Linux驱动学习(三)--字符设备架构与注册

1.内核如何维护设备号的？

chrdevs指针数组

在内核中有一个重要的全局变量：chrdevs指针数组，位于char_dev.c文件中

 chrdevs指针数组的每一个成员指向一个char_device_struct结构体，该结构体中，最重要的变量是cdev指针

cdev结构体

cdev结构体：

 该结构体定义位于cdev.h

在cdev中，kobj和owner成员不用太关注，内核会自己填充；dev填充设备号的成员；我们需要关心的最重要的成员是file_operations *ops

file_operations结构体

file_operations结构（位于fs.h）

struct file_operations {struct module *owner;loff_t (*llseek) (struct file *, loff_t, int);ssize_t (*read) (struct file *, char __user *, size_t, loff_t *);ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *);ssize_t (*aio_read) (struct kiocb *, const struct iovec *, unsigned long, loff_t);ssize_t (*aio_write) (struct kiocb *, const struct iovec *, unsigned long, loff_t);int (*iterate) (struct file *, struct dir_context *);unsigned int (*poll) (struct file *, struct poll_table_struct *);long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long);long (*compat_ioctl) (struct file *, unsigned int, unsigned long);int (*mmap) (struct file *, struct vm_area_struct *);int (*open) (struct inode *, struct file *);int (*flush) (struct file *, fl_owner_t id);int (*release) (struct inode *, struct file *);int (*fsync) (struct file *, loff_t, loff_t, int datasync);int (*aio_fsync) (struct kiocb *, int datasync);int (*fasync) (int, struct file *, int);int (*lock) (struct file *, int, struct file_lock *);ssize_t (*sendpage) (struct file *, struct page *, int, size_t, loff_t *, int);unsigned long (*get_unmapped_area)(struct file *, unsigned long, unsigned long, unsigned long, unsigned long);int (*check_flags)(int);int (*flock) (struct file *, int, struct file_lock *);ssize_t (*splice_write)(struct pipe_inode_info *, struct file *, loff_t *, size_t, unsigned int);ssize_t (*splice_read)(struct file *, loff_t *, struct pipe_inode_info *, size_t, unsigned int);int (*setlease)(struct file *, long, struct file_lock **);long (*fallocate)(struct file *file, int mode, loff_t offset,loff_t len);int (*show_fdinfo)(struct seq_file *m, struct file *f);
};

常用的函数有：

• owner 拥有该结构体的模块的指针，一般设置为 THIS_MODULE

• llseek 函数用于修改文件当前的读写位置

• read 函数用于读取设备文件

• write 函数用于向设备文件写入(发送)数据

• poll 是个轮询函数，用于查询设备是否可以进行非阻塞的读写

• unlocked_ioctl 函数提供对于设备的控制功能，与应用程序中的 ioctl 函数对应

• compat_ioctl 函数与 unlocked_ioctl 函数功能一样，区别在于在 64 位系统上，32 位的应用程序调用将会使用此函数。在 32 位的系统上运行 32 位的应用程序调用的是unlocked_ioctl

• mmap 函数用于将设备的内存映射到进程空间中(也就是用户空间)，一般帧缓冲设备会使用此函数，比如 LCD 驱动的显存，将帧缓冲(LCD 显存)映射到用户空间中以后应用程序就可以直接操作显存了，这样就不用在用户空间和内核空间之间来回复制

• open 函数用于打开设备文件

• release 函数用于释放(关闭)设备文件，与应用程序中的 close 函数对应

• fasync 函数用于刷新待处理的数据，用于将缓冲区中的数据刷新到磁盘中

cdev之间通过链表串起来 

需要我们自己实现的东西有：

• cdev
• read
• write

 2.用户层如何找到驱动的？

 答案：借助文件系统

mknod命令

创建设备节点命令：

mknod /dev/loh c 237 0

• loh：设备名称
• c：字符设备
• 237：主设备号
• 0：次设备号

输入该命令过后，在内核中会产生一个inode结构体，inode会记录文件的相关信息

inode结构体

inode结构体（位于fs.h）

struct inode {umode_t			i_mode;unsigned short		i_opflags;kuid_t			i_uid;kgid_t			i_gid;unsigned int		i_flags;#ifdef CONFIG_FS_POSIX_ACLstruct posix_acl	*i_acl;struct posix_acl	*i_default_acl;
#endifconst struct inode_operations	*i_op;struct super_block	*i_sb;struct address_space	*i_mapping;#ifdef CONFIG_SECURITYvoid			*i_security;
#endif/* Stat data, not accessed from path walking */unsigned long		i_ino;/** Filesystems may only read i_nlink directly.  They shall use the* following functions for modification:**    (set|clear|inc|drop)_nlink*    inode_(inc|dec)_link_count*/union {const unsigned int i_nlink;unsigned int __i_nlink;};dev_t			i_rdev;loff_t			i_size;struct timespec		i_atime;struct timespec		i_mtime;struct timespec		i_ctime;spinlock_t		i_lock;	/* i_blocks, i_bytes, maybe i_size */unsigned short          i_bytes;unsigned int		i_blkbits;blkcnt_t		i_blocks;#ifdef __NEED_I_SIZE_ORDEREDseqcount_t		i_size_seqcount;
#endif/* Misc */unsigned long		i_state;struct mutex		i_mutex;unsigned long		dirtied_when;	/* jiffies of first dirtying */struct hlist_node	i_hash;struct list_head	i_wb_list;	/* backing dev IO list */struct list_head	i_lru;		/* inode LRU list */struct list_head	i_sb_list;union {struct hlist_head	i_dentry;struct rcu_head		i_rcu;};u64			i_version;atomic_t		i_count;atomic_t		i_dio_count;atomic_t		i_writecount;const struct file_operations	*i_fop;	/* former ->i_op->default_file_ops */struct file_lock	*i_flock;struct address_space	i_data;
#ifdef CONFIG_QUOTAstruct dquot		*i_dquot[MAXQUOTAS];
#endifstruct list_head	i_devices;union {struct pipe_inode_info	*i_pipe;struct block_device	*i_bdev;struct cdev		*i_cdev;};__u32			i_generation;#ifdef CONFIG_FSNOTIFY__u32			i_fsnotify_mask; /* all events this inode cares about */struct hlist_head	i_fsnotify_marks;
#endif#ifdef CONFIG_IMAatomic_t		i_readcount; /* struct files open RO */
#endifvoid			*i_private; /* fs or device private pointer */
};

 其中比较重要我们需要关心的是设备号成员：

有了这个设备号，我们就可以找到 chrdevs数组成员

3.汇总图

---

 

 此图参考：Linux字符设备驱动开发(一)

 4.字符设备注册

常用相关函数：

1. void cdev_init(struct cdev *, const struct file_operations *);
2. int cdev_add(struct cdev *, dev_t, unsigned);
3. void cdev_del(struct cdev *);

 实验

加载模块

 mknod创建设备节点

应用程序

驱动代码

/**cdev.c*Original Author: luoyunheng, 2025-02-20** Linux驱动之字符设备注册
*/#include <linux/init.h>
#include <linux/module.h>
#include <linux/kdev_t.h>
#include <linux/fs.h>
#include <linux/cdev.h>static int major = 222;
static int minor = 0;static dev_t devno;
static struct cdev dev;static int hello_open(struct inode *inode, struct file *filep)
{printk("hello_open()\n");return 0;
}static struct file_operations hello_ops = 
{.open = hello_open,
};static int hello_init(void)
{int result;int error;printk("hello_init\n");devno = MKDEV(major, minor);result = register_chrdev_region(devno, 1, "loh");if (result < 0 ) {printk("register dev number failed\n");return result;}cdev_init(&dev, &hello_ops);error = cdev_add(&dev, devno, 1);if (error < 0) {printk("cdev_add fail\n");unregister_chrdev_region(devno, 1);return error;}return 0;
}static void hello_exit(void)
{printk("hello_exit\n");cdev_del(&dev);unregister_chrdev_region(devno, 1);return;
}module_init(hello_init);
module_exit(hello_exit);

 应用程序代码

#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>int main(int argc, char **argv)
{int fd;fd = open("/dev/loh", O_RDWR);if(fd < 0) {perror("");return 0;}	return 0;
}