1. 核心数据结构 ops

static const struct uart_ops cdns_uart_ops = {.set_mctrl    = cdns_uart_set_mctrl,.get_mctrl   = cdns_uart_get_mctrl,.start_tx    = cdns_uart_start_tx,.stop_tx  = cdns_uart_stop_tx,.stop_rx   = cdns_uart_stop_rx,.tx_empty  = cdns_uart_tx_empty,.break_ctl    = cdns_uart_break_ctl,.set_termios = cdns_uart_set_termios,.startup   = cdns_uart_startup,.shutdown  = cdns_uart_shutdown,.pm       = cdns_uart_pm,.type       = cdns_uart_type,.verify_port  = cdns_uart_verify_port,.request_port  = cdns_uart_request_port,.release_port = cdns_uart_release_port,.config_port  = cdns_uart_config_port,
#ifdef CONFIG_CONSOLE_POLL.poll_get_char    = cdns_uart_poll_get_char,.poll_put_char   = cdns_uart_poll_put_char,
#endif
};

2. ops各个接口功能及在流程中的位置

2.1 cdns_uart_startup 即打开串口

static int cdns_uart_startup(struct uart_port *port)
{struct cdns_uart *cdns_uart = port->private_data;bool is_brk_support;int ret;unsigned long flags;unsigned int status = 0;is_brk_support = cdns_uart->quirks & CDNS_UART_RXBS_SUPPORT;spin_lock_irqsave(&port->lock, flags); //这里采用锁，多个进程同时调用支持？为啥禁止中断/* Disable the TX and RX */writel(CDNS_UART_CR_TX_DIS | CDNS_UART_CR_RX_DIS,port->membase + CDNS_UART_CR);   //禁用发送和接收/* Set the Control Register with TX/RX Enable, TX/RX Reset,* no break chars.*/writel(CDNS_UART_CR_TXRST | CDNS_UART_CR_RXRST,port->membase + CDNS_UART_CR);while (readl(port->membase + CDNS_UART_CR) &(CDNS_UART_CR_TXRST | CDNS_UART_CR_RXRST))cpu_relax();/** Clear the RX disable bit and then set the RX enable bit to enable* the receiver.*/status = readl(port->membase + CDNS_UART_CR);status &= ~CDNS_UART_CR_RX_DIS;status |= CDNS_UART_CR_RX_EN;writel(status, port->membase + CDNS_UART_CR);   //使能接收/* Set the Mode Register with normal mode,8 data bits,1 stop bit,* no parity.*/writel(CDNS_UART_MR_CHMODE_NORM | CDNS_UART_MR_STOPMODE_1_BIT| CDNS_UART_MR_PARITY_NONE | CDNS_UART_MR_CHARLEN_8_BIT,port->membase + CDNS_UART_MR);/** Set the RX FIFO Trigger level to use most of the FIFO, but it* can be tuned with a module parameter*/writel(rx_trigger_level, port->membase + CDNS_UART_RXWM); //接收FIFO门限设置。门限的大小和波特率相关，可以控制中断的数目/** Receive Timeout register is enabled but it* can be tuned with a module parameter*/writel(rx_timeout, port->membase + CDNS_UART_RXTOUT); // 设置接收超时，即某些字节在FIFO中，但软件一直没有读取，超过此时间设定后，硬件产生中断，通知软件获取，实际也和波特率相关。/* Clear out any pending interrupts before enabling them */writel(readl(port->membase + CDNS_UART_ISR),port->membase + CDNS_UART_ISR);     //  清除中断spin_unlock_irqrestore(&port->lock, flags);   //释放锁ret = request_irq(port->irq, cdns_uart_isr, 0, CDNS_UART_NAME, port);if (ret) {dev_err(port->dev, "request_irq '%d' failed with %d\n",port->irq, ret);return ret;}//使能接收中断，开始接收数据/* Set the Interrupt Registers with desired interrupts */if (is_brk_support)writel(CDNS_UART_RX_IRQS | CDNS_UART_IXR_BRK,port->membase + CDNS_UART_IER);elsewritel(CDNS_UART_RX_IRQS, port->membase + CDNS_UART_IER);return 0;
}

综上，代码open的主要操作为：

1）禁用发送和接收

2) 使能接收

3) 设置串口工作模式

4）接收FIFO门限设置。门限的大小和波特率相关，可以控制中断的数目

5）设置接收超时，即某些字节在FIFO中，但软件一直没有读取，超过此时间设定后，硬件产生中断，通知软件获取，实际也和波特率相关。

6）清除中断

7）挂接中断处理

8）使能接收中断，开始接收数据

以上代码流程中 spin_lock_irqsave 禁止中断与采取锁的背景考虑是？多线程是lock，那么中断，是怕在操作的过程中有串口中断上来，而此时中断处理程序还没上来？

2.2 start tx 即发送使能

将数据从软件缓存队列里面放到TX FIFO 中，并进行发送及tx fifo 空中断的使能控制

static void cdns_uart_start_tx(struct uart_port *port)
{unsigned int status;if (uart_tx_stopped(port))return;/** Set the TX enable bit and clear the TX disable bit to enable the* transmitter.*/status = readl(port->membase + CDNS_UART_CR);status &= ~CDNS_UART_CR_TX_DIS;status |= CDNS_UART_CR_TX_EN;writel(status, port->membase + CDNS_UART_CR);if (uart_circ_empty(&port->state->xmit))  //如果上层tty给的缓存里面没有数据，即实际没有数据需要发送，因而直接返回。return;cdns_uart_handle_tx(port); //如果软件有数据发送，则在此处填充硬件TX FIFO，此处填充，由于上面已经将TX 发送使能打开了，所以边往fifo里面写，串口边从FIFO读取并发送出去。writel(CDNS_UART_IXR_TXEMPTY, port->membase + CDNS_UART_ISR);/* Enable the TX Empty interrupt */writel(CDNS_UART_IXR_TXEMPTY, port->membase + CDNS_UART_IER); //使能tx fifo空中断，这样
}

2.3 中断发送与软件队列的同步

即硬件将软件缓存中的数据发送的差不多时，此时再次唤醒软件往缓存中增加数据.

static void cdns_uart_handle_tx(void *dev_id)
{struct uart_port *port = (struct uart_port *)dev_id;unsigned int numbytes;//从软件缓存里面取数据，然后写入到TX FIFO中。//以下这个代码用于唤醒软件进程继续往缓存里面写数据，当缓存中的数据小于 WAKUP_CHARS(此处是256)if (uart_circ_chars_pending(&port->state->xmit) < WAKEUP_CHARS)uart_write_wakeup(port);}
}

为什么不是软件一直往缓存里面写，而硬件一直从缓存里面读，而采取这种门限唤醒的机制呢？

serdev-ttyport.c drivers\tty\serdev 7921 2021/10/26 189

// wake up a tty device
void tty_port_tty_wakeup(struct tty_port *port)
{port->client_ops->write_wakeup(port);
}static const struct tty_port_client_operations client_ops = {.receive_buf = ttyport_receive_buf,.write_wakeup = ttyport_write_wakeup, //
};

//等待write_wait 的接口
static void ttyport_wait_until_sent(struct serdev_controller *ctrl, long timeout)
{struct serport *serport = serdev_controller_get_drvdata(ctrl);struct tty_struct *tty = serport->tty;tty_wait_until_sent(tty, timeout);  //此处等待
}

3. 中断处理程序

分为接收和发送中断。

static irqreturn_t cdns_uart_isr(int irq, void *dev_id)
{struct uart_port *port = (struct uart_port *)dev_id; //支持多个串口，获取此中断对应的串口实例，以便获取资源信息，避免数据的混乱。unsigned int isrstatus;spin_lock(&port->lock);  //端口锁，无中断禁止操作/* Read the interrupt status register to determine which* interrupt(s) is/are active and clear them.*/isrstatus = readl(port->membase + CDNS_UART_ISR);writel(isrstatus, port->membase + CDNS_UART_ISR);if (isrstatus & CDNS_UART_IXR_TXEMPTY) {cdns_uart_handle_tx(dev_id);    //针对发送处理isrstatus &= ~CDNS_UART_IXR_TXEMPTY;}/** Skip RX processing if RX is disabled as RXEMPTY will never be set* as read bytes will not be removed from the FIFO.*/if (isrstatus & CDNS_UART_IXR_RXMASK &&!(readl(port->membase + CDNS_UART_CR) & CDNS_UART_CR_RX_DIS))cdns_uart_handle_rx(dev_id, isrstatus);  //针对接收处理spin_unlock(&port->lock);return IRQ_HANDLED;
}

3.1 RX 中断处理

static void cdns_uart_handle_rx(void *dev_id, unsigned int isrstatus)
{struct uart_port *port = (struct uart_port *)dev_id;struct cdns_uart *cdns_uart = port->private_data;char status = TTY_NORMAL;bool is_rxbs_support;is_rxbs_support = cdns_uart->quirks & CDNS_UART_RXBS_SUPPORT;// 主要过程循环读取RX FIFO中的数据，并判断数据的正确性while ((readl(port->membase + CDNS_UART_SR) &CDNS_UART_SR_RXEMPTY) != CDNS_UART_SR_RXEMPTY) {data = readl(port->membase + CDNS_UART_FIFO);port->icount.rx++;tty_insert_flip_char(&port->state->port, data, status); //写入到缓存isrstatus = 0;}tty_flip_buffer_push(&port->state->port); // 写入到缓存。这个接口比较重要
}

The Serial Device Bus (linuxfoundation.org)

Serial TTY overview - stm32mpu (stmicroelectronics.cn)

(3条消息) Linux tty驱动学习 - UART驱动的write操作流程_sbctsp的博客-CSDN博客 (此篇文章，主要参考write_wait，我们在中断处理发送流程中看到对此wakeup的操作)

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