控制台驱动是linux重要的设备驱动之一

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 = tty;

if (!tty->winsize.ws_row && !tty->winsize.ws_col) {

tty->winsize.ws_row = vc_cons[currcons].d->vc_rows;

tty->winsize.ws_col = vc_cons[currcons].d->vc_cols;

}

release_console_sem();

vcs_make_sysfs(tty);

return ret;

}

}

release_console_sem();

return ret;

}

tty->index表示的是tty_driver所对示的设备节点序号.在这里也就是控制台的序列.用alt+fn就可以切换控制终端.

在这里,它主要为vc_cons[ ]数组中的对应项赋值.并将tty和vc建立关联.

四:控制台的read操作

从tty驱动架构中分析可得到,最终的read操作会转入到ldsic->read中进行.

相应tty_ldisc_N_TTY的read操作如下.这个函数代码较长,分段分析如下:

static ssize_t read_chan(struct tty_struct *tty, struct file *file,

unsigned char __user *buf, size_t nr)

{

unsigned char __user *b = buf;

DECLARE_WAITQUEUE(wait, current);

int c;

int minimum, time;

ssize_t retval = 0;

ssize_t size;

long timeout;

unsigned long flags;

do_it_again:

if (!tty->read_buf) {

printk(KERN_ERR "n_tty_read_chan: read_buf == NULL?!?/n");

return -EIO;

}

c = job_control(tty, file);

if (c < 0)

return c;

minimum = time = 0;

timeout = MAX_SCHEDULE_TIMEOUT;if (!tty->icanon) {

time = (HZ / 10) * TIME_CHAR(tty);

minimum = MIN_CHAR(tty);

if (minimum) {

if (time)

tty->minimum_to_wake = 1;

else if (!waitqueue_active(&tty->read_wait) ||

(tty->minimum_to_wake > minimum))

tty->minimum_to_wake = minimum;

} else {

timeout = 0;

if (time) {

timeout = time;

time = 0;

}

tty->minimum_to_wake = minimum = 1;

}

}

首先,检查read操作的合法性,read_buf是否已经建立.然后再根据操作的类型来设置tty-> minimum_to_wake.这个成员的含义即为: 如果读进程在因数据不足而睡眠的情况下,数据到达并超过了minimum_to_wake.就将这个读进程唤醒.具体的唤醒过程我们在遇到的时候再进行分析.

/*

*      Internal serialization of reads.

*/

//不允许阻塞

if (file->f_flags & O_NONBLOCK) {

if (!mutex_trylock(&tty->atomic_read_lock))

return -EAGAIN;

} else {

if (mutex_lock_interruptible(&tty->atomic_read_lock))

return -ERESTARTSYS;

}

add_wait_queue(&tty->read_wait, &wait);

在不允许睡眠的情况下,调用mutex_trylock()去获得锁.如果锁被占用,马上返回.否则用可中断的方式去获取锁,如果取锁错误,返回失败.如果取锁成功,将进程加至等待队列.在没有数据可读的情况下,直接睡眠.如果有数据可读,将其移出等待队列即可.

while (nr) {

/* First test for status change. */

if (tty->packet && tty->link->ctrl_status) {

unsigned char cs;

if (b != buf)

break;

cs = tty->link->ctrl_status;

tty->link->ctrl_status = 0;

if (tty_put_user(tty, cs, b++)) {

retval = -EFAULT;

b--;

break;

}

nr--;

break;

}

接下来就是一个漫长的while循环,用来读取数据,一直到数据取满为止.如果tty->packet被置为1.即为信包模式,通常用在伪终端设备. 如果tty->link->ctrl_status有数据.则说明如果链路状态发生改变,需要提交此信息.在这种情况下,将其直接copy到用户空间即可.

/* This statement must be first before checking for input

so that any interrupt will set the state back to

TASK_RUNNING. */

set_current_state(TASK_INTERRUPTIBLE);

if (((minimum - (b - buf)) < tty->minimum_to_wake) &&

((minimum - (b - buf)) >= 1))

tty->minimum_to_wake = (minimum - (b - buf));

if (!input_available_p(tty, 0)) {

if (test_bit(TTY_OTHER_CLOSED, &tty->flags)) {

retval = -EIO;

break;

}

if (tty_hung_up_p(file))

break;

if (!timeout)

break;

if (file->f_flags & O_NONBLOCK) {

retval = -EAGAIN;

break;

}

if (signal_pending(current)) {

retval = -ERESTARTSYS;

break;

}

n_tty_set_room(tty);

timeout = schedule_timeout(timeout);

continue;

}

__set_current_state(TASK_RUNNING);

先将进程设为TASK_INTERRUPTIBLE状态.再调用input_available_p()来判断可数据供读取.如果没有.则进程睡眠.如果有数据,则将进程状态设为TASK_RUNNING.在终端接收数据的处理过程中,有两种方式,一种是规范模式.一种是原始模式.在规范模式下,终端需要对数据里面的一些特殊字符做处理.在原始模式下.终端不会对接收到的数据做任何的处理.在这里input_available_p()在判断是否有数据可读也分两种情况进行,对于规范模式,看是否有已经转换好的数据,对于原始模式,判断接收的信息总数

/* Deal with packet mode. */

//packet模式`忽略

if (tty->packet && b == buf) {

if (tty_put_user(tty, TIOCPKT_DATA, b++)) {

retval = -EFAULT;

b--;

break;

}

nr--;

}

if (tty->icanon) {

/* N.B. avoid overrun if nr == 0 */

while (nr && tty->read_cnt) {

int eol;

eol = test_and_clear_bit(tty->read_tail,

tty->read_flags);

c = tty->read_buf[tty->read_tail];

spin_lock_irqsave(&tty->read_lock, flags);

tty->read_tail = ((tty->read_tail+1) &

(N_TTY_BUF_SIZE-1));

tty->read_cnt--;

if (eol) {

/* this test should be redundant:

* we shouldn't be reading data if

* canon_data is 0

*/

if (--tty->canon_data < 0)

tty->canon_data = 0;

}

spin_unlock_irqrestore(&tty->read_lock, flags);

//如果没有到结束字符,将字符copy到数据空间

//__DISABLED_CHAR是不需要copy到用户空间的

if (!eol || (c != __DISABLED_CHAR)) {

if (tty_put_user(tty, c, b++)) {

retval = -EFAULT;

b--;

break;

}

nr--;

}

if (eol) {

//如果遇到行结束符.就可以退出了

tty_audit_push(tty);

break;

}

}

if (retval)

break;

} else {

//非加工模式,直接copy

int uncopied;

//环形缓存,copy两次

uncopied = copy_from_read_buf(tty, &b, &nr);

uncopied += copy_from_read_buf(tty, &b, &nr);

if (uncopied) {

retval = -EFAULT;

break;

}

}