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

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

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staticint__initusb_serial_init(void)

{

inti;

intresult;

usb_serial_tty_driver=alloc_tty_driver(SERIAL_TTY_MINORS);

if(!usb_serial_tty_driver)

return-ENOMEM;

/*Initializeourglobaldata*/

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-registeringbusdriver"

"failed\n",__func__);

gotoexit_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_driverfailed\n",

__func__);

gotoexit_reg_driver;

}

/*registertheUSBdriver*/

result=usb_register(&usb_serial_driver);

if(result<0){

printk(KERN_ERR"usb-serial:%s-usb_registerfailed\n",

__func__);

gotoexit_tty;

}

/*registerthegenericdriver,ifweshould*/

result=usb_serial_generic_register(debug);

if(result<0){

printk(KERN_ERR"usb-serial:%s-registeringgeneric"

"driverfailed\n",__func__);

gotoexit_generic;

}

printk(KERN_INFOKBUILD_MODNAME":"DRIVER_DESC"\n");

returnresult;

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-returningwitherror%d\n",

__func__,result);

put_tty_driver(usb_serial_tty_driver);

returnresult;

}

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staticint__initusb_serial_init(void)

{

inti;

intresult;

usb_serial_tty_driver=alloc_tty_driver(SERIAL_TTY_MINORS);

if(!usb_serial_tty_driver)

return-ENOMEM;

/*Initializeourglobaldata*/

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-registeringbusdriver"

"failed\n",__func__);

gotoexit_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_driverfailed\n",

__func__);

gotoexit_reg_driver;

}

/*registertheUSBdriver*/

result=usb_register(&usb_serial_driver);

if(result<0){

printk(KERN_ERR"usb-serial:%s-usb_registerfailed\n",

__func__);

gotoexit_tty;

}

/*registerthegenericdriver,ifweshould*/

result=usb_serial_generic_register(debug);

if(result<0){

printk(KERN_ERR"usb-serial:%s-registeringgeneric"

"driverfailed\n",__func__);

gotoexit_generic;

}

printk(KERN_INFOKBUILD_MODNAME":"DRIVER_DESC"\n");

returnresult;

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-returningwitherror%d\n",

__func__,result);

put_tty_driver(usb_serial_tty_driver);

returnresult;

}

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

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intusb_serial_generic_register(int_debug)

{

intretval=0;

debug=_debug;

#ifdefCONFIG_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;

/*registerourgenericdriverwithourselves*/

retval=usb_serial_register(&usb_serial_generic_device);

if(retval)

gotoexit;

retval=usb_register(&generic_driver);

if(retval)

usb_serial_deregister(&usb_serial_generic_device);

exit:

#endif

returnretval;

}

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intusb_serial_generic_register(int_debug)

{

intretval=0;

debug=_debug;

#ifdefCONFIG_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;

/*registerourgenericdriverwithourselves*/

retval=usb_serial_register(&usb_serial_generic_device);

if(retval)

gotoexit;

retval=usb_register(&generic_driver);

if(retval)

usb_serial_deregister(&usb_serial_generic_device);

exit:

#endif

returnretval;

}

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

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staticintgeneric_probe(structusb_interface*interface,

conststructusb_device_id*id)

{

conststructusb_device_id*id_pattern;

id_pattern=usb_match_id(interface,generic_device_ids);

if(id_pattern!=NULL)

returnusb_serial_probe(interface,id);

return-ENODEV;

}

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staticintgeneric_probe(structusb_interface*interface,

conststructusb_device_id*id)

{

conststructusb_device_id*id_pattern;

id_pattern=usb_match_id(interface,generic_device_ids);

if(id_pattern!=NULL)

returnusb_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

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staticconststructtty_operationsserial_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,

};

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staticconststructtty_operationsserial_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

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staticintserial_write(structtty_struct*tty,constunsignedchar*buf,

intcount)

{

structusb_serial_port*port=tty->driver_data;

intretval=-ENODEV;

if(port->serial->dev->state==USB_STATE_NOTATTACHED)

gotoexit;

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

/*passontothedriverspecificversionofthisfunction*/

retval=port->serial->type->write(tty,port,buf,count);

exit:

returnretval;

}

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staticintserial_write(structtty_struct*tty,constunsignedchar*buf,

intcount)

{

structusb_serial_port*port=tty->driver_data;

intretval=-ENODEV;

if(port->serial->dev->state==USB_STATE_NOTATTACHED)

gotoexit;

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

/*passontothedriverspecificversionofthisfunction*/

retval=port->serial->type->write(tty,port,buf,count);

exit:

returnretval;

}

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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