学步园

根据前面的操作，串口作为字符驱动也已经注册到系统了，/dev目录下也有设备文件节点了。

那接下来uart的操作是如何进行的呢?

操作硬件之前都是要先open设备，先来分析下这里的open函数具体做了那些工作(做了大量工作 ，真的！)。

应用层通过open系统调用open(“/dev/s3c2410_serial0”，)一层一层调用到会调用到tty_open。

因为串口在linux下是作为tty设备的，结合前面的注册过程可以分析这里首先调用的就是tty_open这个函数。

cdev_init(&driver->cdev, &tty_fops);

因为根据注册的时候将s3c2410_serial0注册为一个字符设备，字符设备对应的驱动为tty_fops

static const struct file_operations tty_fops = {
	.llseek		= no_llseek,
	.read		= tty_read,
	.write		= tty_write,
	.poll		= tty_poll,
	.unlocked_ioctl	= tty_ioctl,
	.compat_ioctl	= tty_compat_ioctl,
	.open		= tty_open,
	.release		= tty_release,
	.fasync		= tty_fasync,
};

所以这里就调用的是tty_open函数。

下面具体分析tty_open

static int tty_open(struct inode *inode, struct file *filp)
{
	struct tty_struct *tty = NULL;
	int noctty, retval;
	struct tty_driver *driver;
	int index;
	dev_t device = inode->i_rdev;
	unsigned saved_flags = filp->f_flags;

	nonseekable_open(inode, filp);

retry_open:
	noctty = filp->f_flags & O_NOCTTY;
	index  = -1;
	retval = 0;

	mutex_lock(&tty_mutex);
	tty_lock();

	if (device == MKDEV(TTYAUX_MAJOR, 0)) {
		tty = get_current_tty();
		if (!tty) {
			tty_unlock();
			mutex_unlock(&tty_mutex);
			return -ENXIO;
		}
		driver = tty_driver_kref_get(tty->driver);
		index = tty->index;
		filp->f_flags |= O_NONBLOCK; /* Don't let /dev/tty block */
		/* noctty = 1; */
		/* FIXME: Should we take a driver reference ? */
		tty_kref_put(tty);
		goto got_driver;
	}
#ifdef CONFIG_VT
	if (device == MKDEV(TTY_MAJOR, 0)) {
		extern struct tty_driver *console_driver;
		driver = tty_driver_kref_get(console_driver);
		index = fg_console;
		noctty = 1;
		goto got_driver;
	}
#endif
	if (device == MKDEV(TTYAUX_MAJOR, 1)) {
		struct tty_driver *console_driver = console_device(&index);
		if (console_driver) {
			driver = tty_driver_kref_get(console_driver);
			if (driver) {
				/* Don't let /dev/console block */
				filp->f_flags |= O_NONBLOCK;
				noctty = 1;
				goto got_driver;
			}
		}
		tty_unlock();
		mutex_unlock(&tty_mutex);
		return -ENODEV;
	}

	driver = get_tty_driver(device, &index);
	if (!driver) {
		tty_unlock();
		mutex_unlock(&tty_mutex);
		return -ENODEV;
	}
got_driver:
	if (!tty) {
		/* check whether we're reopening an existing tty */
		tty = tty_driver_lookup_tty(driver, inode, index);

		if (IS_ERR(tty)) {
			tty_unlock();
			mutex_unlock(&tty_mutex);
			return PTR_ERR(tty);
		}
	}

	if (tty) {
		retval = tty_reopen(tty);
		if (retval)
			tty = ERR_PTR(retval);
	} else
		tty = tty_init_dev(driver, index, 0);

	mutex_unlock(&tty_mutex);
	tty_driver_kref_put(driver);
	if (IS_ERR(tty)) {
		tty_unlock();
		return PTR_ERR(tty);
	}

	retval = tty_add_file(tty, filp);
	if (retval) {
		tty_unlock();
		return retval;
	}

	check_tty_count(tty, "tty_open");
	if (tty->driver->type == TTY_DRIVER_TYPE_PTY &&
	    tty->driver->subtype == PTY_TYPE_MASTER)
		noctty = 1;
#ifdef TTY_DEBUG_HANGUP
	printk(KERN_DEBUG "opening %s...", tty->name);
#endif
	if (!retval) {
		if (tty->ops->open)
			retval = tty->ops->open(tty, filp);
		else
			retval = -ENODEV;
	}
	filp->f_flags = saved_flags;

	if (!retval && test_bit(TTY_EXCLUSIVE, &tty->flags) &&
						!capable(CAP_SYS_ADMIN))
		retval = -EBUSY;

	if (retval) {
#ifdef TTY_DEBUG_HANGUP
		printk(KERN_DEBUG "error %d in opening %s...", retval,
		       tty->name);
#endif
		tty_unlock(); /* need to call tty_release without BTM */
		tty_release(inode, filp);
		if (retval != -ERESTARTSYS)
			return retval;

		if (signal_pending(current))
			return retval;

		schedule();
		/*
		 * Need to reset f_op in case a hangup happened.
		 */
		tty_lock();
		if (filp->f_op == &hung_up_tty_fops)
			filp->f_op = &tty_fops;
		tty_unlock();
		goto retry_open;
	}
	tty_unlock();


	mutex_lock(&tty_mutex);
	tty_lock();
	spin_lock_irq(¤t->sighand->siglock);
	if (!noctty &&
	    current->signal->leader &&
	    !current->signal->tty &&
	    tty->session == NULL)
		__proc_set_tty(current, tty);
	spin_unlock_irq(&current->sighand->siglock);
	tty_unlock();
	mutex_unlock(&tty_mutex);
	return 0;
}

函数首先判断打开的设备是否是

5 0(/dev/tty)

5 1(/dev/console)

4 0(/dev/tty0)

此处打开的是/dev/s3c2410_serial0设备号为204 64

所以前面的判断全部失败，直接执行标红的那条语句

driver = get_tty_driver(device, &index);

get_tty_driver函数如下：

static struct tty_driver *get_tty_driver(dev_t device, int *index)
{
	struct tty_driver *p;

	list_for_each_entry(p, &tty_drivers, tty_drivers) {
		dev_t base = MKDEV(p->major, p->minor_start);
		if (device < base || device >= base + p->num)
			continue;
		*index = device - base;
		return tty_driver_kref_get(p);
	}
	return NULL;
}

可见，此函数的作用就是通过设备号来找到设备对应的tty_driver，并且将索引号码保存在index中

因为一个tty_driver对应的是所有此种类型的tty设备，比如所有的串口设备，所以需要通过这个索引号

index来判断打开的是具体哪个设备。并且每个具体的设备对应着一个用来描述自己的tty_struct。

而系统后面的操作全部和这个tty_struct相关。

然后接着open函数会判断是否有tty_struct，假如不存在则创建并初始化一个tty_struct。

tty_struct是tty结构体中最重要的一个数据结构，内核通过tty-struct这个结构体来描述一个具体的tty设备在内核中

的活动状况的，后面对设备的write、read中都需要使用到这个tty_struct，并且严重依赖这个结构体。

由于此处是不存在tty_struct,所以直接调用函数初始化tty_struct

		tty = tty_init_dev(driver, index, 0);

参数driver为tty_driver，是前面通过get_tty_driver获得的，index此处为0，也是通过前面get_tty_driver获得的。

tty_init_dev()函数具体如下

struct tty_struct *tty_init_dev(struct tty_driver *driver, int idx,
								int first_ok)
{
	struct tty_struct *tty;
	int retval;

	/* Check if pty master is being opened multiple times */
	if (driver->subtype == PTY_TYPE_MASTER &&
		(driver->flags & TTY_DRIVER_DEVPTS_MEM) && !first_ok) {
		return ERR_PTR(-EIO);
	}

	/*
	 * First time open is complex, especially for PTY devices.
	 * This code guarantees that either everything succeeds and the
	 * TTY is ready for operation, or else the table slots are vacated
	 * and the allocated memory released.  (Except that the termios
	 * and locked termios may be retained.)
	 */

	if (!try_module_get(driver->owner))
		return ERR_PTR(-ENODEV);

	tty = alloc_tty_struct();
	if (!tty)
		goto fail_no_mem;
	initialize_tty_struct(tty, driver, idx);

	retval = tty_driver_install_tty(driver, tty);
	if (retval < 0) {
		free_tty_struct(tty);
		module_put(driver->owner);
		return ERR_PTR(retval);
	}

	/*
	 * Structures all installed ... call the ldisc open routines.
	 * If we fail here just call release_tty to clean up.  No need
	 * to decrement the use counts, as release_tty doesn't care.
	 */
	retval = tty_ldisc_setup(tty, tty->link);
	if (retval)
		goto release_mem_out;
	return tty;

fail_no_mem:
	module_put(driver->owner);
	return ERR_PTR(-ENOMEM);

	/* call the tty release_tty routine to clean out this slot */
release_mem_out:
	if (printk_ratelimit())
		printk(KERN_INFO "tty_init_dev: ldisc open failed, "
				 "clearing slot %d\n", idx);
	release_tty(tty, idx);
	return ERR_PTR(retval);
}

这个函数首先是给tty_struct分配好内存，然后将其初始化，初始化的工作在函数

	initialize_tty_struct(tty, driver, idx);

中完成。参数tty为待初始化的tty_struct，driver为前面通过get_tty_driver获得的tty_driver，index为具体的索引号，同样是通过get_tty_driver获得的，此处index值为0。

初始化的tty_struct部分信息来自于tty_driver，所以将tty_driver传递进此函数。

具体的initialize_tty_struct函数如下

void initialize_tty_struct(struct tty_struct *tty,
		struct tty_driver *driver, int idx)
{
	memset(tty, 0, sizeof(struct tty_struct));
	kref_init(&tty->kref);
	tty->magic = TTY_MAGIC;
	tty_ldisc_init(tty);
	tty->session = NULL;
	tty->pgrp = NULL;
	tty->overrun_time = jiffies;
	tty->buf.head = tty->buf.tail = NULL;
	tty_buffer_init(tty);
	mutex_init(&tty->termios_mutex);
	mutex_init(&tty->ldisc_mutex);
	init_waitqueue_head(&tty->write_wait);
	init_waitqueue_head(&tty->read_wait);
	INIT_WORK(&tty->hangup_work, do_tty_hangup);
	mutex_init(&tty->atomic_read_lock);
	mutex_init(&tty->atomic_write_lock);
	mutex_init(&tty->output_lock);
	mutex_init(&tty->echo_lock);
	spin_lock_init(&tty->read_lock);
	spin_lock_init(&tty->ctrl_lock);
	INIT_LIST_HEAD(&tty->tty_files);
	INIT_WORK(&tty->SAK_work, do_SAK_work);

	tty->driver = driver;
	tty->ops = driver->ops;
	tty->index = idx;
	tty_line_name(driver, idx, tty->name);
	tty->dev = tty_get_device(tty);
}

初始化中最重要的两步已标红，其中第一步和tty线路规程相关。之前分析过tty线路规程初始化部分的代码，

而这里的初始化就是根据数组的索引号，从前面初始化好的tty线路规程操作方法数组中获取对应的操作方法，然后将其填充到

tty_struct中对应的域中。具体如下

void tty_ldisc_init(struct tty_struct *tty)
{
	struct tty_ldisc *ld = tty_ldisc_get(N_TTY);
	if (IS_ERR(ld))
		panic("n_tty: init_tty");
	tty_ldisc_assign(tty, ld);
}

可见索引号是N_TTY，也就是tty_ldiscs[0]中的tty线路规程操纵方法集，tty_ldiscs[0]对应的具体操作集如下

    struct tty_ldisc_ops 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            = n_tty_read,  
        .write           = n_tty_write,  
        .ioctl           = n_tty_ioctl,  
        .set_termios     = n_tty_set_termios,  
        .poll            = n_tty_poll,  
        .receive_buf     = n_tty_receive_buf,  
        .write_wakeup    = n_tty_write_wakeup  
    };  

在write过程中会调用其中的方法。tty_write->n_tty_write。

最后通过函数

tty_ldisc_assign(tty, ld);

将其填充到tty_struct中。

在这个tty_struct的初始化函数还需要注意的是

	tty->ops = driver->ops;

也就是将tty_struct中的ops设置成tty_driver中的ops。后面write、read的操作调用的是tty_struct中的ops而非tty_driver

中的ops

此处的tty_driver的ops在uart_register_driver中被设置成如下

static const struct tty_operations uart_ops = {
	.open		= uart_open,
	.close		= uart_close,
	.write		= uart_write,
	.put_char	= uart_put_char,
	.flush_chars	= uart_flush_chars,
	.write_room	= uart_write_room,
	.chars_in_buffer= uart_chars_in_buffer,
	.flush_buffer	= uart_flush_buffer,
	.ioctl		= uart_ioctl,
	.throttle	= uart_throttle,
	.unthrottle	= uart_unthrottle,
	.send_xchar	= uart_send_xchar,
	.set_termios	= uart_set_termios,
	.set_ldisc	= uart_set_ldisc,
	.stop		= uart_stop,
	.start		= uart_start,
	.hangup		= uart_hangup,
	.break_ctl	= uart_break_ctl,
	.wait_until_sent= uart_wait_until_sent,
#ifdef CONFIG_PROC_FS
	.proc_fops	= &uart_proc_fops,
#endif
	.tiocmget	= uart_tiocmget,
	.tiocmset	= uart_tiocmset,
	.get_icount	= uart_get_icount,
#ifdef CONFIG_CONSOLE_POLL
	.poll_init	= uart_poll_init,
	.poll_get_char	= uart_poll_get_char,
	.poll_put_char	= uart_poll_put_char,
#endif
};

接着函数返回到tty_init_dev函数中继续往下执行。

调用函数

retval = tty_driver_install_tty(driver, tty);

这个函数的主要作用就是将这个初始化好的

tty_struct存放到tty_driver的tty_structs[]数组中，存放的位置依据tty-struct->index

这样可以tty_driver中每个设备都可以根据index找到对应的tty_struct了。

最后再次返回tty_init_dev函数中执行

	retval = tty_ldisc_setup(tty, tty->link);

这个函数会调用tty线路规程中的open方法n_tty_open函数

n_tty_open中主要完成buff缓冲区大小设置的工作，具体不展开了(其实我也没仔细分析)。

最后这个tty_struct的初始化函数彻底执行完毕，退到tty_open函数中继续执行后续代码。

在tty_open中会被执行到的代码是

	retval = tty->ops->open(tty, filp);

这里就是tty-struct中的ops，在tty_struct初始化的时候被赋值成tty_driver的ops，所以这里调用到的就是

uart_open函数。uart_open函数的重要作用是找到之前初始化的时候保存在tty_driver中的uart_state，因为这里面还有uart_port的重要信息。

找到这个uart_state后将其赋值给tty_struct。具体的uart_open函数如下

static int uart_open(struct tty_struct *tty, struct file *filp)
{
	struct uart_driver *drv = (struct uart_driver *)tty->driver->driver_state;
	struct uart_state *state;
	struct tty_port *port;
	int retval, line = tty->index;

	BUG_ON(!tty_locked());
	pr_debug("uart_open(%d) called\n", line);

	/*
	 * tty->driver->num won't change, so we won't fail here with
	 * tty->driver_data set to something non-NULL (and therefore
	 * we won't get caught by uart_close()).
	 */
	retval = -ENODEV;
	if (line >= tty->driver->num)
		goto fail;

	/*
	 * We take the semaphore inside uart_get to guarantee that we won't
	 * be re-entered while allocating the state structure, or while we
	 * request any IRQs that the driver may need.  This also has the nice
	 * side-effect that it delays the action of uart_hangup, so we can
	 * guarantee that state->port.tty will always contain something
	 * reasonable.
	 */
	state = uart_get(drv, line);
	if (IS_ERR(state)) {
		retval = PTR_ERR(state);
		goto fail;
	}
	port = &state->port;

	/*
	 * Once we set tty->driver_data here, we are guaranteed that
	 * uart_close() will decrement the driver module use count.
	 * Any failures from here onwards should not touch the count.
	 */
	tty->driver_data = state;
	state->uart_port->state = state;
	tty->low_latency = (state->uart_port->flags & UPF_LOW_LATENCY) ? 1 : 0;
	tty->alt_speed = 0;
	tty_port_tty_set(port, tty);

	/*
	 * If the port is in the middle of closing, bail out now.
	 */
	if (tty_hung_up_p(filp)) {
		retval = -EAGAIN;
		port->count--;
		mutex_unlock(&port->mutex);
		goto fail;
	}

	/*
	 * Make sure the device is in D0 state.
	 */
	if (port->count == 1)
		uart_change_pm(state, 0);

	/*
	 * Start up the serial port.
	 */
	retval = uart_startup(tty, state, 0);

	/*
	 * If we succeeded, wait until the port is ready.
	 */
	mutex_unlock(&port->mutex);
	if (retval == 0)
		retval = tty_port_block_til_ready(port, tty, filp);

fail:
	return retval;
}

函数通过

	state = uart_get(drv, line);

来找到保存在tty_driver中的uart_state。

最后将其值赋值给tty_struct。此处特别注意一下，这个uart_state会被放到tty_struct的driver_data中的！因为后面的write、read都是