[linux device driver] Chapter 03:字符设备注册学习

从函数scullc_init中学习register_chrdev_region，进而进入到__register_chrdev_region，主要还是研究__register_chrdev_region这个函数。

__register_chrdev_region：

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 = kmalloc(sizeof(struct char_device_struct), GFP_KERNEL);if (cd == NULL)return ERR_PTR(-ENOMEM);memset(cd, 0, sizeof(struct char_device_struct));write_lock_irq(&chrdevs_lock);/* temporary */ //如果major=0，那么就是动态分配，函数从chrdevs中找到未分配的数组元素分配。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;cd->name = name;i = major_to_index(major); //这个循环的作用是从major对应的散列表中找到一项*cp = null，这样用于分配。for (cp = &chrdevs[i]; *cp; cp = &(*cp)->next) if ((*cp)->major > major || ((*cp)->major == major && (*cp)->baseminor >= baseminor))break;if (*cp && (*cp)->major == major && (*cp)->baseminor < baseminor + minorct) {ret = -EBUSY;goto out;}cd->next = *cp;//此时*cp = null*cp = cd; //把已经赋值好的cd挂到cp上，其实就是chrdevs[i]下对应的某个项write_unlock_irq(&chrdevs_lock);return cd;out:write_unlock_irq(&chrdevs_lock);kfree(cd);return ERR_PTR(ret);}

从上面可以看出，__register_chrdev_region完成的工作主要有三个步骤：

1. 分配出一个char_device_struct 结构，使用对应的入参赋值。

2.从chrdevs上找到对应的major号的chrdevs[major]，然后从这个散列表中一级一级往下查找对应的minor号，最近将已经分配好的char_device_struct挂载到chrdevs全局变量中。

值得注意的是__register_chrdev_region的这个部分：

for (cp = &chrdevs[i]; *cp; cp = &(*cp)->next)if ((*cp)->major > major ||    ((*cp)->major == major && (*cp)->baseminor >= baseminor))break;if (*cp && (*cp)->major == major &&    (*cp)->baseminor < baseminor + minorct) {ret = -EBUSY;goto out;}cd->next = *cp;*cp = cd;

这段代码的主要作用是从chrdevs[i]

chrdevs是一个255个char_device_struct的数组指针：

static struct char_device_struct {struct char_device_struct *next;unsigned int major;unsigned int baseminor;int minorct;const char *name;struct file_operations *fops;struct cdev *cdev;/* will die */} *chrdevs[MAX_PROBE_HASH];

它是一个散列表，取得i项chardevs后，函数不停的判断(*cp)->major是否大于或者是等于且baseminor也大于等于baseminor,这样最后在正常情况下会因为*cp=null而退出，从而cd->next = null,而*cp=cd，将cd挂载到*cd上，其实也就是chrdevs下的某一项。相对来说介绍详细的文章是这个：http://blog.csdn.net/yel617/article/details/5627505