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分类： LINUX
2012-07-12 20:23:21
最近看tty源码，发现一个很不错的源码分析文章，不禁读了一下，稍微做了个注释。下面给大家分享一下。由于原文用的是linux2.6.27的内核，我分析的是2.6.28.8，内核稍有不同，有些函数名有改动，文章中我有注释，不过改动不是很大，相比于3.4.4的内核来说，还是改动比较少的啦。
文章中若有错误，或不足，或有补充，欢迎讨论交流！
网址：http://www.linuxidc.com/Linux/2011-02/32060.htm
本文以linux 2.6.27内核为基础，阅读tty core 源码并作注解，自己接触时间不长，希望与爱好者共同分享，错误之处还望指正。
linux tty core 是建立在字符设备驱动的基础之上，并为tty类型设备（串口、控制台、虚拟终端）提供一个公用的平台。所以任何一个tty设备驱动的注册都是作为一个字符设备驱动而操作的。下面我们看看代码中是如何处理的：
/* 3/2004 jmc: why do these devices exist? */
//tty核心默认在内核中实现的字符型tty设备驱动
在drivers/char/tty_io.c文件中
static struct cdev tty_cdev, console_cdev;
#ifdef CONFIG_UNIX98_PTYS
static struct cdev ptmx_cdev;
#endif
#ifdef CONFIG_VT
static struct cdev vc0_cdev;
#endif
/*
* Ok, now we can initialize the rest of the tty devices and can count
* on memory allocations, interrupts etc..
*/
static int __init tty_init(void)
{
//在字符设备模型中加入注册tty_cdev驱动并加入/dev/tty这样的设备
cdev_init(&tty_cdev, &tty_fops);
if (cdev_add(&tty_cdev, MKDEV(TTYAUX_MAJOR, 0), 1) ||
register_chrdev_region(MKDEV(TTYAUX_MAJOR, 0), 1, "/dev/tty") < 0)
panic("Couldn't register /dev/tty driver\n");
device_create_drvdata(tty_class, NULL, MKDEV(TTYAUX_MAJOR, 0), NULL,
"tty");
//在字符设备模型中加入注册console_cdev驱动并加入/dev/console这样的设备
cdev_init(&console_cdev, &console_fops);
if (cdev_add(&console_cdev, MKDEV(TTYAUX_MAJOR, 1), 1) ||
register_chrdev_region(MKDEV(TTYAUX_MAJOR, 1), 1, "/dev/console") < 0)
panic("Couldn't register /dev/console driver\n");
device_create_drvdata(tty_class, NULL, MKDEV(TTYAUX_MAJOR, 1), NULL,
"console");
//在字符设备模型中加入注册ptmx_cdev驱动并加入/dev/ptmx这样的设备
#ifdef CONFIG_UNIX98_PTYS
cdev_init(&ptmx_cdev, &ptmx_fops);
if (cdev_add(&ptmx_cdev, MKDEV(TTYAUX_MAJOR, 2), 1) ||
register_chrdev_region(MKDEV(TTYAUX_MAJOR, 2), 1, "/dev/ptmx") < 0)
panic("Couldn't register /dev/ptmx driver\n");
device_create_drvdata(tty_class, NULL, MKDEV(TTYAUX_MAJOR, 2), NULL, "ptmx");
#endif
//在字符设备模型中加入注册vc0_cdev驱动并加入/dev/tty0这样的设备
#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_drvdata(tty_class, NULL, MKDEV(TTY_MAJOR, 0), NULL, "tty0");
vty_init(); //这里暂时不做解释
#endif
return 0;
}
//上面是本身加入的，也就是我们系统一般都有的设备，而且这几种设备都是指向其他的设备，在tty_open中我们将看到以后都会指向其
//他具体的设备。对这几种设备大多数人都存在一定的混淆，这里我就自己的理解解释下：
// /dev/console (5,1) 表示系统的控制台
// /dev/tty (5,0) 表示进程的控制终端
// /dev/tty0 (4,0) 表示当前使用的虚拟终端
// 这样的解释也不是很清楚，这得从历史说起，以前计算机还是比较昂贵的时候，一台电脑上一般接有很多键盘与显示器的组合设备用以
//操作计算机这样的组合设备就是所谓的终端，而还存在一种直接和电脑连接键盘和显示器这就是控制台。而现在的应用环境发生了变化，
//一般把能直接显示系统信息的终端称呼为系统控制台，而其他设备则称呼虚拟终端。也就是当前虚拟终端作控制台。
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,
};
#ifdef CONFIG_UNIX98_PTYS
static const struct file_operations ptmx_fops = {
.llseek = no_llseek,
.read = tty_read,
.write = tty_write,
.poll = tty_poll,
.unlocked_ioctl = tty_ioctl,
.compat_ioctl = tty_compat_ioctl,
.open = ptmx_open,
.release = tty_release,
.fasync = tty_fasync,
};
#endif
static const struct file_operations console_fops = {
.llseek = no_llseek,
.read = tty_read,
.write = redirected_tty_write,
.poll = tty_poll,
.unlocked_ioctl = tty_ioctl,
.compat_ioctl = tty_compat_ioctl,
.open = tty_open,
.release = tty_release,
.fasync = tty_fasync,
};
//从上面看几个驱动的操作函数大致相同，只有ptmx_fops的open方法和console_fops的write方法不同其他操作都是相同的所以在其
//他操作上要兼顾各种设备
//下面我们介绍下tty_driver结构和tty_struct 结构。tty_driver表示一个具体的tty设备的驱动程序，而tty_struct 表示tty设备在
//具体的分析中介绍其成员。
源码在 /include/linux/tty_driver.h中(包含tty_driver and tty_operations的定义)
struct tty_driver {
int magic; /* magic number for this structure */
struct kref kref; /* Reference management */
struct cdev cdev;//可见tty设备驱动是一个字符设备设备驱动
struct module *owner;
const char *driver_name;//这里是指驱动程序的名字
const char *name;//tty设备的命名相关
int name_base; /* offset of printed name */
int major; /* major device number */
int minor_start; /* start of minor device number */
int minor_num; /* number of *possible* devices */
int num; /* number of devices allocated */
short type; /* type of tty driver */
short subtype; /* subtype of tty driver */
struct ktermios init_termios; /* Initial termios */
int flags; /* tty driver flags */
struct proc_dir_entry *proc_entry; /* /proc fs entry */
struct tty_driver *other; /* only used for the PTY driver */
/*
* Pointer to the tty data structures
*/
struct tty_struct **ttys;//驱动操作的具体tty设备
struct ktermios **termios;
struct ktermios **termios_locked;
void *driver_state;
/*
* Driver methods
*/
const struct tty_operations *ops;
struct list_head tty_drivers; //用于链接tty驱动全局链表
};
//下面对tty_open函数进行分析，open函数的具体操作就是初始化tty_struct结构并作为赋值filp->private_data,为后续的操作做准备
/**
* tty_open - open a tty device
* @inode: inode of device file
* @filp: file pointer to tty
*
* tty_open and tty_release keep up the tty count that contains the
* number of opens done on a tty. We cannot use the inode-count, as
* different inodes might point to the same tty.
*
* Open-counting is needed for pty masters, as well as for keeping
* track of serial lines: DTR is dropped when the last close happens.
* (This is not done solely through tty->count, now. - Ted 1/27/92)
*
* The termios state of a pty is reset on first open so that
* settings don't persist across reuse.
*
* Locking: tty_mutex protects tty, get_tty_driver and init_dev work.
* tty->count should protect the rest.
* ->siglock protects ->signal/->sighand
*/
static int __tty_open(struct inode *inode, struct file *filp)
{
struct tty_struct *tty;
int noctty, retval;
struct tty_driver *driver;
int index;
dev_t device = inode->i_rdev;//得到设备的设备号
unsigned short saved_flags = filp->f_flags;
nonseekable_open(inode, filp);//主要是设置file->f_mode的一些标志使文件不可定位
retry_open:
noctty = filp->f_flags & O_NOCTTY；//是否终端绑定到指定的进程
index = -1;
retval = 0;
mutex_lock(&tty_mutex);
//这里表示的是设备号为（5,0）及/dev/tty是进程控制终端的别名，因此是一个要找到真正的设备
if (device == MKDEV(TTYAUX_MAJOR, 0)) {
tty = get_current_tty();
if (!tty) {//进程没有控制终端则直接返回
mutex_unlock(&tty_mutex);
return -ENXIO;
}
driver = tty->driver;
index = tty->index;//设备在驱动中的driver->ttys中的位置
filp->f_flags |= O_NONBLOCK; /* Don't let /dev/tty block */
/* noctty = 1; */
goto got_driver;
}
#ifdef CONFIG_VT
//这里表示的是设备号为（4,0）及/dev/tty0 是当前进程使用虚拟终端的别名，因此是一个要找到真正的设备
if (device == MKDEV(TTY_MAJOR, 0)) {
extern struct tty_driver *console_driver;//控制台的驱动
driver = console_driver;
index = fg_console;//表示当前控制台索引
noctty = 1;
goto got_driver;
}
#endif
//这里表示的是设备号为（5,1）及/dev/console 是指系统控制台，因此是一个要找到真正的设备
if (device == MKDEV(TTYAUX_MAJOR, 1)) {
driver = console_device(&index);
if (driver) {
/* Don't let /dev/console block */
filp->f_flags |= O_NONBLOCK;
noctty = 1;
goto got_driver;
}
mutex_unlock(&tty_mutex);
return -ENODEV;
}
//依据设备的设备号来确定设备在具体驱动中的索引。
driver = get_tty_driver(device, &index);
if (!driver) {
mutex_unlock(&tty_mutex);
return -ENODEV;
}
/*到此位置我们的目的是确定设备驱动和设备在驱动中的位置以及设备是否和进程绑定*/
got_driver:
//init_dev 初始化一个tty_struct 结构 具体在下面分析
retval = init_dev(driver, index, &tty);
mutex_unlock(&tty_mutex);
if (retval)
return retval;
filp->private_data = tty;//把tty_struct结构作为file->private_data这也是open函数的主要目的
file_move(filp, &tty->tty_files);
check_tty_count(tty, "tty_open");//统计tty设备打开的次数
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);//调用tty_operations中的回调函数tty驱动中我们要实现的%%%%%
else
retval = -ENODEV;
}
filp->f_flags = saved_flags;
if (!retval && test_bit(TTY_EXCLUSIVE, &tty->flags) && //设备是否为互斥设备
!capable(CAP_SYS_ADMIN))
retval = -EBUSY;
if (retval) {
#ifdef TTY_DEBUG_HANGUP
printk(KERN_DEBUG "error %d in opening %s...", retval,
tty->name);
#endif
release_dev(filp);//init_dev的反操作
if (retval != -ERESTARTSYS)
return retval;
if (signal_pending(current))
return retval;
schedule();
/*
* Need to reset f_op in case a hangup happened.
*/
if (filp->f_op == &hung_up_tty_fops)
filp->f_op = &tty_fops;
goto retry_open;
}
mutex_lock(&tty_mutex);
spin_lock_irq(¤t->sighand->siglock);
if (!noctty && //设备和进程绑定作为进程回话的控制终端
current->signal->leader &&
!current->signal->tty &&
tty->session == NULL)
__proc_set_tty(current, tty);
spin_unlock_irq(¤t->sighand->siglock);
mutex_unlock(&tty_mutex);
return 0;
}
/* BKL pushdown: scary code avoidance wrapper */
static int tty_open(struct inode *inode, struct file *filp)
{
int ret;
lock_kernel();
ret = __tty_open(inode, filp);
unlock_kernel();
return ret;
}
/**
* init_dev - initialise a tty device
* @driver: tty driver we are opening a device on
* @idx: device index
* @tty: returned tty structure
*
* Prepare a tty device. This may not be a "new" clean device but
* could also be an active device. The pty drivers require special
* handling because of this.
*
* Locking:
* The function is called under the tty_mutex, which
* protects us from the tty struct or driver itself going away.
*
* On exit the tty device has the line discipline attached and
* a reference count of 1. If a pair was created for pty/tty use
* and the other was a pty master then it too has a reference count of 1.
*
* WSH 06/09/97: Rewritten to remove races and properly clean up after a
* failed open. The new code protects the open with a mutex, so it's
* really quite straightforward. The mutex locking can probably be
* relaxed for the (most common) case of reopening a tty.
*/
static int init_dev(struct tty_driver *driver, int idx,
struct tty_struct **ret_tty)
{
struct tty_struct *tty, *o_tty;
struct ktermios *tp, **tp_loc, *o_tp, **o_tp_loc;
struct ktermios *ltp, **ltp_loc, *o_ltp, **o_ltp_loc;
int retval = 0;
/* check whether we're reopening an existing tty */
if (driver->flags & TTY_DRIVER_DEVPTS_MEM) {//伪终端初始化，从设备。
tty = devpts_get_tty(idx);
/*
* If we don't have a tty here on a slave open, it's because
* the master already started the close process and there's
* no relation between devpts file and tty anymore.
*/
if (!tty && driver->subtype == PTY_TYPE_SLAVE) {
retval = -EIO;
goto end_init;
}
/*
* It's safe from now on because init_dev() is called with
* tty_mutex held and release_dev() won't change tty->count
* or tty->flags without having to grab tty_mutex
*/
if (tty && driver->subtype == PTY_TYPE_MASTER)
tty = tty->link;
} else {
tty = driver->ttys[idx];
}
if (tty) goto fast_track;//fast_track表示设备不是第一次打开
/*
* 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)) {
retval = -ENODEV;
goto end_init;
}
o_tty = NULL;
tp = o_tp = NULL;
ltp = o_ltp = NULL;
tty = alloc_tty_struct();
if (!tty)
goto fail_no_mem;
initialize_tty_struct(tty);
tty->driver = driver;
tty->ops = driver->ops;
tty->index = idx;
tty_line_name(driver, idx, tty->name);
if (driver->flags & TTY_DRIVER_DEVPTS_MEM) {
tp_loc = &tty->termios;
ltp_loc = &tty->termios_locked;
} else {
tp_loc = &driver->termios[idx];
ltp_loc = &driver->termios_locked[idx];
}
if (!*tp_loc) {
tp = kmalloc(sizeof(struct ktermios), GFP_KERNEL);
if (!tp)
goto free_mem_out;
*tp = driver->init_termios;
}
if (!*ltp_loc) {
ltp = kzalloc(sizeof(struct ktermios), GFP_KERNEL);
if (!ltp)
goto free_mem_out;
}
if (driver->type == TTY_DRIVER_TYPE_PTY) {
o_tty = alloc_tty_struct();
if (!o_tty)
goto free_mem_out;
initialize_tty_struct(o_tty);
o_tty->driver = driver->other;
o_tty->ops = driver->ops;
o_tty->index = idx;
tty_line_name(driver->other, idx, o_tty->name);
if (driver->flags & TTY_DRIVER_DEVPTS_MEM) {
o_tp_loc = &o_tty->termios;
o_ltp_loc = &o_tty->termios_locked;
} else {
o_tp_loc = &driver->other->termios[idx];
o_ltp_loc = &driver->other->termios_locked[idx];
}
if (!*o_tp_loc) {
o_tp = kmalloc(sizeof(struct ktermios), GFP_KERNEL);
if (!o_tp)
goto free_mem_out;
*o_tp = driver->other->init_termios;
}
if (!*o_ltp_loc) {
o_ltp = kzalloc(sizeof(struct ktermios), GFP_KERNEL);
if (!o_ltp)
goto free_mem_out;
}
/*
* Everything allocated ... set up the o_tty structure.
*/
if (!(driver->other->flags & TTY_DRIVER_DEVPTS_MEM))
driver->other->ttys[idx] = o_tty;
if (!*o_tp_loc)
*o_tp_loc = o_tp;
if (!*o_ltp_loc)
*o_ltp_loc = o_ltp;
o_tty->termios = *o_tp_loc;
o_tty->termios_locked = *o_ltp_loc;
driver->other->refcount++;
if (driver->subtype == PTY_TYPE_MASTER)
o_tty->count++;
/* Establish the links in both directions */
tty->link = o_tty;
o_tty->link = tty;
}
/*
* All structures have been allocated, so now we install them.
* Failures after this point use release_tty to clean up, so
* there's no need to null out the local pointers.
*/
if (!(driver->flags & TTY_DRIVER_DEVPTS_MEM))
driver->ttys[idx] = tty;
if (!*tp_loc)
*tp_loc = tp;
if (!*ltp_loc)
*ltp_loc = ltp;
tty->termios = *tp_loc;
tty->termios_locked = *ltp_loc;
/* Compatibility until drivers always set this */
tty->termios->c_ispeed = tty_termios_input_baud_rate(tty->termios);
tty->termios->c_ospeed = tty_termios_baud_rate(tty->termios);
driver->refcount++;
tty->count++;
/*
* 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, o_tty);//打开线路规程
if (retval)
goto release_mem_out;
goto success;
/*
* This fast open can be used if the tty is already open.
* No memory is allocated, and the only failures are from
* attempting to open a closing tty or attempting multiple
* opens on a pty master.
*/
fast_track:
if (test_bit(TTY_CLOSING, &tty->flags)) {
retval = -EIO;
goto end_init;
}
if (driver->type == TTY_DRIVER_TYPE_PTY &&
driver->subtype == PTY_TYPE_MASTER) {
/*
* special case for PTY masters: only one open permitted,
* and the slave side open count is incremented as well.
*/
if (tty->count) {
retval = -EIO;
goto end_init;
}
tty->link->count++;
}
tty->count++;
tty->driver = driver; /* N.B. why do this every time?? */
/* FIXME */
if (!test_bit(TTY_LDISC, &tty->flags))
printk(KERN_ERR "init_dev but no ldisc\n");
success:
*ret_tty = tty;
/* All paths come through here to release the mutex */
end_init:
return retval;
/* Release locally allocated memory ... nothing placed in slots */
free_mem_out:
kfree(o_tp);
if (o_tty)
free_tty_struct(o_tty);
kfree(ltp);
kfree(tp);
free_tty_struct(tty);
fail_no_mem:
module_put(driver->owner);
retval = -ENOMEM;
goto end_init;
/* call the tty release_tty routine to clean out this slot */
release_mem_out:
if (printk_ratelimit())
printk(KERN_INFO "init_dev: ldisc open failed, "
"clearing slot %d\n", idx);
release_tty(tty, idx);
goto end_init;
前面分析了open操作，现在分析读操作tty_read。tty_read直接调用线路规程中的读操作从tty->read_buf中读取数据到用户空间。其中tty.read_head记录已读数据的起始位置，tty.read_tail记录已读数据的末尾位置，tty.read_cnt记录已读数据的数量。至于所读数据从何而来我们在下一篇中分析，下面看具体代码：
/**
* tty_read - read method for tty device files
* @file: pointer to tty file
* @buf: user buffer
* @count: size of user buffer
* @ppos: unused
*
* Perform the read system call function on this terminal device. Checks
* for hung up devices before calling the line discipline method.
*
* Locking:
* Locks the line discipline internally while needed. Multiple
* read calls may be outstanding in parallel.
*/
static ssize_t tty_read(struct file *file, char __user *buf, size_t count,
loff_t *ppos)
{
int i;
struct tty_struct *tty;
struct inode *inode;
struct tty_ldisc *ld;
tty = (struct tty_struct *)file->private_data;//得到open中设置的tty结构
inode = file->f_path.dentry->d_inode;
if (tty_paranoia_check(tty, inode, "tty_read")) //tty及其幻数检查
return -EIO;
if (!tty || (test_bit(TTY_IO_ERROR, &tty->flags)))
return -EIO;
/* We want to wait for the line discipline to sort out in this
situation */
ld = tty_ldisc_ref_wait(tty); //TTY_LDISC 表示tty和线路规程绑定
if (ld->ops->read)
i = (ld->ops->read)(tty, file, buf, count);
else
i = -EIO;
tty_ldisc_deref(ld);
if (i > 0)
inode->i_atime = current_fs_time(inode->i_sb);
return i;
}
读操作的具体细节都在线路规程中实现的,默认的线路规程的读操作时read_chan函数，下面看具体源码：
这个函数在2.6.28.6内核中在中的n_tty_read函数，不同版本的函数名不一样，在2.6.27.62版本下就叫read_chan
在/drivers/char/n_tty.c文件最后将n_tty_read和read联系
/**
* read_chan - read function for tty
* @tty: tty device
* @file: file object
* @buf: userspace buffer pointer
* @nr: size of I/O
*
* Perform reads for the line discipline. We are guaranteed that the
* line discipline will not be closed under us but we may get multiple
* parallel readers and must handle this ourselves. We may also get
* a hangup. Always called in user context, may sleep.
*
* This code must be sure never to sleep through a hangup.
*/
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); //声明等待队列项，每次读操作都加入tty.read_wait等待队列
int c;
int minimum, time;
ssize_t retval = 0;
ssize_t size;
long timeout;
unsigned long flags;
int packet;
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); //tty非控制台而是进程控制终端时的处理
if (c < 0)
return c;
minimum = time = 0;
timeout = MAX_SCHEDULE_TIMEOUT;
//tty设备对数据的处理分为原始模式和规范规范模式tty->icannon表示这种模式
//原始模式时根据c_cc[VTIME]和c_cc[VMIN]设置唤醒用户读读进程的超时时间和数据量
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;
}
}
//tty->atomic_read_lock对读操作的互斥保护
/*
* Internal serialization of reads.
*/
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;
}
//伪终端可以用ioctl将主从两端的通讯方式设置为packet模式(信包模式)，tty->link->ctrl_status非零表明提交的是链路控制信息
packet = tty->packet;
add_wait_queue(&tty->read_wait, &wait); //把读进程加入读等待队列
//进行被唤醒，进程被唤醒时一切情况是不可预测的，因此需要对等待条件进行判断。
while (nr) {
/* First test for status change. */
if (packet && tty->link->ctrl_status) {
unsigned char cs;
if (b != buf)
break;
spin_lock_irqsave(&tty->link->ctrl_lock, flags);
cs = tty->link->ctrl_status;
tty->link->ctrl_status = 0;
spin_unlock_irqrestore(&tty->link->ctrl_lock, flags);
if (tty_put_user(tty, cs, b++)) {
retval = -EFAULT;
b--;
break;
}
nr--;
break;
}
/* 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));
//判断有没有数据可读，原始模式和规范模式有差异，以下表示无数据可读若读条件不成立则退出，否则再次加入到等待队列中
if (!input_available_p(tty, 0)) {
if (test_bit(TTY_OTHER_CLOSED, &tty->flags)) { //伪终端对待端关闭
retval = -EIO;
break;
}
if (tty_hung_up_p(file))
break;
if (!timeout)
break;
if (file->f_flags & O_NONBLOCK) { //非阻塞模式
retval = -EAGAIN;
break;
}
if (signal_pending(current)) {//TIF_SIGPENDING进程有信号要处理
retval = -ERESTARTSYS;
break;
}
/* FIXME: does n_tty_set_room need locking ? */
n_tty_set_room(tty); //设置接收空间
timeout = schedule_timeout(timeout);
continue;
}
//有数据可读，设置进程为可执行状态
__set_current_state(TASK_RUNNING);
/* Deal with packet mode. */
if (packet && b == buf) { //伪终端的信包模式
if (tty_put_user(tty, TIOCPKT_DATA, b++)) {
retval = -EFAULT;
b--;
break;
}
nr--;
}
//规范模式的读处理：规范模式下缓冲区的数据是经过加工的，要积累起一个缓冲行,才唤醒等待读的进程。
if (tty->icanon) {
/* N.B. avoid overrun if nr == 0 */
while (nr && tty->read_cnt) {
int eol;
//tty->read_tail对应的tty->read_flags为1表示缓冲行的终点
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) & //环形缓冲区的处理
(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);
if (!eol || (c != __DISABLED_CHAR)) { //DISABLED '\0'
if (tty_put_user(tty, c, b++)) {
retval = -EFAULT;
b--;
break;
}
nr--;
}
if (eol) {
tty_audit_push(tty);
break;
}
}
if (retval)
break;
} else { //原始模式的处理：直接把数据批量复制，注意调用两次是处理环形缓冲区的回头
int uncopied;
/* The copy function takes the read lock and handles
locking internally for this case */
uncopied = copy_from_read_buf(tty, &b, &nr);
uncopied += copy_from_read_buf(tty, &b, &nr);
if (uncopied) {
retval = -EFAULT;
break;
}
}
/* If there is enough space in the read buffer now, let the
* low-level driver know. We use n_tty_chars_in_buffer() to
* check the buffer, as it now knows about canonical mode.
* Otherwise, if the driver is throttled and the line is
* longer than TTY_THRESHOLD_UNTHROTTLE in canonical mode,
* we won't get any more characters.
*/
//读缓冲区中的可读数据少于某阈值时就调用tty->ops->unthrottle()操作
if (n_tty_chars_in_buffer(tty) <= TTY_THRESHOLD_UNTHROTTLE) {
n_tty_set_room(tty);
check_unthrottle(tty);
}
if (b - buf >= minimum)
break;
if (time)
timeout = time;
}
mutex_unlock(&tty->atomic_read_lock);
remove_wait_queue(&tty->read_wait, &wait);
if (!waitqueue_active(&tty->read_wait))
tty->minimum_to_wake = minimum;
__set_current_state(TASK_RUNNING);
size = b - buf;
if (size) {
retval = size;
if (nr)
clear_bit(TTY_PUSH, &tty->flags);
} else if (test_and_clear_bit(TTY_PUSH, &tty->flags))
goto do_it_again;
n_tty_set_room(tty);
return retval;
}
上面分析了用户空间从tty读数据的过程，读数据时从tty->read_buf那么tty->read_buf中的数据从而而来呢？这就是我们今天要讨论的问题。tty_struct结构中有个 struct tty_bufhead buf 成员，比如当tty串口中有数据过来时就会产生中断，tty就利用tty.buf中的成员保存中断到来的数据，在合适的机会再用tty_flip_buffer_push类函数把tty->buf中的数据保存到tty->read_buf中去，从而就达到数据来源的效果。具体源码如下：
在/include/linux/tty.h中
先看看下面两个数据结构 struct tty_buffer是接收中断数据的一个缓存结构，
struct tty_buffer {
struct tty_buffer *next;
char *char_buf_ptr; //数据缓存
unsigned char *flag_buf_ptr; //数据标志缓存
int used; //已用数据大小
int size; //缓存空间的大小
int commit; //已提交数据大小
int read; //已读走数据大小
/* Data points here */
unsigned long data[0];
};
struct tty_bufhead { //tty_struct结构中临时缓存区的管理结构
struct delayed_work work; //工作队列中的一个工作项
spinlock_t lock;
struct tty_buffer *head; /* Queue head */
struct tty_buffer *tail; /* Active buffer */
struct tty_buffer *free; /* Free queue head */
int memory_used; /* Buffer space used excluding
free queue */
};
在中断程序中我们一般通过int tty_insert_flip_char 把接收到的数据放入tty_struct 的临时缓存中，tty->buf.head指向临时缓存链表的表头，tty->buf.tail指向待操作的临时缓存;
在/include/linux/tty_flip.h
static inline int tty_insert_flip_char(struct tty_struct *tty,
unsigned char ch, char flag)
{
struct tty_buffer *tb = tty->buf.tail;
if (tb && tb->used < tb->size) { //操作缓存存在并有空间
tb->flag_buf_ptr[tb->used] = flag; //数据的标志位
tb->char_buf_ptr[tb->used++] = ch; //接收的数据
return 1;
}
return tty_insert_flip_string_flags(tty, &ch, &flag, 1); //操作缓存不存在或者空间不够时的操作
}
//tty_insert_flip_string_flags 在空间不足的情况下申请临时空间并把接收的数据及标志保存到临时缓存，并返回保存数据大小
在/drivers/char/tty_buffer.c
/**
* tty_insert_flip_string_flags - Add characters to the tty buffer
* @tty: tty structure
* @chars: characters
* @flags: flag bytes
* @size: size
*
* Queue a series of bytes to the tty buffering. For each character
* the flags array indicates the status of the character. Returns the
* number added.
*
* Locking: Called functions may take tty->buf.lock
*/
int tty_insert_flip_string_flags(struct tty_struct *tty,
const unsigned char *chars, const char *flags, size_t size)
{
int copied = 0;
do {
int space = tty_buffer_request_room(tty, size - copied);//申请临时缓存空间,此时buf.tail已经改变了（仔细体会内部的巧妙的指针运用）
struct tty_buffer *tb = tty->buf.tail;
/* If there is no space then tb may be NULL */
if (unlikely(space == 0))
break;
memcpy(tb->char_buf_ptr + tb->used, chars, space);
memcpy(tb->flag_buf_ptr + tb->used, flags, space);
tb->used += space;
copied += space;
chars += space;
flags += space;
/* There is a small chance that we need to split the data over
several buffers. If this is the case we must loop */
} while (unlikely(size > copied));
return copied;
}
当接收到的外界数据保存到临时缓存后通过tty_flip_buffer_push把临时缓存中的数据发送到tty->read_buf中供用户空间读取
在/drivers/char/tty_buffer.c中
/**
* tty_flip_buffer_push - terminal
* @tty: tty to push
*
* Queue a push of the terminal flip buffers to the line discipline. This
* function must not be called from IRQ context if tty->low_is set.
*
* In the event of the queue being busy for flipping the work will be
* held off and retried later.
*
* Locking: tty buffer lock. Driver locks in low latency mode.
*/
void tty_flip_buffer_push(struct tty_struct *tty)
{
unsigned long flags;
spin_lock_irqsave(&tty->buf.lock, flags);
if (tty->buf.tail != NULL)
tty->buf.tail->commit = tty->buf.tail->used;
spin_unlock_irqrestore(&tty->buf.lock, flags);
if (tty->low_latency) //tty->low_latency //设置时直接提交到tty->read_buf 否则采用工作队列方式
flush_to_ldisc(&tty->buf.work.work);
else
schedule_delayed_work(&tty->buf.work, 1);
}
//把临时缓存中的数据提交到线路规程中去即tty->read_buf中去
/**
* flush_to_ldisc
* @work: tty structure passed from work queue.
*
* This routine is called out of the software interrupt to flush data
* from the buffer chain to the line discipline.
*
* Locking: holds tty->buf.lock to guard buffer list. Drops the lock
* while invoking the line discipline receive_buf method. The
* receive_buf method is single threaded for each tty instance.
*/
static void flush_to_ldisc(struct work_struct *work)
{
struct tty_struct *tty =
container_of(work, struct tty_struct, buf.work.work);
unsigned long flags;
struct tty_ldisc *disc;
struct tty_buffer *tbuf, *head;
char *char_buf;
unsigned char *flag_buf; //tty存在线路规程并且设置了线路规程
disc = tty_ldisc_ref(tty);
if (disc == NULL) /* !TTY_LDISC */
return;
spin_lock_irqsave(&tty->buf.lock, flags);
/* So we know a flush is running */
set_bit(TTY_FLUSHING, &tty->flags); //设置数据提交标志
head = tty->buf.head;
if (head != NULL) {
tty->buf.head = NULL;
for (;;) {
int count = head->commit - head->read; //确定要提交数据的大小
if (!count) { //没有数据要提交就继续下个临时缓存
if (head->next == NULL)
break;
tbuf = head;
head = head->next;
tty_buffer_free(tty, tbuf);
continue;
}
/* Ldisc or user is trying to flush the buffers
we are feeding to the ldisc, stop feeding the
line discipline as we want to empty the queue */
if (test_bit(TTY_FLUSHPENDING, &tty->flags))
break;
if (!tty->receive_room) { //tty->read_buf 空间不够时提交到工作队列中去处理
schedule_delayed_work(&tty->buf.work, 1);
break;
}
if (count > tty->receive_room)
count = tty->receive_room;
char_buf = head->char_buf_ptr + head->read;
flag_buf = head->flag_buf_ptr + head->read;
head->read += count;
spin_unlock_irqrestore(&tty->buf.lock, flags);
disc->ops->receive_buf(tty, char_buf,
flag_buf, count); //调用线路规程的ldisc->receive_buf把数据从临时缓存提交到
tty->read_buf 在2.6.27.62内核中通过n_tty.c文件的最后将n_tty_receive_buf函数绑定到此处的receive_buf函数。而在2.6.28.6中也是实现同样的绑定
spin_lock_irqsave(&tty->buf.lock, flags);
}
/* Restore the queue head */
tty->buf.head = head;
}
/* We may have a deferred request to flush the input buffer,
if so pull the chain under the lock and empty the queue */
if (test_bit(TTY_FLUSHPENDING, &tty->flags)) { //提交工作处于等待状态
__tty_buffer_flush(tty);
clear_bit(TTY_FLUSHPENDING, &tty->flags);
wake_up(&tty->read_wait); //唤醒等待读的等待队列，为提交赢得空间
}
clear_bit(TTY_FLUSHING, &tty->flags);
spin_unlock_irqrestore(&tty->buf.lock, flags);
tty_ldisc_deref(disc);
}
/**
* n_tty_receive_buf - data receive
* @tty: terminal device
* @cp: buffer
* @fp: flag buffer
* @count: characters
*
* Called by the terminal driver when a block of characters has
* been received. This function must be called from soft contexts
* not from interrupt context. The driver is responsible for making
* calls one at a time and in order (or using flush_to_ldisc)
*/
static void n_tty_receive_buf(struct tty_struct *tty, const unsigned char *cp,
char *fp, int count)
{
const unsigned char *p;
char *f, flags = TTY_NORMAL;
int i;
char buf[64];
unsigned long cpuflags;
if (!tty->read_buf)
return;
if (tty->real_raw) { //非规范模式的数据处理，这里有两次操作是考虑环形缓存的临界状态
spin_lock_irqsave(&tty->read_lock, cpuflags);
i = min(N_TTY_BUF_SIZE - tty->read_cnt,
N_TTY_BUF_SIZE - tty->read_head);
i = min(count, i);
memcpy(tty->read_buf + tty->read_head, cp, i);
tty->read_head = (tty->read_head + i) & (N_TTY_BUF_SIZE-1);
tty->read_cnt += i;
cp += i;
count -= i;
i = min(N_TTY_BUF_SIZE - tty->read_cnt,
N_TTY_BUF_SIZE - tty->read_head);
i = min(count, i);
memcpy(tty->read_buf + tty->read_head, cp, i); copy两次，可以看出read_buf也是环形缓冲区。
tty->read_head = (tty->read_head + i) & (N_TTY_BUF_SIZE-1);
tty->read_cnt += i;
spin_unlock_irqrestore(&tty->read_lock, cpuflags);
} else { //规范模式具体大家去分析
for (i = count, p = cp, f = fp; i; i--, p++) {
if (f)
flags = *f++;
switch (flags) {
case TTY_NORMAL:
n_tty_receive_char(tty, *p);
break;
case TTY_BREAK:
n_tty_receive_break(tty);
break;
case TTY_PARITY:
case TTY_FRAME:
n_tty_receive_parity_error(tty, *p);
break;
case TTY_OVERRUN:
n_tty_receive_overrun(tty);
break;
default:
printk(KERN_ERR "%s: unknown flag %d\n",
tty_name(tty, buf), flags);
break;
}
}
if (tty->ops->flush_chars)
tty->ops->flush_chars(tty);
}
n_tty_set_room(tty);
if (!tty->icanon && (tty->read_cnt >= tty->minimum_to_wake)) {
kill_fasync(&tty->fasync, SIGIO, POLL_IN);
if (waitqueue_active(&tty->read_wait))
wake_up_interruptible(&tty->read_wait);
}
/*
* Check the remaining room for the input canonicalization
* mode. We don't want to throttle the driver if we're in
* canonical mode and don't have a newline yet!
*/
if (tty->receive_room < TTY_THRESHOLD_THROTTLE) //j接收空间过小可以设置阈值
tty_throttle(tty);
}
tty设备的写操作tty_write首先对写操作的需求做检查，然后调用ldisc->write操作默认即write_chain函数。wrtie_chain通过tty->ops->write或者tty->ops->flush_chars把数据写入到设备中，两者都实现时后者有限。其中write_room函数是用来检测缓存
于空间.
/**
* tty_write - write method for tty device file
* @file: tty file pointer
* @buf: user data to write
* @count: bytes to write
* @ppos: unused
*
* Write data to a tty device via the line discipline.
*
* Locking:
* Locks the line discipline as required
* Writes to the tty driver are serialized by the atomic_write_lock
* and are then processed in chunks to the device. The line discipline
* write method will not be involked in parallel for each device
* The line discipline write method is called under the big
* kernel lock for historical reasons. New code should not rely on this.
*/
static ssize_t tty_write(struct file *file, const char __user *buf,
size_t count, loff_t *ppos)
{
struct tty_struct *tty;
struct inode *inode = file->f_path.dentry->d_inode;
ssize_t ret;
struct tty_ldisc *ld;
tty = (struct tty_struct *)file->private_data;
if (tty_paranoia_check(tty, inode, "tty_write")) //幻数检查及设备结构相关检测
return =-EIO ;
(!tty || !tty->ops->write ||
(test_bit(TTY_IO_ERROR, &tty->flags)))
return -EIO;
/* Short term debug to catch buggy drivers */
if (tty->ops->write_room == NULL)
printk(KERN_ERR "tty driver %s lacks a write_room method.\n",
tty->driver->name);
ld = tty_ldisc_ref_wait(tty);
if (!ld->ops->write)
ret = -EIO;
else
ret = do_tty_write(ld->ops->write, tty, file, buf, count); //调用线路规程write函数即write_chain写入，下面分析
tty_ldisc_deref(ld);
return ret;
}
分配临时缓存给tty->write_buf并把用户空间数据拷贝进去然后调用线路规程写函数即write_chain。
/*
* Split writes up in sane blocksizes to avoid
* denial-of-service type attacks
*/
static inline ssize_t do_tty_write(
ssize_t (*write)(struct tty_struct *, struct file *, const unsigned char *, size_t),
struct tty_struct *tty,
struct file *file,
const char __user *buf,
size_t count)
{
ssize_t ret, written = 0;
unsigned int chunk;
ret = tty_write_lock(tty, file->f_flags & O_NDELAY);
if (ret < 0)
return ret;
/*
* We chunk up writes into a temporary buffer. This
* simplifies low-level drivers immensely, since they
* don't have locking issues and user mode accesses.
*
* But if TTY_NO_WRITE_SPLIT is set, we should use a
* big chunk-size..
*
* The default chunk-size is 2kB, because the NTTY
* layer has problems with bigger chunks. It will
* claim to be able to handle more characters than
* it actually does.
*
* FIXME: This can probably go away now except that 64K chunks
* are too likely to fail unless switched to vmalloc...
*/
chunk = 2048;
if (test_bit(TTY_NO_WRITE_SPLIT, &tty->flags))
chunk = 65536;
if (count < chunk)
chunk = count;
/* write_buf/write_cnt is protected by the atomic_write_lock mutex */
if (tty->write_cnt < chunk) {
unsigned char *buf;
if (chunk < 1024)
chunk = 1024;
buf = kmalloc(chunk, GFP_KERNEL);
if (!buf) {
ret = -ENOMEM;
goto out;
}
kfree(tty->write_buf);
tty->write_cnt = chunk;
tty->write_buf = buf;
}
/* Do the write .. */
for (;;) {
size_t size = count;
if (size > chunk)
size = chunk;
ret = -EFAULT;
if (copy_from_user(tty->write_buf, buf, size))
break;
ret = write(tty, file, tty->write_buf, size);
if (ret <= 0)
break;
written += ret;
buf += ret;
count -= ret;
if (!count)
break;
ret = -ERESTARTSYS;
if (signal_pending(current))
break;
cond_resched();
}
if (written) {
struct inode *inode = file->f_path.dentry->d_inode;
inode->i_mtime = current_fs_time(inode->i_sb);
ret = written;
}
out:
tty_write_unlock(tty);
return ret;
}
//write_chain主要根据数据是否是经过加工的调用tty->ops->flush_chars或者tty->ops->write把数据写入设备
//当写入的空间不足时，且数据没有完全写完则调用schedule()把写操作加入写等待队列
/**
* write_chan - write function for tty
* @tty: tty device
* @file: file object
* @buf: userspace buffer pointer
* @nr: size of I/O
*
* Write function of the terminal device. This is serialized with
* respect to other write callers but not to termios changes, reads
* and other such events. We must be careful with N_TTY as the receive
* code will echo characters, thus calling driver write methods.
*
* This code must be sure never to sleep through a hangup.
*/
static ssize_t write_chan(struct tty_struct *tty, struct file *file,
const unsigned char *buf, size_t nr)
{
const unsigned char *b = buf;
DECLARE_WAITQUEUE(wait, current);
int c;
ssize_t retval = 0;
/* Job control check -- must be done at start (POSIX.1 7.1.1.4). */
//TOSTOP标志设置时若后台进程试图写控制台时将发出SIGTTOU信号，也即执行下面的操作
if (L_TOSTOP(tty) && file->f_op->write != redirected_tty_write) {
retval = tty_check_change(tty); //进程控制终端相关设置
if (retval)
return retval;
}
add_wait_queue(&tty->write_wait, &wait); //代表写进程的等待队列项加入到写等待队列中
while (1) {
set_current_state(TASK_INTERRUPTIBLE);
if (signal_pending(current)) {
retval = -ERESTARTSYS;
break;
}
if (tty_hung_up_p(file) || (tty->link && !tty->link->count)) {
retval = -EIO;
break;
}
//OPOST设置，则操作可以选择加工过的输入
//TTY_HW_COOK_OUT若设置通知线路加工其输出的数据，否则只做拷贝
if (O_OPOST(tty) && !(test_bit(TTY_HW_COOK_OUT, &tty->flags))) {
while (nr > 0) {
//opost_block申请写入空间，并把数据根据加工要求成然后调用tty_operations->write成块写入tty设备
ssize_t num = opost_block(tty, b, nr);
if (num < 0) {
if (num == -EAGAIN)
break;
retval = num;
goto break_out;
}
b += num;
nr -= num;
if (nr == 0)
break;
c = *b;
if (opost(c, tty) < 0)
break;
b++; nr--;
}
if (tty->ops->flush_chars)
tty->ops->flush_chars(tty);
} else {
while (nr > 0) {
c = tty->ops->write(tty, b, nr);
if (c < 0) {
retval = c;
goto break_out;
}
if (!c)
break;
b += c;
nr -= c;
}
}
if (!nr)
break;
if (file->f_flags & O_NONBLOCK) {
retval = -EAGAIN;
break;
}
schedule();
}
break_out:
__set_current_state(TASK_RUNNING);
remove_wait_queue(&tty->write_wait, &wait);
return (b - buf) ? b - buf : retval;
}
tty_ioctl和tty_compat_ioctl都是对设备的控制操作，比较容易理解这里就不做分析，有兴趣的读者可以自己分析。其中tty_compat_ioctl使用在用户空间为32位模式而内核空间为64位模式时将64位转化为32位的操作方式。
剩下的就是最后的操作，当关闭tty设备是调用的tty_release操作，主要是释放前面分配的资费做tty_open的反操作
/**
* tty_release - vfs callback for close
* @inode: inode of tty
* @filp: file pointer for handle to tty
*
* Called the last time each file handle is closed that references
* this tty. There may however be several such references.
*
* Locking:
* Takes bkl. See release_dev
*/
static int tty_release(struct inode *inode, struct file *filp)
{
lock_kernel();
release_dev(filp);
unlock_kernel();
return 0;
}
/*
* Even releasing the tty structures is a tricky business.. We have
* to be very careful that the structures are all released at the
* same time, as interrupts might otherwise get the wrong pointers.
*
* WSH 09/09/97: rewritten to avoid some nasty race conditions that could
* lead to double frees or releasing memory still in use.
*/
static void release_dev(struct file *filp)
{
struct tty_struct *tty, *o_tty;
int pty_master, tty_closing, o_tty_closing, do_sleep;
int devpts;
int idx;
char buf[64];
tty = (struct tty_struct *)filp->private_data;
if (tty_paranoia_check(tty, filp->f_path.dentry->d_inode,
"release_dev"))
return;
check_tty_count(tty, "release_dev"); //对tty设备打开次数进行统计,tty设备的每次打开tty->files创建一个文件描述符
tty_fasync(-1, filp, 0); //从文件队列中删去异步通知结构struct fasync_struct
idx = tty->index;
pty_master = (tty->driver->type == TTY_DRIVER_TYPE_PTY &&
tty->driver->subtype == PTY_TYPE_MASTER); //设备是伪终端主设备
devpts = (tty->driver->flags & TTY_DRIVER_DEVPTS_MEM) != 0;
o_tty = tty->link;
//对设备在驱动对应对对象进行检查
#ifdef TTY_PARANOIA_CHECK
if (idx < 0 || idx >= tty->driver->num) {
printk(KERN_DEBUG "release_dev: bad idx when trying to "
"free (%s)\n", tty->name);
return;
}
if (!(tty->driver->flags & TTY_DRIVER_DEVPTS_MEM)) {
if (tty != tty->driver->ttys[idx]) {
printk(KERN_DEBUG "release_dev: driver.table[%d] not tty "
"for (%s)\n", idx, tty->name);
return;
}
if (tty->termios != tty->driver->termios[idx]) {
printk(KERN_DEBUG "release_dev: driver.termios[%d] not termios "
"for (%s)\n",
idx, tty->name);
return;
}
if (tty->termios_locked != tty->driver->termios_locked[idx]) {
printk(KERN_DEBUG "release_dev: driver.termios_locked[%d] not "
"termios_locked for (%s)\n",
idx, tty->name);
return;
}
}
#endif
#ifdef TTY_DEBUG_HANGUP
printk(KERN_DEBUG "release_dev of %s (tty count=%d)...",
tty_name(tty, buf), tty->count);
#endif
//伪终端设备对等端的检查
#ifdef TTY_PARANOIA_CHECK
if (tty->driver->other &&
!(tty->driver->flags & TTY_DRIVER_DEVPTS_MEM)) {
if (o_tty != tty->driver->other->ttys[idx]) {
printk(KERN_DEBUG "release_dev: other->table[%d] "
"not o_tty for (%s)\n",
idx, tty->name);
return;
}
if (o_tty->termios != tty->driver->other->termios[idx]) {
printk(KERN_DEBUG "release_dev: other->termios[%d] "
"not o_termios for (%s)\n",
idx, tty->name);
return;
}
if (o_tty->termios_locked !=
tty->driver->other->termios_locked[idx]) {
printk(KERN_DEBUG "release_dev: other->termios_locked["
"%d] not o_termios_locked for (%s)\n",
idx, tty->name);
return;
}
if (o_tty->link != tty) {
printk(KERN_DEBUG "release_dev: bad pty pointers\n");
return;
}
}
#endif
if (tty->ops->close)
tty->ops->close(tty, filp); //调用tty->ops->close释放相应资源
/*
* Sanity check: if tty->count is going to zero, there shouldn't be
* any waiters on tty->read_wait or tty->write_wait. We test the
* wait queues and kick everyone out _before_ actually starting to
* close. This ensures that we won't block while releasing the tty
* structure.
*
* The test for the o_tty closing is necessary, since the master and
* slave sides may close in any order. If the slave side closes out
* first, its count will be one, since the master side holds an open.
* Thus this test wouldn't be triggered at the time the slave closes,
* so we do it now.
*
* Note that it's possible for the tty to be opened again while we're
* flushing out waiters. By recalculating the closing flags before
* each iteration we avoid any problems.
*/
while (1) { //循环操作，知道tty设备的的读写操作都完成才退出循环
/* Guard against races with tty->count changes elsewhere and
opens on /dev/tty */
mutex_lock(&tty_mutex);
tty_closing = tty->count <= 1; //是否执行真正的关闭
o_tty_closing = o_tty &&
(o_tty->count <= (pty_master ? 1 : 0));
do_sleep = 0;
if (tty_closing) { //在设备关闭前先完成等待的读写操作
if (waitqueue_active(&tty->read_wait)) {
wake_up(&tty->read_wait);
do_sleep++;
}
if (waitqueue_active(&tty->write_wait)) {
wake_up(&tty->write_wait);
do_sleep++;
}
}
if (o_tty_closing) {
if (waitqueue_active(&o_tty->read_wait)) {
wake_up(&o_tty->read_wait);
do_sleep++;
}
if (waitqueue_active(&o_tty->write_wait)) {
wake_up(&o_tty->write_wait);
do_sleep++;
}
}
if (!do_sleep) //完成所有的读写操作
break;
printk(KERN_WARNING "release_dev: %s: read/write wait queue "
"active!\n", tty_name(tty, buf));
mutex_unlock(&tty_mutex);
schedule();
}
/*
* The closing flags are now consistent with the open counts on
* both sides, and we've completed the last operation that could
* block, so it's safe to proceed with closing.
*/
if (pty_master) {
if (--o_tty->count < 0) {
printk(KERN_WARNING "release_dev: bad pty slave count "
"(%d) for %s\n",
o_tty->count, tty_name(o_tty, buf));
o_tty->count = 0;
}
}
if (--tty->count < 0) {
printk(KERN_WARNING "release_dev: bad tty->count (%d) for %s\n",
tty->count, tty_name(tty, buf));
tty->count = 0;
}
/*
* We've decremented tty->count, so we need to remove this file
* descriptor off the tty->tty_files list; this serves two
* purposes:
* - check_tty_count sees the correct number of file descriptors
* associated with this tty.
* - do_tty_hangup no longer sees this file descriptor as
* something that needs to be handled for hangups.
*/
file_kill(filp); //关闭文件描述符
filp->private_data = NULL;
/*
* Perform some housekeeping before deciding whether to return.
*
* Set the TTY_CLOSING flag if this was the last open. In the
* case of a pty we may have to wait around for the other side
* to close, and TTY_CLOSING makes sure we can't be reopened.
*/
if (tty_closing)
set_bit(TTY_CLOSING, &tty->flags);
if (o_tty_closing)
set_bit(TTY_CLOSING, &o_tty->flags);
/*
* If _either_ side is closing, make sure there aren't any
* processes that still think tty or o_tty is their controlling
* tty.
*/
if (tty_closing || o_tty_closing) {
read_lock(&tasklist_lock);
session_clear_tty(tty->session);
if (o_tty)
session_clear_tty(o_tty->session);
read_unlock(&tasklist_lock);
}
mutex_unlock(&tty_mutex);
/* check whether both sides are closing ... */
if (!tty_closing || (o_tty && !o_tty_closing))
return;
#ifdef TTY_DEBUG_HANGUP
printk(KERN_DEBUG "freeing tty structure...");
#endif
/*
* Ask the line discipline code to release its structures
*/
tty_ldisc_release(tty, o_tty);
/*
* The release_tty function takes care of the details of clearing
* the slots and preserving the termios structure.
*/
release_tty(tty, idx);
/* Make this pty number available for reallocation */
if (devpts)
devpts_kill_index(idx);
}
自此我们分析了linux操作系统中/dev/tty /dev/tty0 /dev/console等设备作为字符设备的驱动程序，同时tty核心也为其他tty设备驱动的注册提供了一个通用的接口和一个通用的管理平台，为其他tty设备驱动的实现提供了一个通用层，下一节中我们将分析tty设备驱动的管理以及如何利用tty核心去实现一个tty设备驱动。
linux tty 核心是构建在标准字符设备驱动之上，提供一系列功能，作为接口为终端类型设备驱动所使用。在前面的分析中我们看到 tty核心控制了通过tty设备的数据流，格式化了这些数据流，所以常规tty驱动的工作不必考虑常规操作方法与用户空间的交互，同时不同的线路规程可以虚拟的插入到任何tty设备上。从前面的各种方法分析上，我们知道tty核心从用户空间得到数据把数据发送到tty线路规程，线路规程驱动把数据传递给tty驱动程序，同时tty设备从硬件那里接收到的数据传递给tty线路规程，线路规程再传递给tty核心。所以tty驱动程序的主要任务放在处理流向或流出硬件的数据上。
tty驱动程序在内核中用struct tty_driver表示，在tty核心中有一个tty_drivers链表，任何tty设备驱动程序都挂在在这个链表上。从tty_open的操作中就有一个get_tty_driver函数就是根据设备号在tty_drivers链表中找到相应的设备驱动程序。
tty类设备主要分为三类：控制台、伪终端、串口。其中前两者前面已经介绍了内核为我们实现好了，所以以后我们写tty设备驱动程序主要就是写串口的设备驱动程序。tty驱动程序通俗的说就是分配一个tty_driver结构初始化后并把其挂载到tty_dirvers链表上。下面我们看看struct tty_driver结构及其成员。
extern struct list_head tty_drivers;
struct tty_driver {
int magic; /* magic number for this structure */
struct cdev cdev; //tty驱动是一个字符设备驱动
struct module *owner;
const char *driver_name; //驱动程序名字
const char *name; //设备名称的一部分
int name_base; /* offset of printed name */
int major; /* major device number */ //主设备号
int minor_start; /* start of minor device number */ //次设备号的起始
int minor_num; /* number of *possible* devices */ //设备号的个数
int num; /* number of devices allocated */ //设备个数
short type; /* type of tty driver */
short subtype; /* subtype of tty driver */
struct ktermios init_termios; /* Initial termios */ //初始化设备的struct ktermmios结构
int flags; /* tty driver flags */
int refcount; /* for loadable tty drivers */ //引用计数
struct proc_dir_entry *proc_entry; /* /proc fs entry */
struct tty_driver *other; /* only used for the PTY driver */ //伪终端时指向通信另一端
/*
* Pointer to the tty data structures
*/
struct tty_struct **ttys; //驱动所属设备的tty_struct 结构
struct ktermios **termios; //每个设备相应的
struct ktermios **termios_locked;
void *driver_state; //驱动状态
/*
* Driver methods
*/
const struct tty_operations *ops; //驱动程序的操作函数（驱动的主要工作）
struct list_head tty_drivers;
};
下面介绍实现一个tty设备驱动的主要流程：
1）首先调用alloc_tty_driver动态分配一个tty_driver结构，lines表示驱动中设备的个数
struct tty_driver *alloc_tty_driver(int lines)
{
struct tty_driver *driver;
driver = kzalloc(sizeof(struct tty_driver), GFP_KERNEL);
if (driver) {
driver->magic = TTY_DRIVER_MAGIC;
driver->num = lines;
/* later we'll move allocation of tables here */
}
return driver;
}
2）初始化tty_driver中各个成员 其中主要是 init_termios成员和ops成员 。如果用户在端口初始化以前使用端口，该变量用来提供一系列安全的设置值，成员的具体含义在termios手册中能找到这里就不具体介绍。其次就是tty设备驱动最根本的操作函数集ops,并用set_tty_operations和驱动关联在前面的分析时我们已经知道这些回调函数的具体调用位置。
struct tty_operations {
int (*open)(struct tty_struct * tty, struct file * filp);
void (*close)(struct tty_struct * tty, struct file * filp);
int (*write)(struct tty_struct * tty,
const unsigned char *buf, int count);
int (*put_char)(struct tty_struct *tty, unsigned char ch);
void (*flush_chars)(struct tty_struct *tty);
int (*write_room)(struct tty_struct *tty);
int (*chars_in_buffer)(struct tty_struct *tty);
int (*ioctl)(struct tty_struct *tty, struct file * file,
unsigned int cmd, unsigned long arg);
long (*compat_ioctl)(struct tty_struct *tty, struct file * file,
unsigned int cmd, unsigned long arg);
void (*set_termios)(struct tty_struct *tty, struct ktermios * old);
void (*throttle)(struct tty_struct * tty);
void (*unthrottle)(struct tty_struct * tty);
void (*stop)(struct tty_struct *tty);
void (*start)(struct tty_struct *tty);
void (*hangup)(struct tty_struct *tty);
int (*break_ctl)(struct tty_struct *tty, int state);
void (*flush_buffer)(struct tty_struct *tty);
void (*set_ldisc)(struct tty_struct *tty);
void (*wait_until_sent)(struct tty_struct *tty, int timeout);
void (*send_xchar)(struct tty_struct *tty, char ch);
int (*read_proc)(char *page, char **start, off_t off,
int count, int *eof, void *data);
int (*tiocmget)(struct tty_struct *tty, struct file *file);
int (*tiocmset)(struct tty_struct *tty, struct file *file,
unsigned int set, unsigned int clear);
int (*resize)(struct tty_struct *tty, struct tty_struct *real_tty,
struct winsize *ws);
#ifdef CONFIG_CONSOLE_POLL
int (*poll_init)(struct tty_driver *driver, int line, char *options);
int (*poll_get_char)(struct tty_driver *driver, int line);
void (*poll_put_char)(struct tty_driver *driver, int line, char ch);
#endif
};
3）初始化好tty_driver就调用tty_register_driver在内核中注册一个字符设备驱动并把tty_driver注册到tty_drivers链表中
/*
* Called by a tty driver to register itself.
*/
int tty_register_driver(struct tty_driver *driver)
{
int error;
int i;
dev_t dev;
void **p = NULL;
if (driver->flags & TTY_DRIVER_INSTALLED)
return 0;
if (!(driver->flags & TTY_DRIVER_DEVPTS_MEM) && driver->num) {
p = kzalloc(driver->num * 3 * sizeof(void *), GFP_KERNEL);
if (!p)
return -ENOMEM;
}
//字符设备驱动设备分配的标准步骤
if (!driver->major) {
error = alloc_chrdev_region(&dev, driver->minor_start,
driver->num, driver->name);
if (!error) {
driver->major = MAJOR(dev);
driver->minor_start = MINOR(dev);
}
} else {
dev = MKDEV(driver->major, driver->minor_start);
error = register_chrdev_region(dev, driver->num, driver->name);
}
if (error < 0) {
kfree(p);
return error;
}
//为每个设备的及其ktermios分配指针空间
if (p) {
driver->ttys = (struct tty_struct **)p;
driver->termios = (struct ktermios **)(p + driver->num);
driver->termios_locked = (struct ktermios **)
(p + driver->num * 2);
} else {
driver->ttys = NULL;
driver->termios = NULL;
driver->termios_locked = NULL;
}
//字符设备驱动注册
cdev_init(&driver->cdev, &tty_fops);
driver->cdev.owner = driver->owner;
error = cdev_add(&driver->cdev, dev, driver->num);
if (error) {
unregister_chrdev_region(dev, driver->num);
driver->ttys = NULL;
driver->termios = driver->termios_locked = NULL;
kfree(p);
return error;
}
//tty_driver加入链表
mutex_lock(&tty_mutex);
list_add(&driver->tty_drivers, &tty_drivers);
mutex_unlock(&tty_mutex);
//动态加载设备时，注册驱动控制的设备
if (!(driver->flags & TTY_DRIVER_DYNAMIC_DEV)) {
for (i = 0; i < driver->num; i++)
tty_register_device(driver, i, NULL);
}
proc_tty_register_driver(driver);
return 0;
}
至此设备驱动注册成功，当卸载模块时调用tty_unregister_driver完成相反的工作
本篇文章来源于 Linux公社网站(www.linuxidc.com) 原文链接：http://www.linuxidc.com/Linux/2011-02/32060p0.htm
