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一、旧接口register_chrdev()函数 

上文说到，旧接口register_chrdev()函数内部同时完成了设备号的分配与驱动的注册，现在我们来分析是否真的如此。

1、函数的调用层次关系

                |..............register_chrdev

                |....................__register_chrdev

                |................................__register_chrdev_region（申请设备号）

                |................................dev_alloc（这下面三个是在注册设备驱动）

                |................................其他代码（效果如同cdev_init）

                |................................cdev_add

2、函数内部代码概览

 （1）register_chrdev()函数

此函数定义在/include/linux/fs.h文件中。函数的内容如下：

static inline int register_chrdev(unsigned int major, const char *name,const struct file_operations *fops)
{return __register_chrdev(major, 0, 256, name, fops);
}

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（2）__register_chrdev()函数

此函数定义在/fs/char_dev.c文件中。此函数的内容如下：

/*** __register_chrdev() - create and register a cdev occupying a range of minors* @major: major device number or 0 for dynamic allocation* @baseminor: first of the requested range of minor numbers* @count: the number of minor numbers required* @name: name of this range of devices* @fops: file operations associated with this devices** If @major == 0 this functions will dynamically allocate a major and return* its number.** If @major > 0 this function will attempt to reserve a device with the given* major number and will return zero on success.** Returns a -ve errno on failure.** The name of this device has nothing to do with the name of the device in* /dev. It only helps to keep track of the different owners of devices. If* your module name has only one type of devices it's ok to use e.g. the name* of the module here.*/
int __register_chrdev(unsigned int major, unsigned int baseminor,unsigned int count, const char *name,const struct file_operations *fops)
{struct char_device_struct *cd;struct cdev *cdev;int err = -ENOMEM;cd = __register_chrdev_region(major, baseminor, count, name);//获取设备号if (IS_ERR(cd))return PTR_ERR(cd);cdev = cdev_alloc(); if (!cdev)goto out2;//对cdev结构体（设备的象征）成员的填充，尤其是ops（驱动的实体函数指针）的填充，//体现了设备与驱动的绑定与关联//后面再用cdev_add函数注册设备时，其实就相当于注册驱动，因为设备已经和驱动关联//这两句代码相当于cdev_init()函数的作用。cdev->owner = fops->owner; cdev->ops = fops;kobject_set_name(&cdev->kobj, "%s", name);err = cdev_add(cdev, MKDEV(cd->major, baseminor), count);if (err)goto out;cd->cdev = cdev;return major ? 0 : cd->major;
out:kobject_put(&cdev->kobj);
out2:kfree(__unregister_chrdev_region(cd->major, baseminor, count));return err;
}

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（3）__register_chrdev_region()函数

此函数定义在/fs/char_dev.c文件中。此函数的内容见本文的第三点。

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（4）dev_alloc()函数、cdev_add()函数

这些函数的说明，见博客：字符设备驱动高级篇1——注册字符设备驱动的新接口

二、新接口函数

新接口函数包括许多，这里只介绍分配设备号的函数，包括register_chrdev_region()函数和alloc_chrdev_region()函数，前者用于获取指定设备号，后者让内核自动分配设备号。

1、函数的调用关系

|..............register_chrdev_region

|....................__register_chrdev_region

或者

|..............alloc_chrdev_region

|....................__register_chrdev_region

可见内部都调用了__register_chrdev_region这个函数。

2、函数内部代码概览

（1）register_chrdev_region()函数

此函数定义在/fs/char_dev.c文件中。此函数的内容如下：

/*** 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)+1, 0);if (next > to)next = to;cd = __register_chrdev_region(MAJOR(n), MINOR(n),next - n, name);if (IS_ERR(cd))goto fail;}return 0;
fail:to = n;for (n = from; n < to; n = next) {next = MKDEV(MAJOR(n)+1, 0);kfree(__unregister_chrdev_region(MAJOR(n), MINOR(n), next - n));}return PTR_ERR(cd);
}

可知此函数最终调用了__register_chrdev_region函数。

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（2）alloc_chrdev_region()函数

此函数定义在/fs/char_dev.c文件中。此函数的内容如下：

/*** 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(0, baseminor, count, name);if (IS_ERR(cd))return PTR_ERR(cd);*dev = MKDEV(cd->major, cd->baseminor);return 0;
}

可知此函数最终调用了__register_chrdev_region函数。

三、分析__register_chrdev_region函数

老接口register_chrdev函数调用关系如下：

                |..............register_chrdev

                |....................__register_chrdev

                |................................__register_chrdev_region

                |................................dev_alloc

                |................................cdev_add

新街口register_chrdev_region、alloc_chrdev_region调用关系如下：

                |..............register_chrdev_region

                |....................__register_chrdev_region

                或者

                |..............alloc_chrdev_region

                |....................__register_chrdev_region

从上面的分析得知，新接口和老接口内部都调用了__register_chrdev_region()函数。此函数用于向内核申请设备号。现在我们来重点分析函数__register_chrdev_region。

__register_chrdev_region()函数定义在/fs/char_dev.c文件中，内容如下：

/** Register a single major with a specified minor range.** If major == 0 this functions will dynamically allocate a major and return* its number.** If major > 0 this function will attempt to reserve the passed range of* minors and will return zero on success.** Returns a -ve errno on failure.*/
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;// 分配一个新的 char_device_struct 结构，并用 0 填充。cd = kzalloc(sizeof(struct char_device_struct), GFP_KERNEL);if (cd == NULL)return ERR_PTR(-ENOMEM);mutex_lock(&chrdevs_lock);/* temporary */if (major == 0) {//如果申请的设备编号范围的主设备号为 0for (i = ARRAY_SIZE(chrdevs)-1; i > 0; i--) {//从最后一个倒着来找if (chrdevs[i] == NULL) //如果为空（这整个指针数组什么时候赋值为空的）break;}if (i == 0) {ret = -EBUSY;goto out;}major = i; //主设备号就是这个ret = major;}//根据参数设置struct char_device_struct变量的值cd->major = major;  //如果为0，则分配刚才得到的//如果不为0，则分配自身设置的cd->baseminor = baseminor;cd->minorct = minorct;strlcpy(cd->name, name, sizeof(cd->name));i = major_to_index(major);//major % BLKDEV_MAJOR_HASH_SIZE;即major%255//这里从自身设置编号时的角度考虑，major为0时这步没意义?for (cp = &chrdevs[i]; *cp; cp = &(*cp)->next)if ((*cp)->major > major ||((*cp)->major == major &&(((*cp)->baseminor >= baseminor) ||((*cp)->baseminor + (*cp)->minorct > baseminor))))break;/* Check for overlapping minor ranges.  */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;/* New driver overlaps from the left.  */if (new_max >= old_min && new_max <= old_max) {ret = -EBUSY;goto out;}/* New driver overlaps from the right.  */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);
}

（1）函数说明

/** Register a single major with a specified minor range.* If major == 0 this functions will dynamically allocate a major and return its number.* If major > 0 this function will attempt to reserve the passed range of* minors and will return zero on success.* Returns a -ve errno on failure.*/

（2）参数说明

1）参数 major

如果major大于0，则major表示想要申请的主设备号，函数返回0表示申请成功。

如果major等于0，则表示让内核自动分配主设备号，函数成功则返回所分配的主设备号。

2）参数 baseminor

表示次设备号的起始编号。

3）参数 minorct

表示次设备号的数目。

4）参数 name

表示设备（或者说驱动）的名字。

5）返回值类型：struct char_device_struct

这个结构体定义如下：

static struct char_device_struct {struct char_device_struct *next;unsigned int major;               //主设备的编号unsigned int baseminor;           //次设备的起始编号int minorct;                      //次设备的数目char name[64];                    //设备（或者说驱动）的名字struct cdev *cdev;	              //指向  字符设备驱动程序描述符  的指针
} *chrdevs[CHRDEV_MAJOR_HASH_SIZE];  //#define CHRDEV_MAJOR_HASH_SIZE	255