自动创建设备结点

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2.6.37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;

 struct class_attribute  *class_attrs;
 struct device_attribute  *dev_attrs;
 struct kobject   *dev_kobj;

 int (*dev_uevent)(struct device *dev, struct kobj_uevent_env *env);
 char *(*devnode)(struct device *dev, mode_t *mode);

 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);

 const struct kobj_ns_type_operations *ns_type;
 const void *(*namespace)(struct device *dev);

 const struct dev_pm_ops *pm;

 struct class_private *p;
};

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.
 * @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)
{
 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, key);
 if (retval)
  goto error;

 return cls;

error:
 kfree(cls);
 return ERR_PTR(retval);
}

在class.c中，还定义了class_destroy(…)函数，用于在模块卸载时删除类。

void class_destroy(struct class *cls)
{
 if ((cls == NULL) || (IS_ERR(cls)))
  return;

 class_unregister(cls);
}

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
 * @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, ...)
{
 va_list vargs;
 struct device *dev;

 va_start(vargs, fmt);
 dev = device_create_vargs(class, parent, devt, drvdata, 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。

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

/*
 * device_destroy - removes a device that was created with device_create()
 * @class: pointer to the struct class that this device was registered with
 * @devt: the dev_t of the device that was previously registered
 *
 * This call unregisters and cleans up a device that was created with a
 * call to device_create().
 */

void device_destroy(struct class *class, dev_t devt)
{
 struct device *dev;

 dev = class_find_device(class, NULL, &devt, __match_devt);
 if (dev) {
  put_device(dev);
  device_unregister(dev);
 }
}

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 = 0;
int HELLO_MINOR = 0;
int NUMBER_OF_DEVICES = 2;

struct class *my_class;
struct cdev cdev;
dev_t devno;

struct file_operations hello_fops = {
 .owner = THIS_MODULE
 };

static int __init hello_init (void)
    {
 int result;
 devno = MKDEV(HELLO_MAJOR, HELLO_MINOR);

 if (HELLO_MAJOR)
  result = register_chrdev_region(devno, 2, "memdev");
 else
 {
  result = alloc_chrdev_region(&devno, 0, 2, "memdev");
  HELLO_MAJOR = MAJOR(devno);
 } 
 printk("MAJOR IS %d\n",HELLO_MAJOR);

 my_class = class_create(THIS_MODULE,"hello_char_class");  //类名为hello_char_class
 if(IS_ERR(my_class))
 {
          printk("Err: failed in creating class.\n");
          return -1;
 }

 device_create(my_class,NULL,devno,NULL,"memdev");      //设备名为memdev

 if (result<0)
 {
           printk (KERN_WARNING "hello: can't get major number %d\n", HELLO_MAJOR);
           return result;
 }

 cdev_init(&cdev, &hello_fops);
 cdev.owner = THIS_MODULE;
 cdev_add(&cdev, devno, NUMBER_OF_DEVICES);

 printk (KERN_INFO "Character driver Registered\n");
 return 0;
    }

static void __exit hello_exit (void)
    {
 cdev_del (&cdev);

 device_destroy(my_class, devno);         //delete device node under /dev//必须先删除设备，再删除class类
 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_init);
module_exit (hello_exit);

这样，模块加载后，就能在/dev目录下找到memdev这个设备节点了。