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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：
- void cdev_init(struct cdev *, const struct file_operations *);
- int cdev_add(struct cdev *, dev_t, unsigned);
- 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;
}