------------------------------------------
本文系本站原創,歡迎轉載!
轉載請注明出處:http://ericxiao.cublog.cn/
一:前言
Tty這個名稱源于電傳打位元組的簡稱。在linux表示各種終端。終端通常都跟硬體相對應。比如對應于輸入裝置鍵盤滑鼠。輸出裝置顯示器的控制終端和序列槽終端.也有對應于不存在裝置的pty驅動。在如此衆多的終端模型之中,linux是怎麼将它們統一模組化的呢?這就是我們今天要讨論的問題.
二:tty驅動概貌
Tty架構如下所示:
如上圖所示,使用者空間主要是通過裝置檔案同tty_core互動.tty_core根據用空間操作的類型再選擇跟line discipline和tty_driver互動.例如設定硬體的ioctl指令就直接交給tty_driver處理。Read和write操作就會交給line discipline處理.
Line discipline是線路規程的意思。正如它的名字一樣,它表示的是這條終端”線程”的輸入與輸出規範設定.主要用來進行輸入/輸出資料的預處理。處理之後。就會将資料交給tty_driver
Tty_driver就是終端對應的驅動了。它将字元轉換成終端可以了解的字串.将其傳給終端裝置。
值得注意的是,這個架構沒有為tty_drivero提供read操作。也就是說tty_core 和line discipline都沒有辦法從tty_driver裡直接讀終端資訊。這是因為tty_driver對就的hardware并不一定是輸入資料和輸出資料的共同負載者。例如控制終端,輸出裝置是顯示器。輸入裝置是鍵盤。基于這樣的原理。在line discipline中有一個輸入緩存區。并提供了一個名叫receive_buf()的接口函數。對應的終端裝置隻要調用line discipine的receiver_buf函數,将資料寫入到輸入緩存區就可以了。
如果一個裝置同時是輸入裝置又是輸出裝置。那在裝置的中斷進行中調用receive_buf()将資料寫入即可.
三:tty驅動接口分析
具體的tty驅動設計可以參考LDD3。這裡隻對它的接口實作做一個分析.tty driver的所有操作都包含在tty_driver中。核心即供了一個名叫alloc_tty_driver()來配置設定這個tty_driver。當然我們也可以在自己的驅動中将它定義成一個靜态的結構。對tty_driver進行一些必要的初始化之後,調用tty_register_driver()将其注冊.
alloc_tty_driver()接口代碼如下所示:
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;
}
這個函數隻有一個參數。這個參數的含義為line的個數。也即次裝置号的個數。注意每個裝置檔案都會對應一個line.
在這個接口裡為tty_driver配置設定記憶體,然後将driver->mage.driver->num初始化之後就傳回了.
tty_register_driver()用來注冊一個tty_driver。代碼如下:
int tty_register_driver(struct tty_driver *driver)
int error;
int i;
dev_t dev;
void **p = NULL;
//TTY_DRIVER_INSTALLED:已安裝的
if (driver->flags & TTY_DRIVER_INSTALLED)
return 0;
//TTY_DRIVER_DEVPTS_MEM:使用devpts進行動态記憶體映射
if (!(driver->flags & TTY_DRIVER_DEVPTS_MEM) && driver->num) {
p = kzalloc(driver->num * 3 * sizeof(void *), GFP_KERNEL);
if (!p)
return -ENOMEM;
//注冊字元裝置号
//如果沒有指定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
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);
driver->ttys = NULL;
driver->termios = NULL;
driver->termios_locked = NULL;
//注冊字元裝置
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->termios = driver->termios_locked = NULL;
return error;
//指定預設的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);
//如果沒有指定TTY_DRIVER_DYNAMIC_DEV.即動态裝置管理
if (!(driver->flags & TTY_DRIVER_DYNAMIC_DEV)) {
for (i = 0; i num; i++)
tty_register_device(driver, i, NULL);
proc_tty_register_driver(driver);
return 0;
這個函數操作比較簡單。就是為tty_driver建立字元裝置。然後将字元裝置的操作集指定為tty_fops.并且将tty_driver挂載到tty_drivers連結清單中.其實這個連結清單的作用跟我們之前分析的input子系統中的input_dev[ ]數組類似。都是以裝置号為關鍵字找到對應的driver.
特别的。如果沒有定義TTY_DRIVER_DYNAMIC_DEV.還會在sysfs中建立一個類裝置.這樣主要是為了udev管理裝置.
以流程圖的方式将上述操作表示如下:
四:裝置檔案的操作
裝置檔案的操作是本節分析的重點。它的主要操作是将各項操作對應到ldsic或者是tty_driver.
4.1:打開tty裝置的操作
從注冊的過程可以看到,所有的操作都會對應到tty_fops中。Open操作對應的操作接口是tty_open()。代碼如下:
static int tty_open(struct inode *inode, struct file *filp)
struct tty_struct *tty;
int noctty, retval;
int index;
dev_t device = inode->i_rdev;
unsigned short saved_flags = filp->f_flags;
nonseekable_open(inode, filp);
retry_open:
//O_NOCTTY 如果路徑名指向終端裝置,不要把這個裝置用作控制終端
//noctty:需不需要更改目前程序的控制終端
noctty = filp->f_flags & O_NOCTTY;
index = -1;
retval = 0;
//裝置号(5,0) 即/dev/tty.表示目前程序的控制終端
if (device == MKDEV(TTYAUX_MAJOR, 0)) {
tty = get_current_tty();
//如果目前程序的控制終端不存在,退出
if (!tty) {
mutex_unlock(&tty_mutex);
return -ENXIO;
//取得目前程序的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
//裝置号(4,0).即/dev/tty0:表示目前的控制台
if (device == MKDEV(TTY_MAJOR, 0)) {
extern struct tty_driver *console_driver;
driver = console_driver;
//fg_console: 表示目前的控制台
index = fg_console;
noctty = 1;
#endif
//裝置号(5,1).即/dev/console.表示外接的控制台. 通過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;
//以檔案的裝置号為關鍵字,到tty_drivers中搜尋所注冊的driver
driver = get_tty_driver(device, &index);
if (!driver) {
got_driver:
//index表示它的次裝置号
retval = init_dev(driver, index, &tty);
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)
#ifdef TTY_DEBUG_HANGUP
printk(KERN_DEBUG "opening %s...", tty->name);
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) {
printk(KERN_DEBUG "error %d in opening %s...", retval,
tty->name);
release_dev(filp);
if (retval != -ERESTARTSYS)
return retval;
if (signal_pending(current))
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;
spin_lock_irq(&current->sighand->siglock);
//設定目前程序的終端
if (!noctty &&
current->signal->leader &&
!current->signal->tty &&
tty->session == NULL)
__proc_set_tty(current, tty);
spin_unlock_irq(&current->sighand->siglock);
tty_audit_opening();
注意在這裡有個容易忽略的操作: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 = 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;
這個函數的操作為到tty_ldiscs[ ]找到對應項.這個數組中的成員是調用tty_register_ldisc()将其設定進去的.
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;
即将取出來的idisc作為tty->ldisc字段.
在這段代碼中涉及到了tty_driver,tty_struct, struct tty_ldisc.這三者之間的關系用下圖表示如下:
在這裡,為tty_struct的ldisc是預設指定為tty_ldiscs[N_TTY].該ldisc對應的是控制終端的線路規範。可以在用空間用帶TIOCSETD的ioctl調用進行更改.
将上述open用流程圖的方式表示如下:
4.2:裝置檔案的write操作
裝置檔案的write操作對應tty_fops->write即tty_write().代碼如下:
static ssize_t tty_write(struct file *file, const char __user *buf,
size_t count, loff_t *ppos)
struct inode *inode = file->f_path.dentry->d_inode;
ssize_t ret;
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的過程中,将tty_struct存放在file的私有區。在write中,從file的私有區中就可以取到要操作的tty_struct.
如果tty_driver中沒有write.如果tty有錯誤都會有效性判斷失敗傳回。如果一切正常,遞增ldsic的引用計數。将用do_tty_wirte()再行寫操作。寫完之後,再遞減ldsic的引用計數.
Do_tty_write代碼分段分析如下:
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
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 = count;
/* write_buf/write_cnt is protected by the atomic_write_lock mutex */
if (tty->write_cnt
unsigned char *buf;
if (chunk
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;
預設一次寫資料的大小為2K.如果設定了TTY_NO_WRITE_SPLIT.則将一次寫的資料量擴大為65536.
Tty->write_buf是寫操作的臨時緩存區。即将使用者空的資料暫時存放到這裡
Tty->write_cnt是臨時緩存區的大小。
在這裡,必須要根據一次寫的資料量對這個臨時緩存區做調整
/* 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
written += ret;
buf += ret;
count -= ret;
if (!count)
ret = -ERESTARTSYS;
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);
後面的操作就比較簡單了。先将使用者空間的資料copy到臨時緩存區,然後再調用ldisc->write()完成這次寫操作.最後再更新裝置結點的時間戳.
Write操作的流程圖如下示:
在這裡,我們隻看到将資料寫放到了ldisc->write().沒有看到與tty_driver相關的部份。實際上在ldisc中對寫入的資料做預處理過後,還是會調用tty_driver->write()将其寫入硬體.
4.3:裝置檔案的read操作
static ssize_t tty_read(struct file *file, char __user *buf, size_t count,
loff_t *ppos)
struct inode *inode;
inode = file->f_path.dentry->d_inode;
if (tty_paranoia_check(tty, inode, "tty_read"))
if (!tty || (test_bit(TTY_IO_ERROR, &tty->flags)))
/* We want to wait for the line discipline to sort out in this
situation */
lock_kernel();
if (ld->read)
i = (ld->read)(tty, file, buf, count);
i = -EIO;
unlock_kernel();
if (i > 0)
inode->i_atime = current_fs_time(inode->i_sb);
return i;
這個read操作就更簡單。直接調用ldsic->read()完成工作
流程圖如下:
五:小結
在tty裝置檔案的操作中。Open操作會進行一系統初始化。然後調用ldsic->open tty_driver->open。在write和read調用中隻tty_core隻會用到ldisc->wirte/ldisc->read.除了上面分析的幾個操作之外,還有一個ioctl操作,以及它封裝的幾個termios。這些ioctl類的操作會直接和tty_driver相關聯.
在這一節裡,隻對tty的構造做一個分析,具體ldisc的操作我們之後以控制終端為例進行分析.
閱讀(78) | 評論(0) | 轉發(0) |
給主人留下些什麼吧!~~
評論熱議
請登入後評論。