一、write过程分析

• App写：
  • 使用行规程来写
  • 数据最终存入uart_state->xmit的buffer里
• 硬件发送：
  • 使用硬件驱动中uart_ops->start_tx开始发送
  • 具体的发送方式有两种：通过DMA、通过中断
• 中断方式：
  • 方法1：直接使能tx empty中断，一开始tx buffer为空，在中断里填入数据
  • 方法2：写不封数据到tx fifo，使能中断，剩下的数据再次中断里继续发送

二、tty_write

文件在driver\tty\tty_io.c

static ssize_t tty_write(struct file *file, const char __user *buf,size_t count, loff_t *ppos)
{struct tty_struct *tty = file_tty(file);struct tty_ldisc *ld;//...if (!ld->ops->write)ret = -EIO;elseret = do_tty_write(ld->ops->write, tty, file, buf, count);	//使用行规程里的惭怍函数，进行写入//tty_ldisc->tty_ldisc_ops->writetty_ldisc_deref(ld);return ret;
}

三、ldisk write

文件drivers\tty\n_tty.c

static struct tty_ldisc_ops n_tty_ops = {		//tty_ldisc_ops惭怍函数，实例结构体n_tty_ops.magic           = TTY_LDISC_MAGIC,.name            = "n_tty",.open            = n_tty_open,.close           = n_tty_close,.flush_buffer    = n_tty_flush_buffer,.read            = n_tty_read,.write           = n_tty_write,		//上面ld->ops->write，调用的是这个函数//...
};static ssize_t n_tty_write(struct tty_struct *tty, struct file *file,const unsigned char *buf, size_t nr)
{//...while (1) {//...if (O_OPOST(tty)) {//...} else {struct n_tty_data *ldata = tty->disc_data;while (nr > 0) {mutex_lock(&ldata->output_lock);c = tty->ops->write(tty, b, nr);		//tty_struct->tty_operationsmutex_unlock(&ldata->output_lock);//...}}if (!nr)break;if (file->f_flags & O_NONBLOCK) {		//非阻塞方式的话，直接返回，不等数据发送完retval = -EAGAIN;break;}up_read(&tty->termios_rwsem);wait_woken(&wait, TASK_INTERRUPTIBLE, MAX_SCHEDULE_TIMEOUT);		//阻塞方式，等待发送完毕down_read(&tty->termios_rwsem);}//...
}

四、uart_write

文件：drivers\tty\serial\serial_core.c

static const struct tty_operations uart_ops = {.open		= uart_open,.close		= uart_close,.write		= uart_write,//....
};static int uart_write(struct tty_struct *tty,const unsigned char *buf, int count)
{struct uart_state *state = tty->driver_data;struct uart_port *port;struct circ_buf *circ;unsigned long flags;int c, ret = 0;//...port = uart_port_lock(state, flags);circ = &state->xmit;		//赋值寄存器地址//...while (port) {//...memcpy(circ->buf + circ->head, buf, c);		//把数据存入xmit buffercirc->head = (circ->head + c) & (UART_XMIT_SIZE - 1);buf += c;count -= c;ret += c;}__uart_start(tty);		//启动串口发送数据uart_port_unlock(port, flags);return ret;
}static void __uart_start(struct tty_struct *tty)
{struct uart_state *state = tty->driver_data;struct uart_port *port = state->uart_port;if (port && !uart_tx_stopped(port))port->ops->start_tx(port);		//uart_port->uart_ops->start_tx
}

五、硬件相关的发送

相关文件：drivers\tty\serial\imx.c

static const struct uart_ops imx_uart_pops = {.tx_empty	= imx_uart_tx_empty,.set_mctrl	= imx_uart_set_mctrl,.get_mctrl	= imx_uart_get_mctrl,.stop_tx	= imx_uart_stop_tx,.start_tx	= imx_uart_start_tx,.stop_rx	= imx_uart_stop_rx,//...
};static void imx_uart_start_tx(struct uart_port *port)
{//...if (!sport->dma_is_enabled) {		//不适用DMA时，比较容易理解，以它为例ucr1 = imx_uart_readl(sport, UCR1);//Transimiter Empty Interrupt Enable，发送buffer为空时，就产生中断//在中断函数里发送字符imx_uart_writel(sport, ucr1 | UCR1_TXMPTYEN, UCR1);}if (sport->dma_is_enabled) {//...}
}

一开始时，发送buffer肯定为空，就会立即产生中断：

static irqreturn_t imx_uart_txint(int irq, void *dev_id)
{struct imx_port *sport = dev_id;unsigned long flags;spin_lock_irqsave(&sport->port.lock, flags);imx_uart_transmit_buffer(sport);spin_unlock_irqrestore(&sport->port.lock, flags);return IRQ_HANDLED;
}static inline void imx_uart_transmit_buffer(struct imx_port *sport)
{struct circ_buf *xmit = &sport->port.state->xmit;//...while (!uart_circ_empty(xmit) &&!(imx_uart_readl(sport, imx_uart_uts_reg(sport)) & UTS_TXFULL)) {/* send xmit->buf[xmit->tail]* out the port here *///UART TX FIFO未满时，从xmit buffer取出数据写入FIFO，更新统计信息imx_uart_writel(sport, xmit->buf[xmit->tail], URTX0);xmit->tail = (xmit->tail + 1) & (UART_XMIT_SIZE - 1);sport->port.icount.tx++;}if (uart_circ_chars_pending(xmit) < WAKEUP_CHARS)uart_write_wakeup(&sport->port);if (uart_circ_empty(xmit))imx_uart_stop_tx(&sport->port);
}