Linux设备模型之tty驱动架构分析

一：前言
Tty这个名称源于电传打字节的简称。在linux表示各种终端。终端通常都跟硬件相对应。比如对应于输入设备键盘鼠标。输出设备显示器的控制终端和串口终端.也有对应于不存在设备的pty驱动。在如此众多的终端模型之中，linux是怎么将它们统一建模的呢?这就是我们今天要讨论的问题.

二：tty驱动概貌
Tty架构如下所示：


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如上图所示,用户空间主要是通过设备文件同tty_core交互.tty_core根据用空间操作的类型再选择跟line discipline和tty_driver交互.例如设置硬件的ioctl指令就直接交给tty_driver处理。Read和write操作就会交给line discipline处理.
Line discipline是线路规程的意思。正如它的名字一样，它表示的是这条终端”线程”的输入与输出规范设置.主要用来进行输入/输出数据的预处理。处理之后。就会将数据交给tty_driver
Tty_driver就是终端对应的驱动了。它将字符转换成终端可以理解的字串.将其传给终端设备。
值得注意的是,这个架构没有为tty_drivero提供read操作。也就是说tty_core 和line discipline都没有办法从tty_driver里直接读终端信息。这是因为tty_driver对就的hardware并不一定是输入数据和输出数据的共同负载者。例如控制终端，输出设备是显示器。输入设备是键盘。基于这样的原理。在line discipline中有一个输入缓存区。并提供了一个名叫receive_buf()的接口函数。对应的终端设备只要调用line discipine的receiver_buf函数，将数据写入到输入缓存区就可以了。
如果一个设备同时是输入设备又是输出设备。那在设备的中断处理中调用receive_buf()将数据写入即可.

三：tty驱动接口分析
具体的tty驱动设计可以参考LDD3。这里只对它的接口实现做一个分析.tty driver的所有操作都包含在tty_driver中。内核即供了一个名叫alloc_tty_driver()来分配这个tty_driver。当然我们也可以在自己的驱动中将它定义成一个静态的结构。对tty_driver进行一些必要的初始化之后，调用tty_register_driver()将其注册.
alloc_tty_driver()接口代码如下所示：
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;
}
这个函数只有一个参数。这个参数的含义为line的个数。也即次设备号的个数。注意每个设备文件都会对应一个line.
在这个接口里为tty_driver分配内存，然后将driver->mage.driver->num初始化之后就返回了.

tty_register_driver()用来注册一个tty_driver。代码如下：
int tty_register_driver(struct tty_driver *driver)
{
int error;
int i;
dev_t dev;
void **p = NULL;

//TTY_DRIVER_INSTALLED:已安装的
if (driver->flags & TTY_DRIVER_INSTALLED)
return 0;

//TTY_DRIVER_DEVPTS_MEM:使用devpts进行动态内存映射
if (!(driver->flags & TTY_DRIVER_DEVPTS_MEM) && driver->num) {
p = kzalloc(driver->num * 3 * sizeof(void *), GFP_KERNEL);
if (!p)
return -ENOMEM;
}

//注册字符设备号
//如果没有指定driver->major
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;
}

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;
}

//注册字符设备
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;
}

//指定默认的put_char
if (!driver->put_char)
driver->put_char = tty_default_put_char;

mutex_lock(&tty_mutex);
list_add(&driver->tty_drivers, &tty_drivers);
mutex_unlock(&tty_mutex);

//如果没有指定TTY_DRIVER_DYNAMIC_DEV.即动态设备管理
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;
}
这个函数操作比较简单。就是为tty_driver创建字符设备。然后将字符设备的操作集指定为tty_fops.并且将tty_driver挂载到tty_drivers链表中.其实这个链表的作用跟我们之前分析的input子系统中的input_dev[ ]数组类似。都是以设备号为关键字找到对应的driver.
特别的。如果没有定义TTY_DRIVER_DYNAMIC_DEV.还会在sysfs中创建一个类设备.这样主要是为了udev管理设备.
以流程图的方式将上述操作表示如下：


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四：设备文件的操作
设备文件的操作是本节分析的重点。它的主要操作是将各项操作对应到ldsic或者是tty_driver.

4．1：打开tty设备的操作
从注册的过程可以看到，所有的操作都会对应到tty_fops中。Open操作对应的操作接口是tty_open()。代码如下：
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;
unsigned short saved_flags = filp->f_flags;

nonseekable_open(inode, filp);

retry_open:
//O_NOCTTY 如果路径名指向终端设备，不要把这个设备用作控制终端

//noctty:需不需要更改当前进程的控制终端
noctty = filp->f_flags & O_NOCTTY;
index = -1;
retval = 0;

mutex_lock(&tty_mutex);

//设备号(5,0) 即/dev/tty.表示当前进程的控制终端
if (device == MKDEV(TTYAUX_MAJOR, 0)) {
tty = get_current_tty();
//如果当前进程的控制终端不存在,退出
if (!tty) {
mutex_unlock(&tty_mutex);
return -ENXIO;
}

//取得当前进程的tty_driver
driver = tty->driver;
index = tty->index;
filp->f_flags |= O_NONBLOCK; /* Don't let /dev/tty block */
/* noctty = 1; */
goto got_driver;
}
#ifdef CONFIG_VT
//设备号(4,0).即/dev/tty0:表示当前的控制台
if (device == MKDEV(TTY_MAJOR, 0)) {
extern struct tty_driver *console_driver;
driver = console_driver;
//fg_console: 表示当前的控制台
index = fg_console;
noctty = 1;
goto got_driver;
}
#endif
//设备号(5,1).即/dev/console.表示外接的控制台. 通过regesit_console()
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;
}

//以文件的设备号为关键字,到tty_drivers中搜索所注册的driver
driver = get_tty_driver(device, &index);
if (!driver) {
mutex_unlock(&tty_mutex);
return -ENODEV;
}
got_driver:
//index表示它的次设备号
retval = init_dev(driver, index, &tty);
mutex_unlock(&tty_mutex);
if (retval)
return retval;

filp->private_data = tty;
file_move(filp, &tty->tty_files);
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->driver->open)
retval = tty->driver->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
release_dev(filp);
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);

//设置当前进程的终端
if (!noctty &&
current->signal->leader &&
!current->signal->tty &&
tty->session == NULL)
__proc_set_tty(current, tty);
spin_unlock_irq(¤t->sighand->siglock);
mutex_unlock(&tty_mutex);
tty_audit_opening();
return 0;
}
注意在这里有个容易忽略的操作：init_dev（）。
Init_dev() -à initialize_tty_struct() à tty_ldisc_assign(tty, tty_ldisc_get(N_TTY));
看一下tty_ldisc_assign(tty, tty_ldisc_get(N_TTY))的操作:
Tty_ldisc_get():
struct tty_ldisc *tty_ldisc_get(int disc)
{
unsigned long flags;
struct tty_ldisc *ld;

if (disc < N_TTY || disc >= NR_LDISCS)
return NULL;

spin_lock_irqsave(&tty_ldisc_lock, flags);

ld = &tty_ldiscs[disc];
/* Check the entry is defined */
if (ld->flags & LDISC_FLAG_DEFINED) {
/* If the module is being unloaded we can't use it */
if (!try_module_get(ld->owner))
ld = NULL;
else /* lock it */
ld->refcount++;
} else
ld = NULL;
spin_unlock_irqrestore(&tty_ldisc_lock, flags);
return ld;
}
这个函数的操作为到tty_ldiscs[ ]找到对应项.这个数组中的成员是调用tty_register_ldisc()将其设置进去的.

tty_ldisc_assign操作如下：
static void tty_ldisc_assign(struct tty_struct *tty, struct tty_ldisc *ld)
{
tty->ldisc = *ld;
tty->ldisc.refcount = 0;
}
即将取出来的idisc作为tty->ldisc字段.
在这段代码中涉及到了tty_driver,tty_struct, struct tty_ldisc.这三者之间的关系用下图表示如下：


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在这里,为tty_struct的ldisc是默认指定为tty_ldiscs[N_TTY].该ldisc对应的是控制终端的线路规范。可以在用空间用带TIOCSETD的ioctl调用进行更改.
将上述open用流程图的方式表示如下：


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4.2:设备文件的write操作
设备文件的write操作对应tty_fops->write即tty_write().代码如下：
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"))
return -EIO;
if (!tty || !tty->driver->write ||
(test_bit(TTY_IO_ERROR, &tty->flags)))
return -EIO;

ld = tty_ldisc_ref_wait(tty);
if (!ld->write)
ret = -EIO;
else
ret = do_tty_write(ld->write, tty, file, buf, count);
tty_ldisc_deref(ld);
return ret;
}
在open的过程中，将tty_struct存放在file的私有区。在write中,从file的私有区中就可以取到要操作的tty_struct.
如果tty_driver中没有write.如果tty有错误都会有效性判断失败返回。如果一切正常，递增ldsic的引用计数。将用do_tty_wirte()再行写操作。写完之后，再递减ldsic的引用计数.
Do_tty_write代码分段分析如下：
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;
}
默认一次写数据的大小为2K.如果设置了TTY_NO_WRITE_SPLIT.则将一次写的数据量扩大为65536.
Tty->write_buf是写操作的临时缓存区。即将用户空的数据暂时存放到这里
Tty->write_cnt是临时缓存区的大小。
在这里，必须要根据一次写的数据量对这个临时缓存区做调整

/* 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;
lock_kernel();
ret = write(tty, file, tty->write_buf, size);
unlock_kernel();
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;
}
后面的操作就比较简单了。先将用户空间的数据copy到临时缓存区，然后再调用ldisc->write()完成这次写操作.最后再更新设备结点的时间戳.
Write操作的流程图如下示：


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在这里，我们只看到将数据写放到了ldisc->write().没有看到与tty_driver相关的部份。实际上在ldisc中对写入的数据做预处理过后，还是会调用tty_driver->write()将其写入硬件.

4.3:设备文件的read操作
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;
inode = file->f_path.dentry->d_inode;
if (tty_paranoia_check(tty, inode, "tty_read"))
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);
lock_kernel();
if (ld->read)
i = (ld->read)(tty, file, buf, count);
else
i = -EIO;
tty_ldisc_deref(ld);
unlock_kernel();
if (i > 0)
inode->i_atime = current_fs_time(inode->i_sb);
return i;
}
这个read操作就更简单。直接调用ldsic->read()完成工作
流程图如下：


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五：小结
在tty设备文件的操作中。Open操作会进行一系统初始化。然后调用ldsic->open tty_driver->open。在write和read调用中只tty_core只会用到ldisc->wirte/ldisc->read.除了上面分析的几个操作之外，还有一个ioctl操作，以及它封装的几个termios。这些ioctl类的操作会直接和tty_driver相关联.
在这一节里，只对tty的构造做一个分析，具体ldisc的操作我们之后以控制终端为例进行分析.