Linux設備驅動申請設備號過程分析
時間：2017-11-22 來源：未知
今天我們通過內核中的源碼共同分析一下Linux設備驅動中申請設備號的過程，首先在Linux內核為我們提供了兩種申請設備號的方式，一是指定設備號注冊，二是動態分配設備號；分別用到一下兩個函數：
register_chrdev_region();
alloc_chrdev_region();
設備號的數據類型是dev_t類型，是一個無符號長整型，在32位操作系統中，它的大小是4個字節，32位，高12位用來存放主設備號，低20位用來存放次設備號。
通過主設備號和次設備號合成設備號的宏如下：
MKDEV(major,minor);
從設備號中提取主設備號和次設備號的宏如下：
MAJOR（dev）；
MINOR（dev）；
接下來我們就來分析設備號注冊過程：
一、自動分配設備號：
1. 調用內核提供的自動分配設備號函數，分配設備號
int alloc_chrdev_region(dev_t *dev, unsigned baseminor, unsigned count,const char *name)
2. alloc_chrdev_region函數中繼續調用如下函數，進行進一步申請
struct char_device_struct *cd = __register_chrdev_region(0, baseminor, count, name);
/*在該函數中進行判斷，如果major==0,執行if內的語句*/
if (major == 0) { //自動分配時major == 0
/*
*chrdevs是一個結構體指針數組，見附錄1-1
*作用是從struct char_device_struct類型的結構體指針數組中找到一個空的結構體指針
*/
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 ,說明，存放設備號的結構體指針數組已經用完
ret = -EBUSY;
goto out;
}
major = i; //否則，主設備號 = i
ret = major;
cd->major = major; // 將主設備號賦值給cd->major
cd->baseminor = baseminor; // 將起始次設備號賦值cd->baseminor
cd->minorct = minorct; // 將傳入的第三個參數賦值給程cd->minorct,
// 表示申請設備號的個數
strlcpy(cd->name, name, sizeof(cd->name)); //設備名拷貝
/*
*struct char_device_struct *cd, **cp;
*cp存放的是結構體指針的地址，基于當前的條件，*cp == NULL ，所以for循環條件不成立
*/
--------------------------------------不執行-------------------------------------
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 結構體在上面賦值的
mutex_unlock(&chrdevs_lock);
return cd;
回到alloc_chrdev_region函數的*dev = MKDEV(cd->major, cd->baseminor);
實際上就申請了一個主設備號，次設備號
二、用戶指定設備號注冊（eg：major = 250；count=3）
1. int register_chrdev_region(dev_t from, unsigned count, const char *name);
2. 在該函數中進行如下操作：
dev_t to = from + count; // dev_t to = MKDEV(dev_major,dev_minor) +3;
dev_t n, next;
/*如果申請的設備編號范圍跨越了主設備號，
*它會把分配范圍內的編號按主設備號分割
*成較小的子范圍，并在每個子范圍上調用
*__register_chrdev_region() 。如果其中
*有一次分配失敗的話，那會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)))) //正常情況下不會成立，即次設備號范圍不會重復
break;
/*判斷次設備號范圍是否重復，如果重復就錯誤返回*/
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;
}
}
/*將設備號的結構體插入到散列表中*/
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
