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Linux设备驱动申请设备号过程分析
时间:2017-11-22作者:华清远见

今天我们通过内核中的源码共同分析一下Linux设备驱动中申请设备号的过程,首先在Linux内核为我们提供了两种申请设备号的方式,一是指定设备号注册,二是动态分配设备号;分别用到一下两个函数:

register_chrdev_region();

alloc_chrdev_region();

设备号的数据类型是dev_t类型,是一个无符号长整型,在32位操作系统中,它的大小是4个字节,32位,高12位用来存放主设备号,低20位用来存放次设备号。

通过主设备号和次设备号合成设备号的宏如下:

MKDEV(major,minor);

从设备号中提取主设备号和次设备号的宏如下:

MAJOR(dev);

MINOR(dev);

接下来我们就来分析设备号注册过程:

一、自动分配设备号:

1.  调用内核提供的自动分配设备号函数,分配设备号 

int alloc_chrdev_region(dev_t *dev, unsigned baseminor, unsigned count,const char *name)  

2. alloc_chrdev_region函数中继续调用如下函数,进行进一步申请 

struct char_device_struct *cd =  __register_chrdev_region(0, baseminor, count, name);

/*在该函数中进行判断,如果major==0,执行if内的语句*/

if (major == 0) {                     //自动分配时major == 0

/*

*chrdevs是一个结构体指针数组,见附录1-1

*作用是从struct char_device_struct类型的结构体指针数组中找到一个空的结构体指针

*/

for (i = ARRAY_SIZE(chrdevs)-1; i > 0; i--) {  // for(i=254;i>0;i--)

if (chrdevs[i] == NULL)

break;

}

if (i == 0) {      //i == 0 ,说明,存放设备号的结构体指针数组已经用完

ret = -EBUSY;

goto out;

}

major = i;         //否则,主设备号 = i

ret = major;

cd->major = major;                       // 将主设备号赋值给cd->major

cd->baseminor = baseminor;       // 将起始次设备号赋值cd->baseminor

cd->minorct = minorct;    // 将传入的第三个参数赋值给程cd->minorct,

                                                     // 表示申请设备号的个数

strlcpy(cd->name, name, sizeof(cd->name));  //设备名拷贝

/*

*struct char_device_struct *cd, **cp;

*cp存放的是结构体指针的地址,基于当前的条件,*cp == NULL ,所以for循环条件不成立

*/

--------------------------------------不执行-------------------------------------

for (cp = &chrdevs[i]; *cp; cp = &(*cp)->next)          

if ((*cp)->major > major ||((*cp)->major == major &&(((*cp)->baseminor >= baseminor) ||((*cp)->baseminor + (*cp)->minorct > baseminor))))

break;

---------------------------------------------------------------------------------------

/*     **cp == NULL ,所以条件不成立*/

---------------------------------------不执行-----------------------------------------

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;                  //cd ->next = NULL

*cp = cd; //*cp[chrdevs[i]] , cd 结构体在上面赋值的

mutex_unlock(&chrdevs_lock);

return cd;

回到alloc_chrdev_region函数的*dev = MKDEV(cd->major, cd->baseminor);

实际上就申请了一个主设备号,次设备号

二、用户指定设备号注册(eg:major = 250;count=3)

1. int register_chrdev_region(dev_t from, unsigned count, const char *name);

2. 在该函数中进行如下操作:

dev_t to = from + count;      // dev_t to =  MKDEV(dev_major,dev_minor) +3;

dev_t n, next;

/*如果申请的设备编号范围跨越了主设备号,

*它会把分配范围内的编号按主设备号分割

 *成较小的子范围,并在每个子范围上调用

 *__register_chrdev_region() 。如果其中

*有一次分配失败的话,那会把之前成功分配的都全部退回*/

// for(n = MKDEV(dev_major,dev_minor); n<to;n=next)循环一次(next = to)

for (n = from; n < to; n = next) {

next = MKDEV(MAJOR(n)+1, 0);         // next = MKDEV(250+1,0);

if (next > to)                       // if(MKDEV(251,0)>MKDEV(250,0)+3) [成立]

next = to;                          // next = MKDEV(250,0)+3

cd = __register_chrdev_region(MAJOR(n), MINOR(n),next - n, name);

//__register_chrdev_region(250, 0,3, "xxx_demo");

      {

 cd->major = major;           //cd->major = 250;

cd->baseminor = baseminor;   //cd->baseminor = 0;

cd->minorct = minorct;           //cd->minorct  = 3;

strlcpy(cd->name, name, sizeof(cd->name));

i = major_to_index(major);   //i = 250;

/*for (cp = &chrdevs[250]; *cp; cp = &(*cp)->next)*/

for (cp = &chrdevs[i]; *cp; cp = &(*cp)->next)        

if((*cp)->major > major || ((*cp)->major == major &&(((*cp)->baseminor >= baseminor) ||((*cp)->baseminor + (*cp)->minorct > baseminor))))    //正常情况下不会成立,即次设备号范围不会重复

break;

 

/*判断次设备号范围是否重复,如果重复就错误返回*/

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;

}

}

/*将设备号的结构体插入到散列表中*/

 cd->next = *cp

 *cp = cd;

 }

附录1-1:

 

#define CHRDEV_MAJOR_HASH_SIZE 255

static struct char_device_struct {

struct char_device_struct *next;

unsigned int major;

unsigned int baseminor;

int minorct;

char name[64];

struct cdev *cdev; /* will die */

} *chrdevs[CHRDEV_MAJOR_HASH_SIZE];

附录1-2


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