drivers/tty/ttynull.c

   1 // SPDX-License-Identifier: GPL-2.0
   2 /*
   3  * Copyright (C) 2019 Axis Communications AB
   4  *
   5  * The console is useful for userspace applications which expect a console
   6  * device to work without modifications even when no console is available
   7  * or desired.
   8  *
   9  * In order to use this driver, you should redirect the console to this
  10  * TTY, or boot the kernel with console=ttynull.
  11  *
  12  * Based on ttyprintk.c:
  13  *  Copyright (C) 2010 Samo Pogacnik
  14  */
  15
  16 #include <linux/console.h>
  17 #include <linux/module.h>
  18 #include <linux/tty.h>
  19
  20 static const struct tty_port_operations ttynull_port_ops;
  21 static struct tty_driver *ttynull_driver;
  22 static struct tty_port ttynull_port;
  23
  24 static int ttynull_open(struct tty_struct *tty, struct file *filp)
  25 {
  26         return tty_port_open(&ttynull_port, tty, filp);
  27 }
  28
  29 static void ttynull_close(struct tty_struct *tty, struct file *filp)
  30 {
  31         tty_port_close(&ttynull_port, tty, filp);
  32 }
  33
  34 static void ttynull_hangup(struct tty_struct *tty)
  35 {
  36         tty_port_hangup(&ttynull_port);
  37 }
  38
  39 static int ttynull_write(struct tty_struct *tty, const unsigned char *buf,
  40                          int count)
  41 {
  42         return count;
  43 }
  44
  45 static int ttynull_write_room(struct tty_struct *tty)
  46 {
  47         return 65536;
  48 }
  49
  50 static const struct tty_operations ttynull_ops = {
  51         .open = ttynull_open,
  52         .close = ttynull_close,
  53         .hangup = ttynull_hangup,
  54         .write = ttynull_write,
  55         .write_room = ttynull_write_room,
  56 };
  57
  58 static struct tty_driver *ttynull_device(struct console *c, int *index)
  59 {
  60         *index = 0;
  61         return ttynull_driver;
  62 }
  63
  64 static struct console ttynull_console = {
  65         .name = "ttynull",
  66         .device = ttynull_device,
  67 };
  68
  69 void __init register_ttynull_console(void)
  70 {
  71         if (!ttynull_driver)
  72                 return;
  73
  74         if (add_preferred_console(ttynull_console.name, 0, NULL))
  75                 return;
  76
  77         register_console(&ttynull_console);
  78 }
  79
  80 static int __init ttynull_init(void)
  81 {
  82         struct tty_driver *driver;
  83         int ret;
  84
  85         driver = tty_alloc_driver(1,
  86                 TTY_DRIVER_RESET_TERMIOS |
  87                 TTY_DRIVER_REAL_RAW |
  88                 TTY_DRIVER_UNNUMBERED_NODE);
  89         if (IS_ERR(driver))
  90                 return PTR_ERR(driver);
  91
  92         tty_port_init(&ttynull_port);
  93         ttynull_port.ops = &ttynull_port_ops;
  94
  95         driver->driver_name = "ttynull";
  96         driver->name = "ttynull";
  97         driver->type = TTY_DRIVER_TYPE_CONSOLE;
  98         driver->init_termios = tty_std_termios;
  99         driver->init_termios.c_oflag = OPOST | OCRNL | ONOCR | ONLRET;
 100         tty_set_operations(driver, &ttynull_ops);
 101         tty_port_link_device(&ttynull_port, driver, 0);
 102
 103         ret = tty_register_driver(driver);
 104         if (ret < 0) {
 105                 put_tty_driver(driver);
 106                 tty_port_destroy(&ttynull_port);
 107                 return ret;
 108         }
 109
 110         ttynull_driver = driver;
 111         register_console(&ttynull_console);
 112
 113         return 0;
 114 }
 115
 116 static void __exit ttynull_exit(void)
 117 {
 118         unregister_console(&ttynull_console);
 119         tty_unregister_driver(ttynull_driver);
 120         put_tty_driver(ttynull_driver);
 121         tty_port_destroy(&ttynull_port);
 122 }
 123
 124 module_init(ttynull_init);
 125 module_exit(ttynull_exit);
 126
 127 MODULE_LICENSE("GPL v2");