User space can not exchange data with kernel space directly.
They need to use system call.
e.g. fopen, ioctl, write, read …… etc.

In Kernel Space ……
1. Register your device with “register_chrdev”, defined in linux/fs.h .
　https://ithelp.ithome.com.tw/articles/10159749
2. Implement driver functions, ioctl, open,…
　struct file_operations fops = {
　　.owner = THIS_MODULE,
　　.read = device1_read,
　　.write = device1_write,
　　.ioctl = device1_ioctl,
　　.open = device1_open,
　　.release = device1_release,
　};

　int ioctl(struct inode *, struct file *, unsigned int, unsigned long);
　is in linux/ioctl.h
　http://ccckmit.wikidot.com/lk:io
　開發 driver 需要的基礎知識
　user space/kernel space 的IO觀念及實作

Copy data ……
　Kernel Space to User Space: copy_to_user()
　User Space to Kernel Space: copy_from_user()

In User Space ……
Use system call to control kernel driver.
　fopen (open)
　write
　read
　close
　seek
　poll / select
　ioctl
　mmap
　fcntl


e.g.

int main(int argc, char *argv[]){
　　int devfd;
　　int num = 0;

　　if (argc > 1) num = atoi(argv[1]);
　　if (num < 0) num = 0xff;

　　devfd = open("/dev/debug", O_RDONLY);
　　if (devfd == -1) {
　　　　printf("Can't open /dev/debug\n");
　　　　return -1;
　　}

　　printf("Write 0x%02x...\n", num);
　　ioctl(devfd, IOCTL_WRITE, num);
　　printf("Done. Wait 5 seconds...\n");
　　sleep(5);
　　close(devfd);

　　return 0;
}