Linux字符设备驱动总结

 

Linux2.6内核中使用cdev结构体描述字符设备：

struct cdev

{

struct kobject kobj; // 内嵌的kobject对象，描述设备引用计数

struct module *owner; // 所属模块，一般赋值为THIS_MODULE

struct file_operations *ops; // 文件操作结构体

struct list_head list;

dev_t dev; // 设备号

unsigned int count;

};

cdev结构体的dev_t定义了设备号，32位。高12位为主设备号，低20位为次设备号。

下列宏可从dev_t获得主、次设备号：

MAJOR(dev_t dev)

MINOR(dev_t dev)

通过主、次设备号生成dev_t：

MKDEV(int major, int minor)

一组函数操作cdev结构体：

void cdev_init(struct cdev *, struct file_operations *); // 初始化cdev成员，建立cdev和file_operations之间的连接

struct cdev *cdev_alloc(void); // 动态申请一个cdev内存

void cdev_put(struct cdev *p); // 减少模块的引用计数，释放结构体空间

int cdev_add(struct cdev *, dev_t, unsigned); // 向系统添加一个dev，完成字符设备的注册，常用于模块加载函数中

void cdev_del(struct cdev *); // 向系统删除一个dev，完成字符设备的注销，常用于模块卸载函数中

调用cdev_add()前，先调用register_chrdev_region()或alloc_chrdev_region():

int register_chrdev_region(dev_t from, unsigned count, const char *name); // 用于已知起始设备号

int alloc_chrdev_region(dev_t *dev, unsigned baseminor, unsigned count, const char *name); // 设备号未知，向系统动态申请未占用设备号放入dev

cdev_del()后，unregister_chrdev_region()应被调用以释放设备号：

void unregister_chrdev_region(dev_t from, unsigned count);

file_operations结构：

1	struct file_operations {
2	struct module *owner;
3	loff_t (*llseek) (struct file *, loff_t, int);
4	ssize_t (*read) (struct file *, char __user *, size_t, loff_t *);
5	ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *);
6	ssize_t (*aio_read) (struct kiocb *, const struct iovec *, unsigned long, loff_t);
7	ssize_t (*aio_write) (struct kiocb *, const struct iovec *, unsigned long, loff_t);
8	int (*readdir) (struct file *, void *, filldir_t);
9	unsigned int (*poll) (struct file *, struct poll_table_struct *);
10	int (*ioctl) (struct inode *, struct file *, unsigned int, unsigned long);
11	long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long);
12	long (*compat_ioctl) (struct file *, unsigned int, unsigned long);
13	int (*mmap) (struct file *, struct vm_area_struct *);
14	int (*open) (struct inode *, struct file *);
15	int (*flush) (struct file *, fl_owner_t id);
16	int (*release) (struct inode *, struct file *);
17	int (*fsync) (struct file *, int datasync);
18	int (*aio_fsync) (struct kiocb *, int datasync);
19	int (*fasync) (int, struct file *, int);
20	int (*lock) (struct file *, int, struct file_lock *);
21	ssize_t (*sendpage) (struct file *, struct page *, int, size_t, loff_t *, int);
22	unsigned long (*get_unmapped_area)(struct file *, unsigned long, unsigned long, unsigned long, unsigned long);
23	int (*check_flags)(int);
24	int (*flock) (struct file *, int, struct file_lock *);
25	ssize_t (*splice_write)(struct pipe_inode_info *, struct file *, loff_t *, size_t, unsigned int);
26	ssize_t (*splice_read)(struct file *, loff_t *, struct pipe_inode_info *, size_t, unsigned int);
27	int (*setlease)(struct file *, long, struct file_lock **);
28	};

Linux字符设备驱动函数模板：

// 设备结构体

struct xxx_dev_t

{

struct cdev cdev;

...

}

// 设备驱动模块加载函数

static int __init xxx_init(void)

{

...

cdev_init(&xxx_cdev.cdev, &xxx_fops); // 初始化dev

xxx_dev.cdev.owner = THIS_MODULE;

//

if(xxx_major)

{

register_chrdev_region(xxx_dev_no, 1, DEV_NAME);

}

else

{

alloc_chrdev_region(&xxx_dev_no, 0, 1, DEV_NAME);

}

ret = cdev_add(&xxx_dev.cdev, xxx_dev_no, 1); // 注册设备

...

}

// 设备驱动模块卸载函数

static void __exit xxx_exit(void)

{

unregister_chrdev_region(xxx_dev_no, 1); // 释放占用的设备号

cdev_del(&xxx_dev.cdev); // 注销设备

}

// 读设备

// filp是文件结构指针，buf是用户空间的内存地址，该地址空间不能直接读写，count是要读的字节数，f_pos是读的位置相对于文件开头的偏移

ssize_t xxx_read(struct file *filp, char __user *buf, size_t count, loff_t *fpos)

{

...

copy_to_user(buf, ..., ...);

...

}

// 写设备

ssize_t xxx_write(struct file *filp, const char __user *buf, size_t count, loff_t *fpos)

{

...

copy_from_user(..., buf, ...);

...

}

// ioctl函数

int xxx_ioctl(struct inode *inode, struct file *filp, unsigned int cmd, unsigned long arg)

{

...

switch(cmd)

{

case xxx_CMD1:

...

break;

case xxx_CMD2:

...

break;

default:

// 不支持的命令

return - ENOTTY;

}

return 0;

}

// 字符设备文件操作结构体，在模块加载函数的cdev_init(&xxx_dev.cdev, &xxx_fops)的语句中被建立与cdev的连接

struct file_operations xxx_fops =

{

.owner = THIS_MODULE,

.read = xxx_read,

.write = xxx_write,

.ioctl = xxx_ioctl,

...

};

其他说明：

内核空间和用户空间的内存不能直接访问，需借助copy_from_user()和copy_to_user():

unsigned long copy_from_user(void *to, const void __user *from, unsigned long count);

unsigned long copy_to_user(void __user *to, const void *from, unsigned long count);

上述函数返回不能被复制的字节数。若完全复制成功，返回0.

若要复制的内存是简单类型，如char、int、long等，则可以使用简单的put_user()和get_user():

int val; // 内核空间整形变量

...

get_user(val, (int *)arg); // 用户空间到内核空间，arg是用户空间的地址

...

put_user(val, (int *)arg); // 内核空间到用户空间，arg是用户空间的地址

读和写函数中的__user是一个宏，表明其后的指针指向用户空间：

#ifdef __CHECKER__

#define __user __attribute__((noderef, address_space(1)))

#else

#define __user

#endif