#include <linux/types.h>
  #include <linux/errno.h>
  #include <linux/tty.h>
  #include <linux/module.h>
  
  /*
   *  n_null.c - Null line discipline used in the failure path
   *
   *  Copyright (C) Intel 2017
   *
   * ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   *
   *  This program is free software; you can redistribute it and/or modify
   *  it under the terms of the GNU General Public License version 2
   *  as published by the Free Software Foundation.
   *
   *  This program is distributed in the hope that it will be useful,
   *  but WITHOUT ANY WARRANTY; without even the implied warranty of
   *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   *  GNU General Public License for more details.
   *
   * ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   */
  
  static int n_null_open(struct tty_struct *tty)
  {
  	return 0;
  }
  
  static void n_null_close(struct tty_struct *tty)
  {
  }
  
  static ssize_t n_null_read(struct tty_struct *tty, struct file *file,
  			   unsigned char __user * buf, size_t nr)
  {
  	return -EOPNOTSUPP;
  }
  
  static ssize_t n_null_write(struct tty_struct *tty, struct file *file,
  			    const unsigned char *buf, size_t nr)
  {
  	return -EOPNOTSUPP;
  }
  
  static void n_null_receivebuf(struct tty_struct *tty,
  				 const unsigned char *cp, char *fp,
  				 int cnt)
  {
  }
  
  static struct tty_ldisc_ops null_ldisc = {
  	.owner		=	THIS_MODULE,
  	.magic		=	TTY_LDISC_MAGIC,
  	.name		=	"n_null",
  	.open		=	n_null_open,
  	.close		=	n_null_close,
  	.read		=	n_null_read,
  	.write		=	n_null_write,
  	.receive_buf	=	n_null_receivebuf
  };
  
  static int __init n_null_init(void)
  {
  	BUG_ON(tty_register_ldisc(N_NULL, &null_ldisc));
  	return 0;
  }
  
  static void __exit n_null_exit(void)
  {
  	tty_unregister_ldisc(N_NULL);
  }
  
  module_init(n_null_init);
  module_exit(n_null_exit);
  
  MODULE_LICENSE("GPL");
  MODULE_AUTHOR("Alan Cox");
  MODULE_ALIAS_LDISC(N_NULL);
  MODULE_DESCRIPTION("Null ldisc driver");