usb_serial_init(void)
{
int i;
int result;
usb_serial_tty_driver = alloc_tty_driver(SERIAL_TTY_MINORS);
if (!usb_serial_tty_driver)
return -ENOMEM;
for (i = 0; i < SERIAL_TTY_MINORS; ++i)
serial_table[i] = NULL;
result = bus_register(&usb_serial_bus_type);
if (result) {
printk(KERN_ERR "usb-serial: %s - registering bus driver "
"failed\n", __func__);
goto exit_bus;
}
usb_serial_tty_driver->owner = THIS_MODULE;
usb_serial_tty_driver->driver_name = "usbserial";
usb_serial_tty_driver->name = "ttyUSB";
usb_serial_tty_driver->major = SERIAL_TTY_MAJOR;
usb_serial_tty_driver->minor_start = 0;
usb_serial_tty_driver->type = TTY_DRIVER_TYPE_SERIAL;
usb_serial_tty_driver->subtype = SERIAL_TYPE_NORMAL;
usb_serial_tty_driver->flags = TTY_DRIVER_REAL_RAW |
TTY_DRIVER_DYNAMIC_DEV;
usb_serial_tty_driver->init_termios = tty_std_termios;
usb_serial_tty_driver->init_termios.c_cflag = B9600 | CS8 | CREAD
| HUPCL | CLOCAL;
usb_serial_tty_driver->init_termios.c_ispeed = 9600;
usb_serial_tty_driver->init_termios.c_ospeed = 9600;
tty_set_operations(usb_serial_tty_driver, &serial_ops);
result = tty_register_driver(usb_serial_tty_driver);
if (result) {
printk(KERN_ERR "usb-serial: %s - tty_register_driver failed\n",
__func__);
goto exit_reg_driver;
}
result = usb_register(&usb_serial_driver);
if (result < 0) {
printk(KERN_ERR "usb-serial: %s - usb_register failed\n",
__func__);
goto exit_tty;
}
result = usb_serial_generic_register(debug);
if (result < 0) {
printk(KERN_ERR "usb-serial: %s - registering generic "
"driver failed\n", __func__);
goto exit_generic;
}
printk(KERN_INFO KBUILD_MODNAME ": " DRIVER_DESC "\n");
return result;
exit_generic:
usb_deregister(&usb_serial_driver);
exit_tty:
tty_unregister_driver(usb_serial_tty_driver);
exit_reg_driver:
bus_unregister(&usb_serial_bus_type);
exit_bus:
printk(KERN_ERR "usb-serial: %s - returning with error %d\n",
__func__, result);
put_tty_driver(usb_serial_tty_driver);
return result;
}

static int __init usb_serial_init(void)
{
	int i;
	int result;

	usb_serial_tty_driver = alloc_tty_driver(SERIAL_TTY_MINORS);
	if (!usb_serial_tty_driver)
		return -ENOMEM;

	/* Initialize our global data */
	for (i = 0; i < SERIAL_TTY_MINORS; ++i)
		serial_table[i] = NULL;

	result = bus_register(&usb_serial_bus_type);
	if (result) {
		printk(KERN_ERR "usb-serial: %s - registering bus driver "
		       "failed\n", __func__);
		goto exit_bus;
	}

	usb_serial_tty_driver->owner = THIS_MODULE;
	usb_serial_tty_driver->driver_name = "usbserial";
	usb_serial_tty_driver->name = 	"ttyUSB";
	usb_serial_tty_driver->major = SERIAL_TTY_MAJOR;
	usb_serial_tty_driver->minor_start = 0;
	usb_serial_tty_driver->type = TTY_DRIVER_TYPE_SERIAL;
	usb_serial_tty_driver->subtype = SERIAL_TYPE_NORMAL;
	usb_serial_tty_driver->flags = TTY_DRIVER_REAL_RAW |
						TTY_DRIVER_DYNAMIC_DEV;
	usb_serial_tty_driver->init_termios = tty_std_termios;
	usb_serial_tty_driver->init_termios.c_cflag = B9600 | CS8 | CREAD
							| HUPCL | CLOCAL;
	usb_serial_tty_driver->init_termios.c_ispeed = 9600;
	usb_serial_tty_driver->init_termios.c_ospeed = 9600;
	tty_set_operations(usb_serial_tty_driver, &serial_ops);
	result = tty_register_driver(usb_serial_tty_driver);
	if (result) {
		printk(KERN_ERR "usb-serial: %s - tty_register_driver failed\n",
		       __func__);
		goto exit_reg_driver;
	}

	/* register the USB driver */
	result = usb_register(&usb_serial_driver);
	if (result < 0) {
		printk(KERN_ERR "usb-serial: %s - usb_register failed\n",
		       __func__);
		goto exit_tty;
	}

	/* register the generic driver, if we should */
	result = usb_serial_generic_register(debug);
	if (result < 0) {
		printk(KERN_ERR "usb-serial: %s - registering generic "
		       "driver failed\n", __func__);
		goto exit_generic;
	}

	printk(KERN_INFO KBUILD_MODNAME ": " DRIVER_DESC "\n");

	return result;

exit_generic:
	usb_deregister(&usb_serial_driver);

exit_tty:
	tty_unregister_driver(usb_serial_tty_driver);

exit_reg_driver:
	bus_unregister(&usb_serial_bus_type);

exit_bus:
	printk(KERN_ERR "usb-serial: %s - returning with error %d\n",
	       __func__, result);
	put_tty_driver(usb_serial_tty_driver);
	return result;
}

很简单

第一步将usb_seria的TTY驱动注册进TTY驱动列表里面，以后调用open,write,read首先会调用tty驱动里面的函数，然后函数指针会指到用户自己定义的驱动里面，这应该是多态的一种应用吧，个人理解求指正

第二步将usb_seria驱动注册进usb_core里面的驱动列表

只有usb_serial模块驱动，设备还不能正常工作，linux很好的把它分成了

分层一： usb_serial驱动，设备的大部分实现都在此

分层二：用户驱动，不需要知道太多的细节，实现几个回调函数就能实现整个驱动功能。

generic.c 就是个通用的用户驱动模型，并且大部分设备都能兼容。

下面generic.c的模块初始化函数

[cpp] view plain copy print ?

int usb_serial_generic_register(int _debug)
{
int retval = 0;
debug = _debug;
#ifdef CONFIG_USB_SERIAL_GENERIC
generic_device_ids[0].idVendor = vendor;
generic_device_ids[0].idProduct = product;
generic_device_ids[0].match_flags =
USB_DEVICE_ID_MATCH_VENDOR | USB_DEVICE_ID_MATCH_PRODUCT;
retval = usb_serial_register(&usb_serial_generic_device);
if (retval)
goto exit;
retval = usb_register(&generic_driver);
if (retval)
usb_serial_deregister(&usb_serial_generic_device);
exit:
#endif
return retval;
}

int usb_serial_generic_register(int _debug)
{
	int retval = 0;

	debug = _debug;
#ifdef CONFIG_USB_SERIAL_GENERIC
	generic_device_ids[0].idVendor = vendor;
	generic_device_ids[0].idProduct = product;
	generic_device_ids[0].match_flags =
		USB_DEVICE_ID_MATCH_VENDOR | USB_DEVICE_ID_MATCH_PRODUCT;

	/* register our generic driver with ourselves */
	retval = usb_serial_register(&usb_serial_generic_device);
	if (retval)
		goto exit;
	retval = usb_register(&generic_driver);
	if (retval)
		usb_serial_deregister(&usb_serial_generic_device);
exit:
#endif
	return retval;
}

第一步首先确定自己的特征码，这里通过vendor 和 product表明自己是哪种设备的驱动

第二步将用户驱动注册进usb_serial_bus总线,linux很喜欢这么搞……，通过总线来管理一类设备

第三步将usb驱动注册进usb_core的驱动列表

有人会问，为什么usb会自动发现对应的驱动，这其实是个匹配的过程,linux里面叫probe

前面第一步是确定了驱动的匹配值，第二步和第三步都把驱动的匹配值注册进去

linux下usb转串口驱动分析

事实上前面第三步代码retval = usb_register(&generic_driver);就是为了把generic驱动注册进usb的驱动列表，当有设备插入时，会轮询到它，然后会调用此驱动的probe，就是下面的函数

[cpp] view plain copy print ?

static int generic_probe(struct usb_interface *interface,
const struct usb_device_id *id)
{
const struct usb_device_id *id_pattern;
id_pattern = usb_match_id(interface, generic_device_ids);
if (id_pattern != NULL)
return usb_serial_probe(interface, id);
return -ENODEV;
}

static int generic_probe(struct usb_interface *interface,
			       const struct usb_device_id *id)
{
	const struct usb_device_id *id_pattern;

	id_pattern = usb_match_id(interface, generic_device_ids);
	if (id_pattern != NULL)
		return usb_serial_probe(interface, id);
	return -ENODEV;
}

最主要的还是调用usb_serial_probe(interface, id)函数

现在我们来看usb_serial_probe()的匹配过程

linux下usb转串口驱动分析

通过probe我们最终将/dev/ttySn设备和usb_serial_port对象绑定起来，我们对/dev/ttySn设备进行操作，对应tty驱动里面的

[cpp] view plain copy print ?

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,
.tiocmget = serial_tiocmget,
.tiocmset = serial_tiocmset,
.cleanup = serial_cleanup,
.install = serial_install,
.proc_fops = &serial_proc_fops,
};

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,
	.tiocmget =		serial_tiocmget,
	.tiocmset =		serial_tiocmset,
	.cleanup = 		serial_cleanup,
	.install = 		serial_install,
	.proc_fops =		&serial_proc_fops,
};

比如我们对/dev/ttySn进行写操作，write->serial_write

[cpp] view plain copy print ?

static int serial_write(struct tty_struct *tty, const unsigned char *buf,
int count)
{
struct usb_serial_port *port = tty->driver_data;
int retval = -ENODEV;
if (port->serial->dev->state == USB_STATE_NOTATTACHED)
goto exit;
dbg("%s - port %d, %d byte(s)", __func__, port->number, count);
retval = port->serial->type->write(tty, port, buf, count);
exit:
return retval;
}

static int serial_write(struct tty_struct *tty, const unsigned char *buf,
								int count)
{
	struct usb_serial_port *port = tty->driver_data;
	int retval = -ENODEV;

	if (port->serial->dev->state == USB_STATE_NOTATTACHED)
		goto exit;

	dbg("%s - port %d, %d byte(s)", __func__, port->number, count);

	/* pass on to the driver specific version of this function */
	retval = port->serial->type->write(tty, port, buf, count);

exit:
	return retval;
}

struct usb_serial_port *port = tty->driver_data;找到了这个tty对象所对应的port对象，

port对象里面有驱动信息，urb的缓冲区信息，最终调用的是我们写的用户驱动里面的write方法。

用户驱动通过信使URB将想要发送的数据发送出去。

总结：

通过分析usb 串口驱动可以推测出其他usb设备大致的工作方式，下一步将分析usb网卡的驱动。

linux复杂的代码结构其实是有面向对象的思想

linux下usb转串口驱动分析

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