linux uart终端 - 调用关系

内核函数调用关系可以通过dump_stack()函数分析出来

uart设备是tty设备的一种，主从设备号的分配依赖于平台实现，在我的mx51板上，主设备号是217，从设备号范围为16-23

终端驱动设备号分配信息可以通过proc文件系统查看：cat /proc/tty/driver

uart serial是tty设备的一种，而tty设备又是char 设备。因此，tty驱动向上层提供file_operations实现，uart core驱动对tty层实现tty_operations，特定的uart驱动比如uart mxc向uart core提供uart_ops实现。uart core和uart driver可以看成特定的tty驱动。

接口关系如下：

从接口关系图可以看出，用户对uart设备操作的调用关系非常简单，

file_operations => [tty_ldisc_ops] => tty_operations => uart ops

其中tty_ldisc_ops线路规程并不是必要的，依赖于应用层设置是否使用ldisc处理数据。

几个常用文件操作调用关系

open 操作

 [<c01a5adc>] (uart_open+0x48/0x4a8) from [<c018d514>] (tty_open+0x320/0x4c4)
 [<c018d514>] (tty_open+0x320/0x4c4) from [<c00acb04>] (chrdev_open+0x20c/0x234)
 [<c00acb04>] (chrdev_open+0x20c/0x234) from [<c00a81c4>] (__dentry_open+0x190/0x2a8)
 [<c00a81c4>] (__dentry_open+0x190/0x2a8) from [<c00a8398>] (nameidata_to_filp+0x3c/0x50)
 [<c00a8398>] (nameidata_to_filp+0x3c/0x50) from [<c00b4070>] (do_last+0x50c/0x650)
 [<c00b4070>] (do_last+0x50c/0x650) from [<c00b5ce0>] (do_filp_open+0x180/0x514)
 [<c00b5ce0>] (do_filp_open+0x180/0x514) from [<c00a7f58>] (do_sys_open+0x58/0x10c)
 [<c00a7f58>] (do_sys_open+0x58/0x10c) from [<c000844c>] (kernel_init+0xcc/0x16c)

write 操作

 [<c01a5c44>] (uart_write+0x48/0x138) from [<c018e750>] (n_tty_write+0x2c8/0x3b4)
 [<c018e750>] (n_tty_write+0x2c8/0x3b4) from [<c018baf8>] (tty_write+0x180/0x214)
 [<c018baf8>] (tty_write+0x180/0x214) from [<c00a9f08>] (vfs_write+0xac/0x154)
 [<c00a9f08>] (vfs_write+0xac/0x154) from [<c00aa05c>] (sys_write+0x3c/0x68)
 [<c00aa05c>] (sys_write+0x3c/0x68) from [<c0026f80>] (ret_fast_syscall+0x0/0x30)

ioctl操作

 [<c01a5470>] (uart_ioctl+0x34/0x358) from [<c018c8e0>] (tty_ioctl+0x8e8/0x990)
 [<c018c8e0>] (tty_ioctl+0x8e8/0x990) from [<c00b7488>] (vfs_ioctl+0x2c/0xac)
 [<c00b7488>] (vfs_ioctl+0x2c/0xac) from [<c00b7b40>] (do_vfs_ioctl+0x540/0x5a0)
 [<c00b7b40>] (do_vfs_ioctl+0x540/0x5a0) from [<c00b7bec>] (sys_ioctl+0x4c/0x6c)
 [<c00b7bec>] (sys_ioctl+0x4c/0x6c) from [<c0026f80>] (ret_fast_syscall+0x0/0x30)

read操作

 [<c018eaa0>] (n_tty_read+0xac/0x72c) from [<c018a3f0>] (tty_read+0x7c/0xc0)
 [<c018a3f0>] (tty_read+0x7c/0xc0) from [<c00aa130>] (vfs_read+0xa8/0x150)
 [<c00aa130>] (vfs_read+0xa8/0x150) from [<c00aa284>] (sys_read+0x3c/0x68)
 [<c00aa284>] (sys_read+0x3c/0x68) from [<c0026f80>] (ret_fast_syscall+0x0/0x30)

串口终端的read操作

从上面的调用关系，我们发现read操作并没有调用uart层的函数，而仅仅调用到n_tty_read中。

这是因为read操作和 write ioctl操作不同，处理流程如下：

1. read操作在没有数据的情况下会阻塞在n_tty_read(或者其他的LDISC read函数)

2. uart 底层驱动通过中断从硬件电路获取数据，然后唤醒阻塞在ty->read_wait上测进程

3. 阻塞的read操作别唤醒后，处理到来的数据

因此n_tty_ldisc和uart 驱动之间是通过等待队列和唤醒操作进行协调的。

首先我们看看n_tty_read的代码中read操作的阻塞过程

 1705 static ssize_t n_tty_read(struct tty_struct *tty, struct file *file,
 1706              unsigned char __user *buf, size_t nr)
 1707 {
 1708     unsigned char __user *b = buf;
 1709     DECLARE_WAITQUEUE(wait, current);

 1759     add_wait_queue(&tty->read_wait, &wait);

 1796             if (file->f_flags & O_NONBLOCK) {
 1797                 retval = -EAGAIN;
 1798                 break;
 1799             }
 1800             if (signal_pending(current)) {
 1801                 retval = -ERESTARTSYS;
 1802                 break;
 1803             }
 1804             /* FIXME: does n_tty_set_room need locking ? */
 1805             n_tty_set_room(tty);
 1806             timeout = schedule_timeout(timeout);
 1807             continue;
 1808         }
 1809         __set_current_state(TASK_RUNNING);

1709 声明一个等待队列元素

1759 把当前进程加入到等待队列tty->read_wait

1796 如果文件打开方式不是block的，那么不需要调度当前进程

1800 ～ 1802 如果当前进程有信号处理，那么我们也不要阻塞进程，而是返回-ERESTARTSYS，当应用层看到这个返回值后，表明有信号需要处理

1806 调度当前进程

1807 进程被重新调度会来

然后我们再看uart driver如何在接收数据后唤醒read进程，以mx51开发板为例，drivers/serial/mxc_uart.c中的接收中断处理函数

  331 static void mxcuart_rx_chars(uart_mxc_port *umxc)
  332 {
  333     volatile unsigned int ch, sr2;
  334     unsigned int status, flag, max_count = 256;
  335
  336     sr2 = readl(umxc->port.membase + MXC_UARTUSR2);
  337     while (((sr2 & MXC_UARTUSR2_RDR) == 1) && (max_count-- > 0)) {
  338         ch = readl(umxc->port.membase + MXC_UARTURXD);

  387         uart_insert_char(&umxc->port, status, MXC_UARTURXD_OVRRUN, ch,
  388                  flag);
  389           ignore_char:
  390         sr2 = readl(umxc->port.membase + MXC_UARTUSR2);
  391     }
  392     tty_flip_buffer_push(umxc->port.state->port.tty);
  393 }

337 while循环从uart寄存器读取数据

387 把读入的字符ch保存到flip buffer中

392 已经接收完数据或者接收到足够多的数据，翻转flip_buffer。tty_flip_buffer_push ==> flush_to_ldisc ==> disc->ops->receive_buf，disc->ops->receive_buf对于N_TTY线路规程来说，正是n_tty_receive_buf

函数n_tty_receive_buf代码

 1352 static void n_tty_receive_buf(struct tty_struct *tty, const unsigned char *cp,
 1353                   char *fp, int count)
 1354 {

 1412     if (!tty->icanon && (tty->read_cnt >= tty->minimum_to_wake)) {
 1413         kill_fasync(&tty->fasync, SIGIO, POLL_IN);
 1414         if (waitqueue_active(&tty->read_wait))
 1415             wake_up_interruptible(&tty->read_wait);
 1416     }

这里代码比较多，不要废话直奔主题

1415 唤醒tty->read_wait上的等待进程，n_tty_read就可以继续处理了