一. misc結構體

struct miscdevice  {
	int minor;		//次裝置號
	const char *name;		//裝置名
	const struct file_operations *fops;	//操作函式集
	struct list_head list;	//連結串列頭
	struct device *parent;	//父裝置裝置檔案
	struct device *this_device;	//裝置檔案
	const char *nodename;	//
	mode_t mode;		//
};

二. 裝置號
    misc裝置的主裝置號為10

#define MISC_MAJOR		10
 

三. misc裝置系統的初始化 misc_init

static int __init misc_init(void)
{
	int err;

#ifdef CONFIG_PROC_FS
	proc_create("misc", 0, NULL, &misc_proc_fops);
#endif
	misc_class = class_create(THIS_MODULE, "misc");	//建立目錄"/sys/class/misc"
	err = PTR_ERR(misc_class);
	if (IS_ERR(misc_class))
		goto fail_remove;

	err = -EIO;
	if (register_chrdev(MISC_MAJOR,"misc",&misc_fops))	//註冊為字元裝置,捆綁操作函式集
		goto fail_printk;
	misc_class->devnode = misc_devnode;
	return 0;

fail_printk:
	printk("unable to get major %d for misc devices\n", MISC_MAJOR);
	class_destroy(misc_class);
fail_remove:
	remove_proc_entry("misc", NULL);
	return err;
}
 

再看一下register_chrdev

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

這裡直接根據主裝置號建立256個裝置,256=2^8,所以它是把所有的雜項裝置都給註冊成字元裝置了
misc_fops作為這所有misc裝置捆綁的操作函式集

static const struct file_operations misc_fops = {
	.owner		= THIS_MODULE,
	.open		= misc_open,
	.llseek		= noop_llseek,
};
 

在misc_open中,也就是我們開啟misc裝置的時候,misc_open方法會為我們捆綁自定義的操作函式集

static int misc_open(struct inode * inode, struct file * file)
{
	int minor = iminor(inode);		//根據裝置節點獲取次裝置號
	struct miscdevice *c;
	int err = -ENODEV;
	const struct file_operations *old_fops, *new_fops = NULL;

	mutex_lock(&misc_mtx);
	
	list_for_each_entry(c, &misc_list, list) {	//遍歷misc_list檢視次裝置是否註冊(misc_list元素在設備註冊時新增)
		if (c->minor == minor) {
			new_fops = fops_get(c->fops);	//獲取自定義的操作函式集		
			break;
		}
	}
		
	if (!new_fops) {			//若找不到則再找一次,還不行error
		mutex_unlock(&misc_mtx);
		request_module("char-major-%d-%d", MISC_MAJOR, minor);
		mutex_lock(&misc_mtx);

		list_for_each_entry(c, &misc_list, list) {
			if (c->minor == minor) {
				new_fops = fops_get(c->fops);
				break;
			}
		}
		if (!new_fops)
			goto fail;
	}

	err = 0;
	old_fops = file->f_op;	//獲取指向舊的裝置操作函式集(misc_fops)指標
	file->f_op = new_fops;	//檔案操作函式集指標指向新的操作函式集
	if (file->f_op->open) {	//若存在open方法
		file->private_data = c;
		err=file->f_op->open(inode,file);	//則呼叫其自定義的open方法
		if (err) {
			fops_put(file->f_op);
			file->f_op = fops_get(old_fops);
		}
	}
	fops_put(old_fops);	//釋放舊的函式集指標
fail:
	mutex_unlock(&misc_mtx);
	return err;
}

四. misc設備註冊 misc_register

int misc_register(struct miscdevice * misc)
{
	struct miscdevice *c;
	dev_t dev;
	int err = 0;

	INIT_LIST_HEAD(&misc->list);	//初始化連結串列頭

	mutex_lock(&misc_mtx);
	list_for_each_entry(c, &misc_list, list) {	//遍歷misc_list
		if (c->minor == misc->minor) {	//檢視裝置號是否被佔用
			mutex_unlock(&misc_mtx);
			return -EBUSY;
		}
	}

	if (misc->minor == MISC_DYNAMIC_MINOR) {	//判斷是否指定要動態分配次裝置號
		int i = find_first_zero_bit(misc_minors, DYNAMIC_MINORS);	//查詢可用次裝置號
		if (i >= DYNAMIC_MINORS) {
			mutex_unlock(&misc_mtx);
			return -EBUSY;
		}
		misc->minor = DYNAMIC_MINORS - i - 1;	//分配次裝置號	
		set_bit(i, misc_minors);		//設定佔用標誌位
	}

	dev = MKDEV(MISC_MAJOR, misc->minor);	//根據主次裝置號獲取裝置號

	misc->this_device = device_create(misc_class, misc->parent, dev,misc, "%s", misc->name);	//新增裝置檔案"/dev/xxx"
	if (IS_ERR(misc->this_device)) {
		int i = DYNAMIC_MINORS - misc->minor - 1;
		if (i < DYNAMIC_MINORS && i >= 0)
			clear_bit(i, misc_minors);
		err = PTR_ERR(misc->this_device);
		goto out;
	}

	/*
	 * Add it to the front, so that later devices can "override"
	 * earlier defaults
	 */
	list_add(&misc->list, &misc_list);	//將裝置連結串列頭新增到misc_list中
 out:
	mutex_unlock(&misc_mtx);
	return err;
}

五. misc設備註銷 misc_deregister

int misc_deregister(struct miscdevice *misc)
{
	int i = DYNAMIC_MINORS - misc->minor - 1;	//通過次裝置號計算出位標誌

	if (WARN_ON(list_empty(&misc->list)))
		return -EINVAL;

	mutex_lock(&misc_mtx);
	list_del(&misc->list);	//從misc_list刪除裝置連結串列頭
	device_destroy(misc_class, MKDEV(MISC_MAJOR, misc->minor));	//刪除裝置節點
	if (i < DYNAMIC_MINORS && i >= 0)
		clear_bit(i, misc_minors);	//清除位標誌
	mutex_unlock(&misc_mtx);
	return 0;
}

六. misc簡單例項

#include <linux/fs.h>
#include <linux/module.h>
#include <linux/miscdevice.h>

static int test_open(struct inode * inode, struct file * filp)
{
	return 0;
}

static const struct file_operations test_fops = {
	.owner = THIS_MODULE,
	.open  = test_open,
};

static struct miscdevice misc_dev={
	.minor=MISC_DYNAMIC_MINOR,
	.name="misc_test",
	.fops=&test_fops
};

static int __init test_init(void)
{
	return misc_register(&misc_dev);
}

static void __exit  test_exit(void)
{
	misc_deregister(&misc_dev);
}

module_init(test_init);
module_exit(test_exit);
MODULE_LICENSE("GPL");

七. 檢視misc資訊的方法

載入模組後

lsmod |grep test
test                     829  0 

ls -l /dev/misc_test 
crw-rw---- 1 root root 10, 54 2012-12-18 10:28 /dev/misc_test

ls -l /sys/class/misc/ |grep misc_test
lrwxrwxrwx 1 root root 0 2012-12-18 10:33 misc_test -> ../../devices/virtual/misc/misc_test

cat /proc/misc |grep misc_test
 54 misc_test