linux下usb转串口驱动分析

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

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

 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;
 }

 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

 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;
 }

 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

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

 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
 

 

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

 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

 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;
 }

 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