#include <linux/cdev.h> //for struct cdev
#include <linux/fs.h>  //for struct file
#include <asm-generic/uaccess.h>  //for copy_to_user
#include <linux/errno.h>      //for error number


/* 准备操作方法集 */
/* 
struct file_operations {
  struct module *owner;  //THIS_MODULE
  
  //读设备
  ssize_t (*read) (struct file *, char __user *, size_t, loff_t *);
  //写设备
  ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *);

  //映射内核空间到用户空间
  int (*mmap) (struct file *, struct vm_area_struct *);

  //读写设备参数、读设备状态、控制设备
  long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long);

  //打开设备
  int (*open) (struct inode *, struct file *);
  //关闭设备
  int (*release) (struct inode *, struct file *);

  //刷新设备
  int (*flush) (struct file *, fl_owner_t id);

  //文件定位
  loff_t (*llseek) (struct file *, loff_t, int);

  //异步通知
  int (*fasync) (int, struct file *, int);
  //POLL机制
  unsigned int (*poll) (struct file *, struct poll_table_struct *);
  。。。
};
*/

ssize_t myread(struct file *filep, char __user * user_buf, size_t size, loff_t* offset)
{
  return 0;
}

struct file fops = {
  .owner = THIS_MODULE,
  .read = myread，
  ...
};


/* 字符设备对象类型 */
struct cdev {
  //public  
  struct module *owner;        //模块所有者（THIS_MODULE），用于模块计数
  const struct file_operations *ops; //操作方法集（分工:打开、关闭、读/写、...）
  dev_t dev;             //设备号（第一个）
  unsigned int count;         //设备数量
  //private
  ...
};

static int __init chrdev_init(void)
{
  ...
  /* 构造cdev设备对象 */
  struct cdev *cdev_alloc(void);

  /* 初始化cdev设备对象 */
  void cdev_init(struct cdev*, const struct file_opeartions*);

  /* 为字符设备静态申请设备号 */
  int register_chedev_region(dev_t from, unsigned count, const char* name);

  /* 为字符设备动态申请主设备号 */
  int alloc_chedev_region(dev_t* dev, unsigned baseminor, unsigned count, const char* name);

  MKDEV(ma,mi)  //将主设备号和次设备号组合成设备号
  MAJOR(dev)   //从dev_t数据中得到主设备号
  MINOR(dev)   //从dev_t数据中得到次设备号

  /* 注册字符设备对象cdev到内核 */
  int cdev_add(struct cdev* , dev_t, unsigned);
  ...
}

static void __exit chrdev_exit(void)
{
  ...
  /* 从内核注销cdev设备对象 */
  void cdev_del(struct cdev* );

  /* 从内核注销cdev设备对象 */
  void cdev_put(stuct cdev *);

  /* 回收设备号 */
  void unregister_chrdev_region(dev_t from, unsigned count);
  ...
}

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

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

ssize_t myread(struct file *filep, char __user * user_buf, size_t size, loff_t* offset)
{
  long ret = 0;
  size = size > MAX_KBUF?MAX_KBUF:size;
  if(copy_to_user(user_buf, kbuf,size)
    return -EAGAIN;
  }
  return 0;
}

//mycmd.h
...
#include <asm/ioctl.h>
#define CMDT 'A'
#define KARG_SIZE 36
struct karg{
  int kval;
  char kbuf[KARG_SIZE];
};
#define CMD_OFF _IO(CMDT,0)
#define CMD_ON _IO(CMDT,1)
#define CMD_R  _IOR(CMDT,2,struct karg)
#define CMD_W  _IOW(CMDT,3,struct karg)
...

//chrdev.c
static long demo_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
{
  static struct karg karg = {
    .kval = 0,
    .kbuf = {0},
  };
  struct karg *usr_arg;

  switch(cmd){
  case CMD_ON:
    /* 开灯 */
    break;
  case CMD_OFF:
    /* 关灯 */
    break;
  case CMD_R:
    if(_IOC_SIZE(cmd) != sizeof(karg)){
      return -EINVAL;
    }
    usr_arg = (struct karg *)arg;
    if(copy_to_user(usr_arg, &karg, sizeof(karg))){
      return -EAGAIN;
    }
    break;
  case CMD_W:   
    if(_IOC_SIZE(cmd) != sizeof(karg)){
      return -EINVAL;
    }
    usr_arg = (struct karg *)arg;
    if(copy_from_user(&karg, usr_arg, sizeof(karg))){
      return -EAGAIN;
    }
    break;
  default:
    ;
  };
  return 0;
}

Device Drivers --->
    Generic Driver Options --->
      [*]Maintain a devtmpfs filesystem to mount at /dev
      [*] Automount devtmpfs at /dev,after the kernel mounted the rootfs
OR

mount -t sysfs none sysfs /sys
mdev -s //udev也行

class_create(owner,name);
struct device *device_create_vargs(struct class *cls, struct device *parent,dev_t devt, void *drvdata,const char *fmt, va_list vargs);

void class_destroy(struct class *cls);  
void device_destroy(struct class *cls, dev_t devt);

 /* 在/sys中导出设备类信息 */
  cls = class_create(THIS_MODULE,DEV_NAME);

  /* 在cls指向的类中创建一组(个)设备文件 */
  for(i= minor;i<(minor+cnt);i++){
    devp = device_create(cls,NULL,MKDEV(major,i),NULL,"%s%d",DEV_NAME,i);
  } 

/* 在cls指向的类中删除一组(个)设备文件 */
  for(i= minor;i<(minor+cnt);i++){
    device_destroy(cls,MKDEV(major,i));
  }

  /* 在/sys中删除设备类信息 */
  class_destroy(cls);       //一定要先卸载device再卸载class