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linux设备驱动之控制台驱动(1)

  • 2023-08-14 05:06:48
  • 来源:本站整理
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  一:前言

  我们在之前分析过input子系统和tty设备驱动架构.今天需要将两者结合起来.看看linux中的控制台是怎么样实现的.

  二:控制台驱动的初始化

  之前在分析tty驱动架构的时候曾分析到.主设备为4,次设备为0的设备节点,即/dev/tty0为当前的控制终端.

  有tty_init()中,有以下代码段:

  static int __init tty_init(void)

  {

  ……

  ……

  #ifdef CONFIG_VT

  cdev_init(&vc0_cdev, &console_fops);

  if (cdev_add(&vc0_cdev, MKDEV(TTY_MAJOR, 0), 1) ||

  register_chrdev_region(MKDEV(TTY_MAJOR, 0), 1, “/dev/vc/0“) < 0)

  panic(“Couldn't register /dev/tty0 driver\n“);

  device_create(tty_class, NULL, MKDEV(TTY_MAJOR, 0), “tty0“);

  vty_init();

  #endif

  return 0;

  }

  CONFIG_VT:是指配置虚拟终端.即我们所说的控制台.在此可以看到TTY_MAJOR(4),0对应的设备节点操作集为console_fops.

  继续跟进vty_init()

  int __init vty_init(void)

  {

  vcs_init();

  console_driver = alloc_tty_driver(MAX_NR_CONSOLES);

  if (!console_driver)

  panic(“Couldn't allocate console driver\n“);

  console_driver->owner = THIS_MODULE;

  console_driver->name = “tty“;

  console_driver->name_base = 1;

  console_driver->major = TTY_MAJOR;

  console_driver->minor_start = 1;

  console_driver->type = TTY_DRIVER_TYPE_CONSOLE;

  console_driver->init_termios = tty_std_termios;

  console_driver->flags = TTY_DRIVER_REAL_RAW | TTY_DRIVER_RESET_TERMIOS;

  tty_set_operations(console_driver, &con_ops);

  if (tty_register_driver(console_driver))

  panic(“Couldn't register console driver\n“);

  kbd_init();

  console_map_init();

  #ifdef CONFIG_PROM_CONSOLE

  prom_con_init();

  #endif

  #ifdef CONFIG_MDA_CONSOLE

  mda_console_init();

  #endif

  return 0;

  }

  经过我们之前的tty驱动架构分析,这段代码看起来就比较简单了,它就是注册了一个tty驱动.这个驱动对应的操作集是位于con_ops里面的.

  仔细看.在之后还会调用kbd_init().顾名思义,这个是一个有关键盘的初始化.控制终端跟键盘有什么关系呢?在之前分析tty的时候,曾提到过,. 对于控制台而言,它的输入设备是键盘鼠标,它的输出设备是当前显示器.这两者是怎么关联起来的呢?不着急.请看下面的分析.

  三:控制台的open操作

  在前面分析了,对应console的操作集为con_ops.定义如下:

  static const struct file_operations console_fops = {

  .llseek = no_llseek,

  .read = tty_read,

  .write = redirected_tty_write,

  .poll = tty_poll,

  .ioctl = tty_ioctl,

  .compat_ioctl = tty_compat_ioctl,

  .open = tty_open,

  .release = tty_release,

  .fasync = tty_fasync,

  };

  里面的函数指针值我们都不陌生了,在之前分析的tty驱动中已经分析过了.

  结合前面的tty驱动分析.我们知道在open的时候,会调用ldisc的open和tty_driver.open.

  对于ldisc默认是tty_ldiscs[0].我们来看下它的具体赋值.

  console_init():

  void __init console_init(void)

  {

  initcall_t *call;

  /* Setup the default TTY line discipline. */

  (void) tty_register_ldisc(N_TTY, &tty_ldisc_N_TTY);

  /*

  * set up the console device so that later boot sequences can

  * inform about problems etc..

  */

  call = __con_initcall_start;

  while (call < __con_initcall_end) {

  (*call)();

  call++;

  }

  }

  在这里,通过tty_register_ldisc.将tty_ldisc_N_TTY注册为了第N_TTY项.即第1项. tty_ldisc_N_TTY定义如下:

  struct tty_ldisc tty_ldisc_N_TTY = {

  .magic = TTY_LDISC_MAGIC,

  .name = “n_tty“,

  .open = n_tty_open,

  .close = n_tty_close,

  .flush_buffer = n_tty_flush_buffer,

  .chars_in_buffer = n_tty_chars_in_buffer,

  .read = read_chan,

  .write = write_chan,

  .ioctl = n_tty_ioctl,

  .set_termios = n_tty_set_termios,

  .poll = normal_poll,

  .receive_buf = n_tty_receive_buf,

  .write_wakeup = n_tty_write_wakeup

  }

  对应的open操作为n_tty_open:

  static int n_tty_open(struct tty_struct *tty)

  {

  if (!tty)

  return -EINVAL;

  /* This one is ugly. Currently a malloc failure here can panic */

  if (!tty->read_buf) {

  tty->read_buf = alloc_buf();

  if (!tty->read_buf)

  return -ENOMEM;

  }

  memset(tty->read_buf, 0, N_TTY_BUF_SIZE);

  reset_._flags(tty);

  tty->column = 0;

  n_tty_set_termios(tty, NULL);

  tty->minimum_to_wake = 1;

  tty->closing = 0;

  return 0;

  }

  它为tty->read_buf分配内存.这个buffer空间大小为N_TTY_BUF_SIZE.read_buf实际上就是从按键的缓存区.然后调用reset_flags()来初始化tty中的一些字段:

  static void reset_buffer_flags(struct tty_struct *tty)

  {

  unsigned long flags;

  spin_lock_irqsave(&tty->read_lock, flags);

  tty->read_head = tty->read_tail = tty->read_cnt = 0;

  spin_unlock_irqrestore(&tty->read_lock, flags);

  tty->canon_head = tty->canon_data = tty->erasing = 0;

  memset(&tty->read_flags, 0, sizeof tty->read_flags);

  n_tty_set_room(tty);

  check_unthrottle(tty);

  }

  这里比较简,不再详细分析.在这里要注意几个tty成员的含义:

  Tty->read_head, tty->read_tail , tty->read_cnt分别代表read_buf中数据的写入位置,读取位置和数据总数.read_buf是一个环形缓存区.

  n_tty_set_room()是设备read_buf中的可用缓存区

  check_unthrottle():是用来判断是否需要打开“阀门“,允许输入数据流入

  对于console tty_driver对应的open函数如下示:

  static int con_open(struct tty_struct *tty, struct file *filp)

  {

  unsigned int currcons = tty->index;

  int ret = 0;

  acquire_console_sem();

  if (tty->driver_data == NULL) {

  ret = vc_allocate(currcons);

  if (ret == 0) {

  struct vc_data *vc = vc_cons[currcons].d;

  tty->driver_data = vc;

  vc->vc_tty = t

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