Linux字符设备驱动基础（一）

素颜马尾好姑娘i 2022-01-21 20:17 439阅读 0赞

# Linux字符设备驱动基础（一） #

## 1 Linux驱动分类 ##

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*  字符设备：按照字节的顺序读写的设备，其不能随机的读取设备内存中的某一数据。字符设备是面向流的设备，常见的字符设备有串口、控制台、LED等。
 *  块设备：通常是指可从设备的任意位置读取到一定长度数据的设备。常见的块设备有：SD卡、U盘、磁盘等。

## 2 字符设备、字符设备驱动与用户空间三者之间的关系 ##

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### 1）在Linux内核中： ###

*  a – 使用cdev结构体来描述字符设备;
 *  b – 通过其成员dev\_t来定义设备号（分为主、次设备号）以确定字符设备的唯一性;
 *  c – 通过其成员file\_operations来定义字符设备驱动提供给VFS的接口函数，如常见的open()、read()、write()等;

### 2) 在Linux字符设备驱动中: ###

*  a – 模块加载函数通过 register\_chrdev\_region( ) 或 alloc\_chrdev\_region( )来静态或者动态获取设备号;
 *  b – 通过 cdev\_init( ) 建立cdev与 file\_operations之间的连接，通过 cdev\_add( ) 向系统添加一个cdev以完成注册;
 *  c – 模块卸载函数通过cdev\_del( )来注销cdev，通过 unregister\_chrdev\_region( )来释放设备号;

### 3) 用户空间访问该设备的程序: ###

*  a – 通过Linux系统调用，如open( )、read( )、write( )，来“调用”file\_operations来定义字符设备驱动提供给VFS的接口函数;

## 3 字符设备驱动模型 ##

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3UwMTM4MzY5MDk_size_16_color_FFFFFF_t_70 2]

## 4 Cdev结构体解析 ##

struct cdev {
    	struct kobject kobj;		//内嵌的内核对象
    	struct module *owner;	//该字符设备锁在的内核模块的对象指针
    	const struct file_operations *ops;	//该字符设备的实现方法
    	struct list_head list;		//链接已在内核注册的所有字符设备
    	dev_t dev;			//该字符设备的设备号，由主设备号与次设备号构成
    	unsigned int count;		//同一主设备号下的次设备号的个数
    } __randomize_layout;

Cdev的相关操作接口：

### 1）cdev\_init ###

/**
     * cdev_init() - initialize a cdev structure
     * @cdev: the structure to initialize
     * @fops: the file_operations for this device
     *
     * Initializes @cdev, remembering @fops, making it ready to add to the
     * system with cdev_add().
     */
    void cdev_init(struct cdev *cdev, const struct file_operations *fops)
    {
    	memset(cdev, 0, sizeof *cdev);	//将cdev清零
    	INIT_LIST_HEAD(&cdev->list);		//初始化list链表，指向其本身
    	kobject_init(&cdev->kobj, &ktype_cdev_default);	//初始化kobj成员
    	cdev->ops = fops;				//初始化ops成员
    }

### 2）cdev\_alloc ###

/**
     * cdev_alloc() - allocate a cdev structure
     *
     * Allocates and returns a cdev structure, or NULL on failure.
     */
    struct cdev *cdev_alloc(void)
    {
    	struct cdev *p = kzalloc(sizeof(struct cdev), GFP_KERNEL);	//动态申请cdev结构体
    	if (p) {
    		INIT_LIST_HEAD(&p->list);					//初始化list链表
    		kobject_init(&p->kobj, &ktype_cdev_dynamic);	//初始化kobj成员
    	}
    	return p;
    }

### 3）cdev\_add ###

该函数向内核注册一个cdev结构体。

/**
     * cdev_add() - add a char device to the system
     * @p: the cdev structure for the device
     * @dev: the first device number for which this device is responsible
     * @count: the number of consecutive minor numbers corresponding to this
     *         device
     *
     * cdev_add() adds the device represented by @p to the system, making it
     * live immediately.  A negative error code is returned on failure.
     */
    int cdev_add(struct cdev *p, dev_t dev, unsigned count)
    {
    	int error;
    
    	p->dev = dev;
    	p->count = count;
    
    	error = kobj_map(cdev_map, dev, count, NULL,
    			 exact_match, exact_lock, p);
    	if (error)
    		return error;
    
    	kobject_get(p->kobj.parent);
    
    	return 0;
    }

### 4）cdev\_del ###

该函数向内核注销一个cdev结构体。

/**
     * cdev_del() - remove a cdev from the system
     * @p: the cdev structure to be removed
     *
     * cdev_del() removes @p from the system, possibly freeing the structure
     * itself.
     *
     * NOTE: This guarantees that cdev device will no longer be able to be
     * opened, however any cdevs already open will remain and their fops will
     * still be callable even after cdev_del returns.
     */
    void cdev_del(struct cdev *p)
    {
    	cdev_unmap(p->dev, p->count);
    	kobject_put(&p->kobj);
    }

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