cdev结构体

在内核源码include/linux/cdev.h里对cdev结构体的定义:

 struct cdev {
     struct kobject kobj;                    // 内嵌的kobject对象 
     struct module *owner;                   // 所属模块
     const struct file_operations *ops;      // 文件操作结构体
     struct list_head list;                  //linux内核所维护的链表指针
     dev_t dev;                              //设备号
     unsigned int count;                     //设备数目
 };

1. 重要成员：

1.1 dev_t dev;设备号，31位，高12位是主设备号，低20位是次设备号。以下函数可以操作设备号：

 #define MINORBITS    20
 #define MINORMASK    ((1U << MINORBITS) - 1)

 #define MAJOR(dev)    ((unsigned int) ((dev) >> MINORBITS))   //获得主设备号
 #define MINOR(dev)    ((unsigned int) ((dev) & MINORMASK))    //获得此设备号
 #define MKDEV(ma,mi)    (((ma) << MINORBITS) | (mi))          //由主次设备号得到设备号

1.2 struct file_operations *ops；

2. 初始化cdev

2.1 静态初始化

 /**
  * cdev_init() - initialize a cdev structure
  * @cdev: the structure to initialize
  * @fops: the file_operations for this device
  *
  * Initializes @cdev, remembering @fops, making it ready to add to the
  * system with cdev_add().
  */
 void cdev_init(struct cdev *cdev, const struct file_operations *fops)
 {
     memset(cdev, , sizeof *cdev);
     INIT_LIST_HEAD(&cdev->list);
     kobject_init(&cdev->kobj, &ktype_cdev_default);
     cdev->ops = fops;
 }

2.2 动态初始化

 /**
  * cdev_alloc() - allocate a cdev structure
  *
  * Allocates and returns a cdev structure, or NULL on failure.
  */
 struct cdev *cdev_alloc(void)
 {
     struct cdev *p = kzalloc(sizeof(struct cdev), GFP_KERNEL);
     if (p) {
         INIT_LIST_HEAD(&p->list);
         kobject_init(&p->kobj, &ktype_cdev_dynamic);
     }
     return p;
 }

3. 分配设备号

3.1 指定主设备号

 /**
  * register_chrdev_region() - register a range of device numbers
  * @from: the first in the desired range of device numbers; must include
  *        the major number.
  * @count: the number of consecutive device numbers required
  * @name: the name of the device or driver.
  *
  * Return value is zero on success, a negative error code on failure.
  */
 int register_chrdev_region(dev_t from, unsigned count, const char *name)
 {
     struct char_device_struct *cd;
     dev_t to = from + count;
     dev_t n, next;

     for (n = from; n < to; n = next) {
         next = MKDEV(MAJOR(n)+, );
         if (next > to)
             next = to;
         cd = __register_chrdev_region(MAJOR(n), MINOR(n),
                    next - n, name);
         if (IS_ERR(cd))
             goto fail;
     }
     return ;
 fail:
     to = n;
     for (n = from; n < to; n = next) {
         next = MKDEV(MAJOR(n)+, );
         kfree(__unregister_chrdev_region(MAJOR(n), MINOR(n), next - n));
     }
     return PTR_ERR(cd);
 }

3.2 系统自动分配主设备号

 /**
  * alloc_chrdev_region() - register a range of char device numbers
  * @dev: output parameter for first assigned number
  * @baseminor: first of the requested range of minor numbers
  * @count: the number of minor numbers required
  * @name: the name of the associated device or driver
  *
  * Allocates a range of char device numbers.  The major number will be
  * chosen dynamically, and returned (along with the first minor number)
  * in @dev.  Returns zero or a negative error code.
  */
 int alloc_chrdev_region(dev_t *dev, unsigned baseminor, unsigned count,
             const char *name)
 {
     struct char_device_struct *cd;
     cd = __register_chrdev_region(, baseminor, count, name);
     if (IS_ERR(cd))
         return PTR_ERR(cd);
     *dev = MKDEV(cd->major, cd->baseminor);
     return ;
 }

4 注销设备号

与上述两个分配设备号对应的注销设备号的函数如下：

 /**
  * unregister_chrdev_region() - return a range of device numbers
  * @from: the first in the range of numbers to unregister
  * @count: the number of device numbers to unregister
  *
  * This function will unregister a range of @count device numbers,
  * starting with @from.  The caller should normally be the one who
  * allocated those numbers in the first place...
  */
 void unregister_chrdev_region(dev_t from, unsigned count)
 {
     dev_t to = from + count;
     dev_t n, next;

     for (n = from; n < to; n = next) {
         next = MKDEV(MAJOR(n)+, );
         if (next > to)
             next = to;
         kfree(__unregister_chrdev_region(MAJOR(n), MINOR(n), next - n));
     }
 }