2416开发记录九：在驱动模块初始化函数中实现设备节点的自动创建（转）

转自：http://blog.csdn.net/zhenwenxian/article/details/5424434
我们在刚开始写Linux设备驱动程序的时候，很多时候都是利用mknod命令手动创建设备节点，实际上Linux内核为我们提供了一组函数，可以用来在模块加载的时候自动在/dev目录下创建相应设备节点，并在卸载模块时删除该节点，当然前提条件是用户空间移植了udev。

内核中定义了struct class结构体，顾名思义，一个struct class结构体类型变量对应一个类，内核同时提供了class_create(…)函数，可以用它来创建一个类，这个类存放于sysfs下面，一旦创建好了这个类，再调用device_create(…)函数来在/dev目录下创建相应的设备节点。这样，加载模块的时候，用户空间中的udev会自动响应device_create(…)函数，去/sysfs下寻找对应的类从而创建设备节点。

注意，在2.6较早的内核版本中，device_create(…)函数名称不同，是class_device_create(…)，所以在新的内核中编译以前的模块程序有时会报错，就是因为函数名称不同，而且里面的参数设置也有一些变化。

struct class和device_create(…) 以及device_create(…)都定义在/include/linux/device.h中，使用的时候一定要包含这个头文件，否则编译器会报错。

在2.6.26.6内核版本中，struct class定义在头文件include/linux/device.h中：

/*      * device classes      */    struct class {      const char        *name;      struct module     *owner;  nbsp;struct kset         subsys;      struct list_head         devices;      struct list_head         interfaces;      struct kset              class_dirs;      struct semaphore sem;    /* locks children, devices, interfaces */      struct class_attribute   *class_attrs;      struct device_attribute      *dev_attrs;  int (*dev_uevent)(struct device *dev, struct kobj_uevent_env *env);  void (*class_release)(struct class *class);      void (*dev_release)(struct device *dev);  int (*suspend)(struct device *dev, pm_message_t state);      int (*resume)(struct device *dev);};class_create(…)在/drivers/base/class.c中实现：     /**    * class_create - create a struct class structure    * @owner: pointer to the module that is to "own" this struct class    * @name: pointer to a string for the name of this class.    *    * This is used to create a struct class pointer that can then be used    * in calls to device_create().    *    * Note, the pointer created here is to be destroyed when finished by    * making a call to class_destroy().    */   struct class *class_create(struct module *owner, const char *name)   {      struct class *cls;      int retval;      cls = kzalloc(sizeof(*cls), GFP_KERNEL);      if (!cls) {           retval = -ENOMEM;           goto error;      }  cls->name = name;      cls->owner = owner;      cls->class_release = class_create_release;  retval = class_register(cls);      if (retval)           goto error;  return cls;error:      kfree(cls);      return ERR_PTR(retval);    }

第一个参数指定类的所有者是哪个模块，第二个参数指定类名。在class.c中，还定义了class_destroy(…)函数，用于在模块卸载时删除类。

device_create(…)函数在/drivers/base/core.c中实现：

  /**     * device_create - creates a device and registers it with sysfs     * @class: pointer to the struct class that this device should be registered to     * @parent: pointer to the parent struct device of this new device, if any     * @devt: the dev_t for the char device to be added     * @fmt: string for the device's name     *     * This function can be used by char device classes. A struct device     * will be created in sysfs, registered to the specified class.     *     * A "dev" file will be created, showing the dev_t for the device, if     * the dev_t is not 0,0.     * If a pointer to a parent struct device is passed in, the newly created     * struct device will be a child of that device in sysfs.     * The pointer to the struct device will be returned from the call.     * Any further sysfs files that might be required can be created using this     * pointer.     *     * Note: the struct class passed to this function must have previously     * been created with a call to class_create().     */    struct device *device_create(struct class *class, struct device *parent,                        dev_t devt, const char *fmt, ...)    {         va_list vargs;         struct device *dev;     va_start(vargs, fmt);         dev = device_create_vargs(class, parent, devt, NULL, fmt, vargs);         va_end(vargs);         return dev;    }

第一个参数指定所要创建的设备所从属的类，第二个参数是这个设备的父设备，如果没有就指定为NULL，第三个参数是设备号，第四个参数是设备名称，第五个参数是从设备号。

下面以一个简单字符设备驱动来展示如何使用这几个函数

 #include <linux/module.h>    #include <linux/kernel.h>    #include <linux/init.h>    #include <linux/fs.h>    #include <linux/cdev.h>    #include <linux/device.h>MODULE_LICENSE ("GPL");int hello_major = 555;    int hello_minor = 0;    int number_of_devices = 1;struct cdev cdev;    dev_t dev = 0;struct file_operations hello_fops = {      .owner = THIS_MODULE    };static void char_reg_setup_cdev (void)    {       int error, devno = MKDEV (hello_major, hello_minor);       cdev_init (&cdev, &hello_fops);       cdev.owner = THIS_MODULE;       cdev.ops = &hello_fops;       error = cdev_add (&cdev, devno , 1);       if (error)           printk (KERN_NOTICE "Error %d adding char_reg_setup_cdev", error);}struct class *my_class;static int __init hello_2_init (void)    {       int result;       dev = MKDEV (hello_major, hello_minor);       result = register_chrdev_region (dev, number_of_devices, "hello");       if (result<0) {           printk (KERN_WARNING "hello: can't get major number %d/n", hello_major);           return result;     } char_reg_setup_cdev (); /* create your own class under /sysfs */     my_class = class_create(THIS_MODULE, "my_class");     if(IS_ERR(my_class))     {          printk("Err: failed in creating class./n");          return -1;      }  /* register your own device in sysfs, and this will cause udev to create corresponding device node */      device_create( my_class, NULL, MKDEV(hello_major, 0), "hello" "%d", 0 );  printk (KERN_INFO "Registered character driver/n");      return 0;    }static void __exit hello_2_exit (void)    {       dev_t devno = MKDEV (hello_major, hello_minor);       cdev_del (&cdev);   device_destroy(my_class, MKDEV(adc_major, 0));         //delete device node under /dev       class_destroy(my_class);                               //delete class created by us   unregister_chrdev_region (devno, number_of_devices);   printk (KERN_INFO "char driver cleaned up/n");    }module_init (hello_2_init);    module_exit (hello_2_exit);

这样，模块加载后，就能在/dev目录下找到hello0这个设备节点了。

例子2

drivers/i2c/i2c-dev.c

/* * module load/unload record keeping */static int __init i2c_dev_init(void){ int res; printk(KERN_INFO "i2c /dev entries driver/n"); res = register_chrdev(I2C_MAJOR, "i2c", &i2cdev_fops); if (res)  goto out; i2c_dev_class = class_create(THIS_MODULE, "i2c-dev"); if (IS_ERR(i2c_dev_class)) {  res = PTR_ERR(i2c_dev_class);  goto out_unreg_chrdev; } res = i2c_add_driver(&i2cdev_driver); if (res)  goto out_unreg_class; return 0;out_unreg_class: class_destroy(i2c_dev_class);out_unreg_chrdev: unregister_chrdev(I2C_MAJOR, "i2c");out: printk(KERN_ERR "%s: Driver Initialisation failed/n", __FILE__); return res;}在 i2c_dev_init创建类i2c_dev_classi2c_dev_class = class_create(THIS_MODULE, "i2c-dev");static int i2cdev_attach_adapter(struct i2c_adapter *adap){ struct i2c_dev *i2c_dev; int res; i2c_dev = get_free_i2c_dev(adap); if (IS_ERR(i2c_dev))  return PTR_ERR(i2c_dev); /* register this i2c device with the driver core */ i2c_dev->dev = device_create(i2c_dev_class, &adap->dev,         MKDEV(I2C_MAJOR, adap->nr), NULL,         "i2c-%d", adap->nr); if (IS_ERR(i2c_dev->dev)) {  res = PTR_ERR(i2c_dev->dev);  goto error; } res = device_create_file(i2c_dev->dev, &dev_attr_name); if (res)  goto error_destroy; pr_debug("i2c-dev: adapter [%s] registered as minor %d/n",   adap->name, adap->nr); return 0;error_destroy: device_destroy(i2c_dev_class, MKDEV(I2C_MAJOR, adap->nr));error: return_i2c_dev(i2c_dev); return res;}在i2cdev_attach_adapter调用device_create(i2c_dev_class, &adap->dev,         MKDEV(I2C_MAJOR, adap->nr), NULL,         "i2c-%d", adap->nr);

这样在dev目录就产生i2c-0 或i2c-1节点