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2012-07-09 16:44:09

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转载请注明出处:http://ericxiao.cublog.cn/
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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;
                            }
                   }
对于规范模式,要读满一行才会返回用户空间.例如我们在shell上输入指令的时候,要按下enter键指令才会进行处理.在tty->read_flags数组中定义了一些满行的标志,如果read_buf中对应的数据在tty->read_flags中被置位.就会认为这次读入已经到结尾了.在这里还要注意的是,不要将__DISABLED_CHAR即’/0’拷贝到用户空间.
对于原始模式,只需要将read_buf中的数据读入到用户空间就可以返回了.在这里需要注意read_buf是一个环形缓存,需要copy两次.例如tail在head之前的情况.
 
                   /* 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.
                    */
                   if (n_tty_chars_in_buffer(tty) <= TTY_THRESHOLD_UNTHROTTLE) {
                            n_tty_set_room(tty);
                            check_unthrottle(tty);
                   }
OK.到这里,read_buf中或多或少已经有数据被取出了.如果当前的数据量少于TTY_THRESHOLD_UNTHROTTLE.就可以调用check_unthrottle()将其它的写进程唤醒了
 
                   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);
 
已经读完了数据,是该到清理的时候了.将进程移出等待队列,并当进程状态设为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;
 
         //更新剩余空间数
         n_tty_set_room(tty);
 
         return retval;
}
TTY_PUSH:是由底层驱动程序在读到一个EOF字符并将其放入缓存区造成的,表示用户要尽快将缓存区数据取走.
如果本次操作没有读取任何数据,且被设置了TTY_PUSH,则跳转到do_it_again,继续执行.如果本次操作读取了数据,可以等到下一次read的时候再来取.
最后,更新read_buf的剩余空间数.
 
五:控制终端数据的来源
从这个函数里面我们可以看到,数据是从read_buf中取出来的,但是谁将数据放入到read_buf中的呢?为了探究出它的根源.我们还得要从vty_init()说起.
在之前分析过. vty_init()会调用一个表面字义看起来与键盘相关的一个子函数: kbd_init().跟踪这个函数:
int __init kbd_init(void)
{
         int i;
         int error;
 
        for (i = 0; i < MAX_NR_CONSOLES; i++) {
                   kbd_table[i].ledflagstate = KBD_DEFLEDS;
                   kbd_table[i].default_ledflagstate = KBD_DEFLEDS;
                   kbd_table[i].ledmode = LED_SHOW_FLAGS;
                   kbd_table[i].lockstate = KBD_DEFLOCK;
                   kbd_table[i].slockstate = 0;
                   kbd_table[i].modeflags = KBD_DEFMODE;
                   kbd_table[i].kbdmode = default_utf8 ? VC_UNICODE : VC_XLATE;
         }
 
         error = input_register_handler(&kbd_handler);
         if (error)
                   return error;
 
         tasklet_enable(&keyboard_tasklet);
         tasklet_schedule(&keyboard_tasklet);
 
         return 0;
}
暂时用不到的部份我们先不与分析。 在这里注册了一个input handler。结合前面我们分析的input子系统,在handler里会处理input device上报的事件。跟进这个handler看一下:
kbd_handler定义如下:
static struct input_handler kbd_handler = {
         .event                 = kbd_event,
         .connect   = kbd_connect,
         .disconnect       = kbd_disconnect,
         .start                   = kbd_start,
         .name                = "kbd",
         .id_table   = kbd_ids,
};
Id_table是用来匹配input device的。跟进去看一下,看哪些device的事件,才会交给它处理:
static const struct input_device_id kbd_ids[] = {
         {
                .flags = INPUT_DEVICE_ID_MATCH_EVBIT,
                .evbit = { BIT_MASK(EV_KEY) },
        },
 
         {
                .flags = INPUT_DEVICE_ID_MATCH_EVBIT,
                .evbit = { BIT_MASK(EV_SND) },
        },
 
         { },    /* Terminating entry */
};
 
从这个id_table中看来,只要是能支持EV_KEY或者是EV_SND的设备都会被这个hnadler匹配到。相应的。也就能够处理input device上报的事件了.
根据之前的input子系统分析,在input device和handler 进行匹配的时候会调用handler->connect.即kbd_connect().代码如下:
static int kbd_connect(struct input_handler *handler, struct input_dev *dev,
                            const struct input_device_id *id)
{
         struct input_handle *handle;
         int error;
         int i;
 
         for (i = KEY_RESERVED; i < BTN_MISC; i++)
                   if (test_bit(i, dev->keybit))
                            break;
 
         if (i == BTN_MISC && !test_bit(EV_SND, dev->evbit))
                   return -ENODEV;
 
         handle = kzalloc(sizeof(struct input_handle), GFP_KERNEL);
         if (!handle)
                   return -ENOMEM;
 
         handle->dev = dev;
         handle->handler = handler;
         handle->name = "kbd";
 
         error = input_register_handle(handle);
         if (error)
                   goto err_free_handle;
 
         error = input_open_device(handle);
         if (error)
                   goto err_unregister_handle;
 
         return 0;
 
 err_unregister_handle:
         input_unregister_handle(handle);
 err_free_handle:
         kfree(handle);
         return error;
}
在这段代码里,它申请分初始化了一个hande结构,并将其注册。Open。这些都是我们之前分析过的东东。在注册handle的时候。又会调用到hande->start.函数如下:
static void kbd_start(struct input_handle *handle)
{
         unsigned char leds = ledstate;
 
         tasklet_disable(&keyboard_tasklet);
         if (leds != 0xff) {
                   input_inject_event(handle, EV_LED, LED_SCROLLL, !!(leds & 0x01));
                   input_inject_event(handle, EV_LED, LED_NUML,    !!(leds & 0x02));
                   input_inject_event(handle, EV_LED, LED_CAPSL,   !!(leds & 0x04));
                   input_inject_event(handle, EV_SYN, SYN_REPORT, 0);
         }
         tasklet_enable(&keyboard_tasklet);
}
这里就是对键盘上的LED进行操作。启用了tasklent。这些都不是我们所关心的重点。
来看下它的事件处理过程:
static void kbd_event(struct input_handle *handle, unsigned int event_type,
                         unsigned int event_code, int value)
{
         if (event_type == EV_MSC && event_code == MSC_RAW && HW_RAW(handle->dev))
                   kbd_rawcode(value);
         if (event_type == EV_KEY)
                   kbd_keycode(event_code, value, HW_RAW(handle->dev));
         tasklet_schedule(&keyboard_tasklet);
         do_poke_blanked_console = 1;
         schedule_console_callback();
}
不管对应键盘的那一种模式。后面的数据流程都会转入到input_queue()进等处理。
实际上。控制终端由vc_cons[ ]数组表示。数组中的每一个项都表示一个控制终端。由全局变量fg_console来指示当前所用的cosole/另外。对于键盘等输出设备也对应一个数组。即kbd_table[ ].用来表示当前终端的控制信息.
其余的都不是我们想关心的。来跟踪一下这个函数的实现:
static void put_queue(struct vc_data *vc, int ch)
{
         struct tty_struct *tty = vc->vc_tty;
 
         if (tty) {
                   tty_insert_flip_char(tty, ch, 0);
                   con_schedule_flip(tty);
         }
}
这里的参数vc就是指的在vc_cons[ ]中的当前项。回忆在console open的时候。初始化了这一项。并建立了VC和tty的关联。就这样。在vc中可以寻着关联关系找到tty了.
Tty_insert_filp_char( )将数据ch存入tty的一个缓存中,具体代码如下示:
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) {
                   tb->flag_buf_ptr[tb->used] = flag;
                   tb->char_buf_ptr[tb->used++] = ch;
                   return 1;
         }
         return tty_insert_flip_string_flags(tty, &ch, &flag, 1);
}
在这里,将数存先存进了tty->buf中。后面的tty_insert_flip_string_flags是在当前buf不够的情况下,扩张buf使用的。代码比较简单,请自行分析。
 
将数据暂存之后,会调用con_schedule_flip(tty)去唤醒一个软中断的工作队列.代码如下:
static inline void con_schedule_flip(struct tty_struct *t)
{
         unsigned long flags;
         spin_lock_irqsave(&t->buf.lock, flags);
         if (t->buf.tail != NULL)
                   t->buf.tail->commit = t->buf.tail->used;
         spin_unlock_irqrestore(&t->buf.lock, flags);
         schedule_delayed_work(&t->buf.work, 0);
}
对应的工作队列为t->buf.work.这个工作队列是怎么定义的呢?这就要回到我们之前分析的tty驱动的tty_struct的初始化.
代码片段如下所示:
static void initialize_tty_struct(struct tty_struct *tty)
{
         。。。。。。
         。。。。。。。
         INIT_DELAYED_WORK(&tty->buf.work, flush_to_ldisc);
         。。。。。。
}
这就是这个工作队列的定义了.
在这里,特别提醒一下。在上面的put_queue()处理是处于一个中断环境。回想一想整个事件的流程。是键盘中断àinput device上报事件àhandler处理这个事件àput_queue()
在中断中,将工作队列唤醒。将比较繁重的工作交由这个工作队列处理。虽然工作队列也是工作在中断状态。但它是开中断执行的.这也就是软中断存在的目的.
 
跟进flush_to_ldisc():
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;
 
         disc = tty_ldisc_ref(tty);
         if (disc == NULL)       /*  !TTY_LDISC */
                   return;
工作队列所调用的参数是它本身所表示的work_queue.而它本身又是封装在tty_strcut里面的。调用container_of()宏就可以获取到封装它的tty_struct.然后增加tty->ldisc的引用计数
 
         spin_lock_irqsave(&tty->buf.lock, flags);
         /* So we know a flush is running */
         set_bit(TTY_FLUSHING, &tty->flags);
         head = tty->buf.head;
         if (head != NULL) {
                   tty->buf.head = NULL;
                   for (;;) {
                            int count = head->commit - head->read;
                            if (!count) {
                                     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) {
                                     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->receive_buf(tty, char_buf, flag_buf, count);
                            spin_lock_irqsave(&tty->buf.lock, flags);
                   }
                   /* Restore the queue head */
                   tty->buf.head = head;
         }
对于tty->buf中的每个缓存区,如果缓存区中没有数据,则将其释放,这个释放是有优化的。如果数据少于512就将其放到tty->buf->free中。下次要放分存放空间的时候可以直接到这里面取。如果设置了TTY_ FLUSHPENDING就会跳出循环。
如果tty的接收缓存区不够,则跳出循环,定时器到达过后再来调用这个工作队列.
最后调用tty->receive_buf()来处理这个数据了.
 
         /* 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)) {
                   __tty_buffer_flush(tty);
                   clear_bit(TTY_FLUSHPENDING, &tty->flags);
                   wake_up(&tty->read_wait);
         }
         clear_bit(TTY_FLUSHING, &tty->flags);
         spin_unlock_irqrestore(&tty->buf.lock, flags);
 
         tty_ldisc_deref(disc);
}
数据最终会通过tty-> receive_buf()将数据放入read_buf.
在这段代码中,有几个很有意思的处理。在进入工作队列的时候,首先会置TTY_FLUSHING标志.如果有进程在读read_buf的时候,如果此标志被置位,就会设置TTY_FLUSHPENDING标志,并进行睡眠。在数据处理完成之后,判断是否有TTY_FLUSHPENDING标志。如果有,则将读进程唤醒.并清除TTY_FLUSHPENDING和TTY_FLUSHING
想一想。为什么会这么处理呢?为什么这里需要两个缓存区,一个buf.一个read_buf。为什么要这样麻烦呢?
首先,对于缓存区的数目问题:我们在后面会看到。对接收数据还有一系列的预处理过程,这些过程是比较费时的。不宜在中断中进行费时的操作。所以需要选用软中断机制。这就需要将数据先放置一个buf.再由软中断进行预处理之后,再将它放入到read_buf.这就是两个缓存区的原因.
另外:在存数据到read_buf的时候。会有进程从read_buf中读数据。这样就会造成一个竞争。注意到在软中断情况下是不可睡眠的。我们只能选用自旋锁一类的机制。而这种机制是禁止中断和抢占的。这又违背了软中断机制的初衷。怎么办呢?这就是这样标志的作用了。在设计中,我们必须首先得要保证软中断处理机制的快速完成。所以一进入软中断,就置了一个标志。如果有进程来读数据了,也就是说竞争条件发生了,先将读进程置睡眠。不管怎样,先让软中断处理完之后再说。软中断的工作over这后,再唤醒读进程。
我们之前讲的一系统加锁机制是在两者同样平等的情况。而原子置位与判断置位一般是为了保证一方的工作先完成。
 
好了,到这一步,我们终于看到跟踪read_buf中数据来源问题的一丝曙光了。数据经过tty->receive_buf之后,这个过程就清晰明朗了。
对于tty_ldisc_N_TTY. receive_buf接口如下所示:
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) {
                   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);
                   tty->read_head = (tty->read_head + i) & (N_TTY_BUF_SIZE-1);
                   tty->read_cnt += i;
                   spin_unlock_irqrestore(&tty->read_lock, cpuflags);
         } else {
                   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->driver->flush_chars)
                            tty->driver->flush_chars(tty);
         }
对于原始模式。直接将数据copy到read_buf中。对于加工模式,将数据预处理之后,再加入到read_buf中。这个预处理过程比较繁杂,这里先忽略.
         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) {
                   /* check TTY_THROTTLED first so it indicates our state */
                   if (!test_and_set_bit(TTY_THROTTLED, &tty->flags) &&
                       tty->driver->throttle)
                            tty->driver->throttle(tty);
         }
}
重新计数read_buf的剩余空间量。如果可读数据大于tty->minimum_to_wake.就将它的读进程唤醒。
如果当前read_buf剩余空间不足TTY_THRESHOLD_THROTTLE.就调用tty->driver->throttle(tty)将数程流入进程先阻塞.
 
六:控制终端的write操作
在输入shell指令的时候,屏幕上会出现我们键入的字符。在输入密码的时候,屏幕上一般不会显示我们当前按入了什么键。就就是终端的两种模式,回显和非回显(ECHO)。当设置为回显模式的时候,会将键入的值在屏幕上面显示出来。这个显示的过程就是通过tty driver->write来实现的。
屏幕上的显示操作跟显示驱动有很重要的联系。一般就是调用显卡驱动的显示接口来实现。在切换终端的时候。设置显示区域。由于这部份跟显卡驱动关联较深,而功能又比较单一。在这里不做详细分析。
 
:总结
在这一节里,将之前分析过的input子系统,tty驱动架构联系在了一起。我们渐渐体会到,Linux中大量的使用分层架构。层与层之前的联系很紧密而维护也很简单。深入体会其中的架构思想。对于我们平时做开发是很有裨益的.
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