驱动是用来控制和操作硬件的软件。

在linux下，一切皆文件。当我们write一个文件时，内核通过文件的file_operations结构体(include/linux/fs.h)来找到对应的驱动函数，最终调用的是存储介质(ssd，硬盘等)驱动提供的write函数(这中间有很多步骤暂时先不关注)。不同的驱动要提供自己的open，read，write，ioctl等接口。

首先要提供驱动的open，read等函数，填充file_operations这个结构体。

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 (*read_iter) (struct kiocb *, struct iov_iter *);
	ssize_t (*write_iter) (struct kiocb *, struct iov_iter *);
	int (*iterate) (struct file *, struct dir_context *);
	int (*iterate_shared) (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 (*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 **, void **);
	long (*fallocate)(struct file *file, int mode, loff_t offset,
			  loff_t len);
	void (*show_fdinfo)(struct seq_file *m, struct file *f);
#ifndef CONFIG_MMU
	unsigned (*mmap_capabilities)(struct file *);
#endif
	ssize_t (*copy_file_range)(struct file *, loff_t, struct file *,
			loff_t, size_t, unsigned int);
	int (*clone_file_range)(struct file *, loff_t, struct file *, loff_t,
			u64);
	ssize_t (*dedupe_file_range)(struct file *, u64, u64, struct file *,
			u64);
};

要把这个结构体注册到内核中，通过register_chrdev函数

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

有注册就有注销，当驱动卸载后通过unregister_chrdev释放资源

static inline void unregister_chrdev(unsigned int major, const char *name)
{
	__unregister_chrdev(major, 0, 256, name);
}

当驱动insmod的时候，通过module_init(xxx_init)宏来调用驱动提供的初始化函数来初始化驱动。

驱动要实现这个xxx_init()函数来注册驱动。

当驱动rmmod的时候，通过module_exit(xxx_exit)宏来清理驱动。驱动要实现这个xxx_exit()函数来释放资源。

总结下来就三步：

/*构建file_operations结构体*/

/*init函数,实现register_chrdev*/

/*exit函数unregister_chrdev*/