Linux内核register_chrdev_region()系列函数

Linux内核中所有已分配的字符设备编号都记录在一个名为 chrdevs 散列表里。该散列表中的每一个元素是一个 char_device_struct 结构，它的定义如下：

   static struct char_device_struct {
       struct char_device_struct *next;    // 指向散列冲突链表中的下一个元素的指针
       unsigned int major;                 // 主设备号
       unsigned int baseminor;             // 起始次设备号
       int minorct;                        // 设备编号的范围大小
       char name[64];                      // 处理该设备编号范围内的设备驱动的名称
       struct file_operations *fops;       // 没有使用
       struct cdev *cdev;                  // 指向字符设备驱动程序描述符的指针
   } *chrdevs[CHRDEV_MAJOR_HASH_SIZE];

注册
内核提供了三个函数来注册一组字符设备编号，这三个函数分别是 register_chrdev_region()、alloc_chrdev_region() 和 register_chrdev()。这三个函数都会调用一个共用的 __register_chrdev_region() 函数来注册一组设备编号范围（即一个 char_device_struct 结构）。

register_chrdev_region(dev_t first,unsigned int count,char *name)
First :要分配的设备编号范围的初始值(次设备号常设为0);
Count:连续编号范围.
Name:编号相关联的设备名称. (/proc/devices);
动态分配:
Int alloc_chrdev_region(dev_t *dev,unsigned int firstminor,unsigned int count,char *name);
Firstminor : 通常为0;
*dev:存放返回的设备号;
释放:
Void unregist_chrdev_region(dev_t first,unsigned int count);
调用Documentation/devices.txt中能够找到已分配的设备号．

所以下面先来看一下 __register_chrdev_region() 函数的实现代码。

static struct char_device_struct * __register_chrdev_region(unsigned int major, unsigned int baseminor, int minorct, const char *name)
{
   struct char_device_struct *cd, **cp;
   int ret = 0;
   int i;

   cd = kzalloc(sizeof(struct char_device_struct), GFP_KERNEL);
   if (cd == NULL)
       return ERR_PTR(-ENOMEM);

   mutex_lock(&chrdevs_lock);

   if (major == 0) {
        for (i = ARRAY_SIZE(chrdevs)-1; i > 0; i--)
           if (chrdevs[i] == NULL)
               break;

       if (i == 0) {
           ret = -EBUSY;
           goto out;
       }
       major = i;
       ret = major;
   }

   cd->major = major;
   cd->baseminor = baseminor;
   cd->minorct = minorct;
   strncpy(cd->name,name, 64);

   i = major_to_index(major);

   for (cp = &chrdevs[i]; *cp; cp = &(*cp)->next)
       if ((*cp)->major > major ||
            ((*cp)->major == major && ( ((*cp)->baseminor >= baseminor) || ((*cp)->baseminor + (*cp)->minorct > baseminor)) ))
           break;

 
   if (*cp && (*cp)->major == major) {
       int old_min = (*cp)->baseminor;
       int old_max = (*cp)->baseminor + (*cp)->minorct - 1;
       int new_min = baseminor;
       int new_max = baseminor + minorct - 1;

     
       if (new_max >= old_min && new_max <= old_max) {
           ret = -EBUSY;
           goto out;
       }

     
       if (new_min <= old_max && new_min >= old_min) {
           ret = -EBUSY;
           goto out;
       }
   }

   cd->next = *cp;
   *cp = cd;
   mutex_unlock(&chrdevs_lock);
   return cd;
out:
   mutex_unlock(&chrdevs_lock);
   kfree(cd);
   return ERR_PTR(ret);
}