Linux串口驱动程序(5)-串口接收分析

本文主要是介绍Linux串口驱动程序(5)-串口接收分析，希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！

1、tty数据接收流程分析

在用户调用read函数来读取设备的文件的数据时，首先得到响应的是tty_read，这个函数又是由n_tty_read来实现的


/***	n_tty_read		-	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 n_tty_read(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;int packet;do_it_again:BUG_ON(!tty->read_buf);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;}}/**	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;}packet = tty->packet;add_wait_queue(&tty->read_wait, &wait);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 inputso that any interrupt will set the state back toTASK_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;}/* FIXME: does n_tty_set_room need locking ? */n_tty_set_room(tty);timeout = schedule_timeout(timeout);continue;}__set_current_state(TASK_RUNNING);/* Deal with packet mode. */if (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);if (!eol || (c != __DISABLED_CHAR)) {if (tty_put_user(tty, c, b++)) {retval = -EFAULT;b--;break;}nr--;}if (eol) {tty_audit_push(tty);break;}}if (retval)break;} else {int uncopied;/* The copy function takes the read lock and handleslocking 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.*/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;
}

在这段代码里，首先把通过set_current_state(TASK_INTERRUPTIBLE);应用程序的标志位设置为阻塞，然后判断是否有数据可以给应用程序读取if (!input_available_p(tty, 0))，如果没有数据就会通过调度使阻塞生效，如果有数据就从read_buf里面读走数据。这里面read_buf就是串口驱动和tty重要的纽带，串口驱动通过中断接收数据，然后把它送到read_buf里面。

2、串口驱动接收分析

串口接收数据是通过s3c24xx_serial_rx_chars在中断里面进行的。

static irqreturn_t
s3c24xx_serial_rx_chars(int irq, void *dev_id)
{struct s3c24xx_uart_port *ourport = dev_id;struct uart_port *port = &ourport->port;struct tty_struct *tty = port->state->port.tty;unsigned int ufcon, ch, flag, ufstat, uerstat;int max_count = 64;while (max_count-- > 0) {ufcon = rd_regl(port, S3C2410_UFCON);ufstat = rd_regl(port, S3C2410_UFSTAT);if (s3c24xx_serial_rx_fifocnt(ourport, ufstat) == 0)break;uerstat = rd_regl(port, S3C2410_UERSTAT);ch = rd_regb(port, S3C2410_URXH);if (port->flags & UPF_CONS_FLOW) {int txe = s3c24xx_serial_txempty_nofifo(port);if (rx_enabled(port)) {if (!txe) {rx_enabled(port) = 0;continue;}} else {if (txe) {ufcon |= S3C2410_UFCON_RESETRX;wr_regl(port, S3C2410_UFCON, ufcon);rx_enabled(port) = 1;goto out;}continue;}}/* insert the character into the buffer */flag = TTY_NORMAL;port->icount.rx++;if (unlikely(uerstat & S3C2410_UERSTAT_ANY)) {dbg("rxerr: port ch=0x%02x, rxs=0x%08x\n",ch, uerstat);/* check for break */if (uerstat & S3C2410_UERSTAT_BREAK) {dbg("break!\n");port->icount.brk++;if (uart_handle_break(port))goto ignore_char;}if (uerstat & S3C2410_UERSTAT_FRAME)port->icount.frame++;if (uerstat & S3C2410_UERSTAT_OVERRUN)port->icount.overrun++;uerstat &= port->read_status_mask;if (uerstat & S3C2410_UERSTAT_BREAK)flag = TTY_BREAK;else if (uerstat & S3C2410_UERSTAT_PARITY)flag = TTY_PARITY;else if (uerstat & (S3C2410_UERSTAT_FRAME |S3C2410_UERSTAT_OVERRUN))flag = TTY_FRAME;}if (uart_handle_sysrq_char(port, ch))goto ignore_char;uart_insert_char(port, uerstat, S3C2410_UERSTAT_OVERRUN,ch, flag);ignore_char:continue;}tty_flip_buffer_push(tty);out:return IRQ_HANDLED;
}

分析函数的流程如下：

1、读取UFCON寄存器

2、然后读取接收fifo的数据量s3c24xx_serial_rx_fifocnt，如果数据量为0，则退出处理

3、读取错误状态寄存器uerstat = rd_regl(port, S3C2410_UERSTAT);

4、然后取出接收到的字符ch = rd_regb(port, S3C2410_URXH);

5、if (port->flags & UPF_CONS_FLOW)，这一段代码其实是在做流控的处理。

6、if (unlikely(uerstat & S3C2410_UERSTAT_ANY)) ，这段代码判断错误发生的类型

7、if (uart_handle_sysrq_char(port, ch))如果接收到的是sysrq这个特殊字符，则进行特殊处理

8、uart_insert_char(port, uerstat, S3C2410_UERSTAT_OVERRUN,ch, flag);这一步把接收到的字符送到串口驱动的buf中，这里面大循环就结束了

9、tty_flip_buffer_push(tty);把串口驱动中的数据送到read_buf中