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今天我們通過(guò)內(nèi)核中的源碼共同分析一下Linux設(shè)備驅(qū)動(dòng)中申請(qǐng)?jiān)O(shè)備號(hào)的過(guò)程，首先在Linux內(nèi)核為我們提供了兩種申請(qǐng)?jiān)O(shè)備號(hào)的方式，一是指定設(shè)備號(hào)注冊(cè)，二是動(dòng)態(tài)分配設(shè)備號(hào)；分別用到一下兩個(gè)函數(shù)：

 register_chrdev_region();

 alloc_chrdev_region();

設(shè)備號(hào)的數(shù)據(jù)類(lèi)型是dev_t類(lèi)型，是一個(gè)無(wú)符號(hào)長(zhǎng)整型，在32位操作系統(tǒng)中，它的大小是4個(gè)字節(jié)，32位，高12位用來(lái)存放主設(shè)備號(hào)，低20位用來(lái)存放次設(shè)備號(hào)。

通過(guò)主設(shè)備號(hào)和次設(shè)備號(hào)合成設(shè)備號(hào)的宏如下：

MKDEV(major,minor);

從設(shè)備號(hào)中提取主設(shè)備號(hào)和次設(shè)備號(hào)的宏如下：

MAJOR（dev）；

MINOR（dev）；

接下來(lái)我們就來(lái)分析設(shè)備號(hào)注冊(cè)過(guò)程：

一、自動(dòng)分配設(shè)備號(hào)：

1.  調(diào)用內(nèi)核提供的自動(dòng)分配設(shè)備號(hào)函數(shù)，分配設(shè)備號(hào) 

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

2. alloc_chrdev_region函數(shù)中繼續(xù)調(diào)用如下函數(shù)，進(jìn)行進(jìn)一步申請(qǐng) 

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

/*在該函數(shù)中進(jìn)行判斷，如果major==0,執(zhí)行if內(nèi)的語(yǔ)句*/

if (major == 0) {                     //自動(dòng)分配時(shí)major == 0

/*

*chrdevs是一個(gè)結(jié)構(gòu)體指針數(shù)組，見(jiàn)附錄1-1

*作用是從struct char_device_struct類(lèi)型的結(jié)構(gòu)體指針數(shù)組中找到一個(gè)空的結(jié)構(gòu)體指針

*/

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 ,說(shuō)明，存放設(shè)備號(hào)的結(jié)構(gòu)體指針數(shù)組已經(jīng)用完

ret = -EBUSY;

goto out;

}

major = i;         //否則，主設(shè)備號(hào) = i

ret = major;

cd->major = major;                       // 將主設(shè)備號(hào)賦值給cd->major

cd->baseminor = baseminor;       // 將起始次設(shè)備號(hào)賦值cd->baseminor

cd->minorct = minorct;    // 將傳入的第三個(gè)參數(shù)賦值給程cd->minorct,

                                                     // 表示申請(qǐng)?jiān)O(shè)備號(hào)的個(gè)數(shù)

strlcpy(cd->name, name, sizeof(cd->name));  //設(shè)備名拷貝

/*

*struct char_device_struct *cd, **cp;

*cp存放的是結(jié)構(gòu)體指針的地址，基于當(dāng)前的條件，*cp == NULL ，所以for循環(huán)條件不成立

*/

--------------------------------------不執(zhí)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 ，所以條件不成立*/

---------------------------------------不執(zhí)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 結(jié)構(gòu)體在上面賦值的

mutex_unlock(&chrdevs_lock);

return cd;

回到alloc_chrdev_region函數(shù)的*dev = MKDEV(cd->major, cd->baseminor);

實(shí)際上就申請(qǐng)了一個(gè)主設(shè)備號(hào)，次設(shè)備號(hào)

二、用戶指定設(shè)備號(hào)注冊(cè)（eg：major = 250；count=3）

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

2. 在該函數(shù)中進(jìn)行如下操作：

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

dev_t n, next;

/*如果申請(qǐng)的設(shè)備編號(hào)范圍跨越了主設(shè)備號(hào)，

*它會(huì)把分配范圍內(nèi)的編號(hào)按主設(shè)備號(hào)分割

 *成較小的子范圍，并在每個(gè)子范圍上調(diào)用

 *__register_chrdev_region() 。如果其中

*有一次分配失敗的話，那會(huì)把之前成功分配的都全部退回*/

// for(n = MKDEV(dev_major,dev_minor); n<to;n=next)循環(huán)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))))    //正常情況下不會(huì)成立，即次設(shè)備號(hào)范圍不會(huì)重復(fù)

break;

 

/*判斷次設(shè)備號(hào)范圍是否重復(fù)，如果重復(fù)就錯(cuò)誤返回*/

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;

}

}

/*將設(shè)備號(hào)的結(jié)構(gòu)體插入到散列表中*/

 cd->next = *cp

 *cp = cd;

 }

附錄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];

附錄1-2