一、class_create

官方说明：

/* This is a #define to keep the compiler from merging different* instances of the __key variable */
#define class_create(owner, name)       \
({                      \static struct lock_class_key __key;    \__class_create(owner, name, &__key);   \
})/*** 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.* @key: the lock_class_key for this class; used by mutex lock debugging** This is used to create a struct class pointer that can then be used* in calls to device_create().** Returns &struct class pointer on success, or ERR_PTR() on error.** 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 lock_class_key *key)

关键的一句是：

 * This is used to create a struct class pointer that can then be used* in calls to device_create().-->这个函数用来创建一个struct class的结构体指针，这个指针可用作device_create()函数的参数。

也就是说，这个函数主要是在调用device_create()前使用，创建一个struct class类型的变量，并返回其指针。

二、device_create

官方说明：

/*** 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* @drvdata: the data to be added to the device for callbacks* @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.** Returns &struct device pointer on success, or ERR_PTR() on error.** 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, void *drvdata, const char *fmt, ...)

首先解释一下"sysfs"：sysfs是linux2.6所提供的一种虚拟档案系统；在设备模型中，sysfs文件系统用来表示设备的结构，将设备的层次结构形象的反应到用户空间中，从而可以通过修改sysfs中的文件属性来修改设备的属性值；sysfs被挂载到根目录下的"/sys"文件夹下。

三、device_create_file

官方说明：

/*** device_create_file - create sysfs attribute file for device.* @dev: device.* @attr: device attribute descriptor.*/
int device_create_file(struct device *dev,const struct device_attribute *attr)

使用这个函数时要引用 device_create所返回的device*指针，作用是在/sys/class/下创建一个属性文件，从而通过对这个属性文件进行读写就能完成对应的数据操作。

如：

a.在驱动程序中使用 device_create_file创建属性文件

static DEVICE_ATTR(val, S_IRUGO | S_IWUSR, hello_val_show, hello_val_store);  /*读取寄存器val的值到缓冲区buf中，内部使用*/
static ssize_t __hello_get_val(struct xxx_dev* dev, char* buf) {  int val = 0;          /*同步访问*/  if(down_interruptible(&(dev->sem))) {                  return -ERESTARTSYS;          }          val = dev->val;          up(&(dev->sem));          return snprintf(buf, PAGE_SIZE, "%d/n", val);
}  /*把缓冲区buf的值写到设备寄存器val中去，内部使用*/
static ssize_t __hello_set_val(struct xxx_dev* dev, const char* buf, size_t count) {  int val = 0;          /*将字符串转换成数字*/          val = simple_strtol(buf, NULL, 10);          /*同步访问*/          if(down_interruptible(&(dev->sem))) {                  return -ERESTARTSYS;          }          dev->val = val;          up(&(dev->sem));  return count;
}  /*读取设备属性val*/
static ssize_t hello_val_show(struct device* dev, struct device_attribute* attr, char* buf) {  struct xxx_dev* hdev = (struct xxx_dev*)dev_get_drvdata(dev);          return __hello_get_val(hdev, buf);
}  /*写设备属性val*/
static ssize_t hello_val_store(struct device* dev, struct device_attribute* attr, const char* buf, size_t count) {   struct xxx_dev* hdev = (struct xxx_dev*)dev_get_drvdata(dev);    return __hello_set_val(hdev, buf, count);
} /*模块加载方法*/
static int __init xxx_init(void){   ... /*在/sys/class/xxx/xxx目录下创建属性文件val*/  err = device_create_file(temp, &dev_attr_val);  if(err < 0) {  printk(KERN_ALERT"Failed to create attribute val.");                  goto destroy_device;  }...
} 

b.在用户空间读取属性

...
read(dev->fd, val, sizeof(*val));
...
write(dev->fd, &val, sizeof(val));
...

四、使用示例

    /*在/sys/class/目录下创建设备类别目录xxx*/ g_vircdev_class = class_create(THIS_MODULE, VIRCDEV_CLASS_NAME);if(IS_ERR(g_vircdev_class)) {  err = PTR_ERR(g_vircdev_class);  printk(KERN_ALERT "Failed to create class.\n");  goto CLASS_CREATE_ERR;  }/*在/dev/目录和/sys/class/xxx目录下分别创建设备文件xxx*/dev = device_create(g_vircdev_class, NULL, devt, NULL, VIRCDEV_DEVICE_NAME);if(IS_ERR(dev)) {  err = PTR_ERR(dev);  printk(KERN_ALERT "Failed to create device.\n");  goto DEVICE_CREATE_ERR;  }/*在/sys/class/xxx/xxx目录下创建属性文件val*/ err = device_create_file(dev, attr);  if(err < 0) {  printk(KERN_ALERT"Failed to create attribute file.");                  goto DEVICE_CREATE_FILE_ERR;  }