linux内核中串口驱动注册过程(tty驱动)

       最近闲来无事情做，想到以前项目中遇到串口硬件流控制的问题，蓝牙串口控制返回错误，上层读写串口buffer溢出的问题等，也折腾了一阵子，虽然最终证明与串口驱动无关，但是排查问题时候毫无疑问会查看串口驱动的相关代码，所以把串口驱动的流程过了一遍，方便以后再用到时拿来用。分析的是全志代码A20。直接开始代码分析吧。

串口驱动代码在linux-3.3/drivers/tty/serial目录下，全志把自己平台相关的代码集中到了一个文件中，叫sw_uart.c，那就从它的__init开始了:

static int __init sw_uart_init(void){    int ret;     u32 i;    struct sw_uart_pdata *pdata;    SERIAL_MSG("driver initializied\n");    ret = sw_uart_get_devinfo();    if (unlikely(ret))        return ret;     ret = uart_register_driver(&sw_uart_driver);    if (unlikely(ret)) {        SERIAL_MSG("driver initializied\n");        return ret;     }        for (i=0; i<SW_UART_NR; i++) {        pdata = &sw_uport_pdata[i];        if (pdata->used)            platform_device_register(&sw_uport_device[i]);    }        return platform_driver_register(&sw_uport_platform_driver);}

sw_uart_get_devinfo是解析全志的sys配置脚本中的串口配置，一共有八个串口，要用到那个直接在sys脚本中设置1就行了，这个用过全志平台的都知道

接着sw_uart_driver结构体定义如下：

static struct uart_driver sw_uart_driver = {                                                                                                                       .owner = THIS_MODULE,    .driver_name = "sw_serial",    .dev_name = "ttyS",    .nr = SW_UART_NR,    .cons = SW_CONSOLE,};

这里SW_UART_NR为8，ttyS就是将要显示在/dev/目录下的名字了，从0～7

接着看uart注册函数,顾名思义是把全志自己平台的串口注册到串口核心serial_core中去：

int uart_register_driver(struct uart_driver *drv){    struct tty_driver *normal;    int i, retval;    BUG_ON(drv->state);    /*     * Maybe we should be using a slab cache for this, especially if     * we have a large number of ports to handle.     */    drv->state = kzalloc(sizeof(struct uart_state) * drv->nr, GFP_KERNEL);    if (!drv->state)        goto out;    normal = alloc_tty_driver(drv->nr);    if (!normal)        goto out_kfree;    drv->tty_driver = normal;    normal->owner       = drv->owner;    normal->driver_name = drv->driver_name;    normal->name        = drv->dev_name;//名字为ttyS    normal->major       = drv->major;    normal->minor_start = drv->minor;    normal->type        = TTY_DRIVER_TYPE_SERIAL;    normal->subtype     = SERIAL_TYPE_NORMAL;    normal->init_termios    = tty_std_termios;    normal->init_termios.c_cflag = B9600 | CS8 | CREAD | HUPCL | CLOCAL;                                                                                           normal->init_termios.c_ispeed = normal->init_termios.c_ospeed = 9600;    normal->flags       = TTY_DRIVER_REAL_RAW | TTY_DRIVER_DYNAMIC_DEV;    normal->driver_state    = drv;    tty_set_operations(normal, &uart_ops);    /*     * Initialise the UART state(s).     */    for (i = 0; i < drv->nr; i++) {        struct uart_state *state = drv->state + i;        struct tty_port *port = &state->port;        tty_port_init(port);        port->ops = &uart_port_ops;        port->close_delay     = HZ / 2; /* .5 seconds */        port->closing_wait    = 30 * HZ;/* 30 seconds */    }    retval = tty_register_driver(normal);    if (retval >= 0)        return retval;    put_tty_driver(normal);out_kfree:    kfree(drv->state);out:    return -ENOMEM;}

先列出来吧，以用到时候再回来看，这里先创建了NR个state, 并为每个state做一些初始化，但是这些state还没有和端口(uart_port)对应起来；初始化完port口后，调用tty_register_driver：

int tty_register_driver(struct tty_driver *driver){    int error;    int i;    dev_t dev;    void **p = NULL;    struct device *d;    if (!(driver->flags & TTY_DRIVER_DEVPTS_MEM) && driver->num) {        p = kzalloc(driver->num * 2 * sizeof(void *), GFP_KERNEL);        if (!p)            return -ENOMEM;    }    if (!driver->major) {        error = alloc_chrdev_region(&dev, driver->minor_start,                        driver->num, driver->name);        if (!error) {            driver->major = MAJOR(dev);            driver->minor_start = MINOR(dev);        }    } else {        dev = MKDEV(driver->major, driver->minor_start);        error = register_chrdev_region(dev, driver->num, driver->name);    }    if (error < 0) {        kfree(p);                                                                                                                                                      return error;    }    if (p) {        driver->ttys = (struct tty_struct **)p;        driver->termios = (struct ktermios **)(p + driver->num);    } else {        driver->ttys = NULL;        driver->termios = NULL;    }    cdev_init(&driver->cdev, &tty_fops);    driver->cdev.owner = driver->owner;    error = cdev_add(&driver->cdev, dev, driver->num);    if (error) {        unregister_chrdev_region(dev, driver->num);        driver->ttys = NULL;        driver->termios = NULL;        kfree(p);        return error;    }    mutex_lock(&tty_mutex);    list_add(&driver->tty_drivers, &tty_drivers);    mutex_unlock(&tty_mutex);    if (!(driver->flags & TTY_DRIVER_DYNAMIC_DEV)) {        for (i = 0; i < driver->num; i++) {            d = tty_register_device(driver, i, NULL);            if (IS_ERR(d)) {                error = PTR_ERR(d);                goto err;            }        }    }    proc_tty_register_driver(driver);    driver->flags |= TTY_DRIVER_INSTALLED;    return 0;

这里alloc_chrdev_region是动态分配了主从设备号，接着cdev_init，它file_operations结构提和它关联了起来，以后我们open /dev/ttyS节点时候会调用他的open函数，先看看这个结构体：

static const struct file_operations tty_fops = {                                                                                                                   .llseek     = no_llseek,    .read       = tty_read,    .write      = tty_write,    .poll       = tty_poll,    .unlocked_ioctl = tty_ioctl,    .compat_ioctl   = tty_compat_ioctl,    .open       = tty_open,    .release    = tty_release,    .fasync     = tty_fasync,};

接着cdev_add就把file_operations和设备号关联起来了，我们现在还没有创建设备节点，不过看到有为driver->major，driver->minor_start赋值的，后面创建节点就是用这两个主从设备号。接着list_add把这个tty_driver添加到链表中来，方便后续查找。

接着if语句判断TTY_DRIVER_DYNAMIC_DEV标志，我们前面有赋值：

     normal->flags       = TTY_DRIVER_REAL_RAW | TTY_DRIVER_DYNAMIC_DEV;

所以这里的if条件不成立的，最后创建proc下的节点，就返回了。按我的理解，tty_register_driver是注册了一个tty的驱动，这个驱动有了逻辑能力，但是这个时候这个驱动还没有对应任何设备，所以后续还要添加对应的端口(也就是芯片的物理串口)，并创建/dev/下的设备节点，上层用tty_driver驱动的逻辑来操作对应的端口。

回到sw_uart.c中，继续__init函数，platform_device_register函数，如果sys配置文件中那个串口配置了1，才会注册相应的平台设备；

接着platform_driver_register，看它的probe函数了：

static int __devinit sw_uart_probe(struct platform_device *pdev){    u32 id = pdev->id;    struct uart_port *port;    struct sw_uart_port *sw_uport;    struct clk *apbclk;    int ret = -1;    if (unlikely(pdev->id < 0 || pdev->id >= SW_UART_NR))        return -ENXIO;    port = &sw_uart_port[id].port;    port->dev = &pdev->dev;    sw_uport = UART_TO_SPORT(port);    sw_uport->id = id;    sw_uport->ier = 0;    sw_uport->lcr = 0;    sw_uport->mcr = 0;    sw_uport->fcr = 0;    sw_uport->dll = 0;    sw_uport->dlh = 0;    /* request system resource and init them */    ret = sw_uart_request_resource(sw_uport);    if (unlikely(ret)) {                                                                                                                                               SERIAL_MSG("uart%d error to get resource\n", id);        return -ENXIO;    }    apbclk = clk_get(&pdev->dev, CLK_SYS_APB1);    if (IS_ERR(apbclk)) {        SERIAL_MSG("uart%d error to get source clock\n", id);        return -ENXIO;    }    ret = clk_set_parent(sw_uport->mclk, apbclk);    if (ret) {        SERIAL_MSG("uart%d set mclk parent error\n", id);        clk_put(apbclk);        return -ENXIO;    }    port->uartclk = clk_get_rate(apbclk);    clk_put(apbclk);    port->type = PORT_SW;    port->flags = UPF_BOOT_AUTOCONF;    port->mapbase = sw_uport->pdata->base;    port->irq = sw_uport->pdata->irq;    platform_set_drvdata(pdev, port);#ifdef CONFIG_PROC_FS    sw_uart_procfs_attach(sw_uport);#endif    SERIAL_DBG("add uart%d port, port_type %d, uartclk %d\n",            id, port->type, port->uartclk);    return uart_add_one_port(&sw_uart_driver, port);}

sw_uart_request_resource是申请配置GPIO，接着uart_add_one_port：

int uart_add_one_port(struct uart_driver *drv, struct uart_port *uport){    struct uart_state *state;    struct tty_port *port;    int ret = 0;     struct device *tty_dev;    BUG_ON(in_interrupt());    if (uport->line >= drv->nr)        return -EINVAL;    state = drv->state + uport->line;//state是在函数 uart_register_driver中kmalloc初始化    port = &state->port;    mutex_lock(&port_mutex);    mutex_lock(&port->mutex);    if (state->uart_port) {        ret = -EINVAL;        goto out;     }        state->uart_port = uport;    state->pm_state = -1;                                                                                                                                                                   uport->cons = drv->cons;    uport->state = state;    /*        * If this port is a console, then the spinlock is already     * initialised.     */    if (!(uart_console(uport) && (uport->cons->flags & CON_ENABLED))) {        spin_lock_init(&uport->lock);        lockdep_set_class(&uport->lock, &port_lock_key);    }        uart_configure_port(drv, state, uport);    /*        * Register the port whether it's detected or not.  This allows     * setserial to be used to alter this ports parameters.     */    tty_dev = tty_register_device(drv->tty_driver, uport->line, uport->dev);    if (likely(!IS_ERR(tty_dev))) {        device_set_wakeup_capable(tty_dev, 1);    } else {        printk(KERN_ERR "Cannot register tty device on line %d\n",               uport->line);    }    /*     * Ensure UPF_DEAD is not set.     */    uport->flags &= ~UPF_DEAD; out:    mutex_unlock(&port->mutex);    mutex_unlock(&port_mutex);    return ret;}

这个函数看名字就猜到是为uart_driver添加端口，前面说过state的状态还没有和uart_port对应起来，那么这里state->uart_port = uport就对应了，port的配置就不去关心它了，这样，uart_driver就可以通过port对ops结构体的操作函数控制底层了，最后调用tty_register_device：

struct device *tty_register_device(struct tty_driver *driver, unsigned index,                   struct device *device){       char name[64];    dev_t dev = MKDEV(driver->major, driver->minor_start) + index;    if (index >= driver->num) {        printk(KERN_ERR "Attempt to register invalid tty line number "               " (%d).\n", index);        return ERR_PTR(-EINVAL);    }           if (driver->type == TTY_DRIVER_TYPE_PTY)        pty_line_name(driver, index, name);    else        tty_line_name(driver, index, name);    return device_create(tty_class, device, dev, NULL, name);}

这里的tty_line_name函数定义如下：

static void tty_line_name(struct tty_driver *driver, int index, char *p)                                                                                       {    sprintf(p, "%s%d", driver->name, index + driver->name_base);//名字在uart_register_driver中赋值}

可以看到就是前面说的名字ttyS0～ttyS7。

这样，如果上层open节点，会调用到前面的file_operations结构体函数tty_open,这是tty核心层的调用：

static int tty_open(struct inode *inode, struct file *filp){    struct tty_struct *tty;    int noctty, retval;    struct tty_driver *driver = NULL;    int index;    dev_t device = inode->i_rdev;    unsigned saved_flags = filp->f_flags;    nonseekable_open(inode, filp);retry_open:    retval = tty_alloc_file(filp);    if (retval)        return -ENOMEM;    noctty = filp->f_flags & O_NOCTTY;    index  = -1;    retval = 0;    mutex_lock(&tty_mutex);    tty_lock();    tty = tty_open_current_tty(device, filp);     ...................................    if (tty->ops->open)        retval = tty->ops->open(tty, filp);    else    ........................................            return retval;        schedule();        /*         * Need to reset f_op in case a hangup happened.         */        tty_lock();        if (filp->f_op == &hung_up_tty_fops)            filp->f_op = &tty_fops;                                                                                                                                    tty_unlock();        goto retry_open;     .................................

可以看到，首先查找tty_driver链表，找到前面添加的tty_driver，然后调用他的ops->open函数，这个ops赋值是在前面的uart_register_driver函数中：

tty_set_operations(normal, &uart_ops);

所以进入uart_ops结构体的open函数，这里就是从tty核心转到serial核心，往下走了一层：

static int uart_open(struct tty_struct *tty, struct file *filp){    struct uart_driver *drv = (struct uart_driver *)tty->driver->driver_state;    int retval, line = tty->index;    struct uart_state *state = drv->state + line;    struct tty_port *port = &state->port;           ................................    /*     * Start up the serial port.     */    retval = uart_startup(tty, state, 0);    ..................................... 

uart_startup:

static int uart_startup(struct tty_struct *tty, struct uart_state *state,                                                                                              int init_hw){    struct tty_port *port = &state->port;    int retval;    if (port->flags & ASYNC_INITIALIZED)        return 0;    /*     * Set the TTY IO error marker - we will only clear this     * once we have successfully opened the port.     */    set_bit(TTY_IO_ERROR, &tty->flags);    retval = uart_port_startup(tty, state, init_hw);    if (!retval) {        set_bit(ASYNCB_INITIALIZED, &port->flags);        clear_bit(TTY_IO_ERROR, &tty->flags);    } else if (retval > 0)        retval = 0;    return retval;}

uart_port_startup：

static int uart_port_startup(struct tty_struct *tty, struct uart_state *state,                                                                                         int init_hw){    struct uart_port *uport = state->uart_port;    struct tty_port *port = &state->port;    unsigned long page;    int retval = 0;    retval = uport->ops->startup(uport);    if (retval == 0) {        if (uart_console(uport) && uport->cons->cflag) {            tty->termios->c_cflag = uport->cons->cflag;            uport->cons->cflag = 0;        }        /*         * Initialise the hardware port settings.         */        uart_change_speed(tty, state, NULL);        if (init_hw) {            /*             * Setup the RTS and DTR signals once the             * port is open and ready to respond.             */            if (tty->termios->c_cflag & CBAUD)                uart_set_mctrl(uport, TIOCM_RTS | TIOCM_DTR);        }        if (port->flags & ASYNC_CTS_FLOW) {            spin_lock_irq(&uport->lock);            if (!(uport->ops->get_mctrl(uport) & TIOCM_CTS))                tty->hw_stopped = 1;            spin_unlock_irq(&uport->lock);        }    } 

可以看到，最终调用了 uport->ops->startup，这样就从serical核心层往下走到了平台的串口驱动层，也就是最底层的驱动了，这个函数定义在sw_uart.c的sw_uart_port结构体中：

static struct sw_uart_port sw_uart_port[] = {                                                                                                                      { .port = { .iotype = UPIO_MEM, .ops = &sw_uart_ops, .fifosize = 64, .line = 0, },      .pdata = &sw_uport_pdata[0], },    { .port = { .iotype = UPIO_MEM, .ops = &sw_uart_ops, .fifosize = 64, .line = 1, },      .pdata = &sw_uport_pdata[1], },    { .port = { .iotype = UPIO_MEM, .ops = &sw_uart_ops, .fifosize = 64, .line = 2, },      .pdata = &sw_uport_pdata[2], },    { .port = { .iotype = UPIO_MEM, .ops = &sw_uart_ops, .fifosize = 64, .line = 3, },      .pdata = &sw_uport_pdata[3], },    { .port = { .iotype = UPIO_MEM, .ops = &sw_uart_ops, .fifosize = 64, .line = 4, },      .pdata = &sw_uport_pdata[4], },    { .port = { .iotype = UPIO_MEM, .ops = &sw_uart_ops, .fifosize = 64, .line = 5, },      .pdata = &sw_uport_pdata[5], },    { .port = { .iotype = UPIO_MEM, .ops = &sw_uart_ops, .fifosize = 64, .line = 6, },      .pdata = &sw_uport_pdata[6], },    { .port = { .iotype = UPIO_MEM, .ops = &sw_uart_ops, .fifosize = 64, .line = 7, },      .pdata = &sw_uport_pdata[7], },};

看他的.startup函数：

static int sw_uart_startup(struct uart_port *port)                                                                                                             {    struct sw_uart_port *sw_uport = UART_TO_SPORT(port);    int ret;    SERIAL_DBG("start up ...\n");    snprintf(sw_uport->name, sizeof(sw_uport->name),         "sw_serial%d", port->line);    ret = request_irq(port->irq, sw_uart_irq, 0, sw_uport->name, port);    if (unlikely(ret)) {        SERIAL_MSG("uart%d cannot get irq %d\n", sw_uport->id, port->irq);        return ret;    }    sw_uport->msr_saved_flags = 0;    /*     * PTIME mode to select the THRE trigger condition:     * if PTIME=1(IER[7]), the THRE interrupt will be generated when the     * the water level of the TX FIFO is lower than the threshold of the     * TX FIFO. and if PTIME=0, the THRE interrupt will be generated when     * the TX FIFO is empty.     * In addition, when PTIME=1, the THRE bit of the LSR register will not     * be set when the THRE interrupt is generated. You must check the     * interrupt id of the IIR register to decide whether some data need to     * send.     */    sw_uport->ier = SW_UART_IER_RLSI | SW_UART_IER_RDI;    #ifdef CONFIG_SW_UART_PTIME_MODE    sw_uport->ier |= SW_UART_IER_PTIME;    #endif    return 0;}

这里最终就是初始化ARM芯片的寄存器操作了，可以看到申请了中断函数，后续的读写操作就和中断服务函数密切相关了。

接着open之后，看看上层的write函数调用过程,首先调用了tty核心层的write：

static ssize_t tty_write(struct file *file, const char __user *buf,                                                                                                                    size_t count, loff_t *ppos){    struct inode *inode = file->f_path.dentry->d_inode;    struct tty_struct *tty = file_tty(file);    struct tty_ldisc *ld;    ssize_t ret;    if (tty_paranoia_check(tty, inode, "tty_write"))        return -EIO;    if (!tty || !tty->ops->write ||        (test_bit(TTY_IO_ERROR, &tty->flags)))            return -EIO;    /* Short term debug to catch buggy drivers */    if (tty->ops->write_room == NULL)        printk(KERN_ERR "tty driver %s lacks a write_room method.\n",            tty->driver->name);    ld = tty_ldisc_ref_wait(tty);    if (!ld->ops->write)        ret = -EIO;    else        ret = do_tty_write(ld->ops->write, tty, file, buf, count);    tty_ldisc_deref(ld);    return ret;}

看这里do_tty_write函数：

static inline ssize_t do_tty_write(    ssize_t (*write)(struct tty_struct *, struct file *, const unsigned char *, size_t),    struct tty_struct *tty,    struct file *file,    const char __user *buf,    size_t count){        ............................    for (;;) {        size_t size = count;        if (size > chunk)            size = chunk;        ret = -EFAULT;        if (copy_from_user(tty->write_buf, buf, size))            break;        ret = write(tty, file, tty->write_buf, size);        if (ret <= 0)            break;    ...............................

copy_from_user就把要写的数据拷贝到了内核空间的write_buf中来，接着write是函数指针，指向ld->ops->write。

这里的ld->ops指向的是n_tty.c 中结构体：

struct tty_ldisc_ops 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            = n_tty_read,    .write           = n_tty_write,                                                                                                                                .ioctl           = n_tty_ioctl,    .set_termios     = n_tty_set_termios,    .poll            = n_tty_poll,    .receive_buf     = n_tty_receive_buf,    .write_wakeup    = n_tty_write_wakeup};

所以调用了调用的是线路规程的write:

static ssize_t n_tty_write(struct tty_struct *tty, struct file *file,               const unsigned char *buf, size_t nr){     ..........................                b++; nr--;            }            if (tty->ops->flush_chars)                tty->ops->flush_chars(tty);        } else {            while (nr > 0) {                c = tty->ops->write(tty, b, nr);                if (c < 0) {                    retval = c;                    goto break_out;                }                if (!c)                    break;                b += c;                nr -= c;            }        }    ............................

从线路规程转到tty驱动层的write:

static int uart_write(struct tty_struct *tty,                    const unsigned char *buf, int count){    ......................    if (!circ->buf)        return 0;    spin_lock_irqsave(&port->lock, flags);    while (1) {        c = CIRC_SPACE_TO_END(circ->head, circ->tail, UART_XMIT_SIZE);        if (count < c)             c = count;        if (c <= 0)            break;        memcpy(circ->buf + circ->head, buf, c);        circ->head = (circ->head + c) & (UART_XMIT_SIZE - 1);        buf += c;        count -= c;        ret += c;    }    spin_unlock_irqrestore(&port->lock, flags);    uart_start(tty);    return ret;}

可以看到，把数据memcpy到环形队列中来，这样数据就保存到了该端口对应的state的xmit的buf中，这是一个环形队列。接着调用uart_start:

static void uart_start(struct tty_struct *tty){    struct uart_state *state = tty->driver_data;    struct uart_port *port = state->uart_port;    unsigned long flags;    spin_lock_irqsave(&port->lock, flags);    __uart_start(tty);    spin_unlock_irqrestore(&port->lock, flags);}

看来要开始传输数据了，所以对这个操作加锁了：

static void __uart_start(struct tty_struct *tty)                                                                                                               {    struct uart_state *state = tty->driver_data;    struct uart_port *port = state->uart_port;    if (port->ops->wake_peer)        port->ops->wake_peer(port);    if (!uart_circ_empty(&state->xmit) && state->xmit.buf &&        !tty->stopped && !tty->hw_stopped)        port->ops->start_tx(port);}

最终调用驱动层的操作函数，也就是该端口对应的传输函数来触发数据发送：

static void sw_uart_start_tx(struct uart_port *port)                                                                                                           {    struct sw_uart_port *sw_uport = UART_TO_SPORT(port);    if (!(sw_uport->ier & SW_UART_IER_THRI)) {        sw_uport->ier |= SW_UART_IER_THRI;        SERIAL_DBG("start tx, ier %x\n", sw_uport->ier);        serial_out(port, sw_uport->ier, SW_UART_IER);    }    }

serial_out函数对应的就是最底层的寄存器操作了，这里#define SW_UART_IER (0x04)，SW_UART_IER_THRI它对应的是使能中断，具体要参考全志的A20 CPU手册：

static inline void serial_out(struct uart_port *port, unsigned char value, int offs)                                                                           {    __raw_writeb(value, port->membase + offs);}

把配置写到了寄存器中，使能中断后，中断服务函数就自动把buf的数据发送出去了，前面分析看到数据是memcpy到了该端口对应的state的circ_buf结构体中的，所以进入前面open时候分析的中断服务函数中去：

static irqreturn_t sw_uart_irq(int irq, void *dev_id){    struct uart_port *port = dev_id;    struct sw_uart_port *sw_uport = UART_TO_SPORT(port);    unsigned int iir = 0, lsr = 0;    unsigned long flags;    spin_lock_irqsave(&port->lock, flags);    iir = serial_in(port, SW_UART_IIR) & SW_UART_IIR_IID_MASK;                                                                                                     lsr = serial_in(port, SW_UART_LSR);    SERIAL_DBG("irq: iir %x lsr %x\n", iir, lsr);    if (iir == SW_UART_IIR_IID_BUSBSY) {        /* handle busy *///      SERIAL_MSG("uart%d busy...\n", sw_uport->id);        serial_in(port, SW_UART_USR);        #ifdef CONFIG_SW_UART_FORCE_LCR        sw_uart_force_lcr(sw_uport, 10);        #else        serial_out(port, sw_uport->lcr, SW_UART_LCR);        #endif    } else {        if (lsr & (SW_UART_LSR_DR | SW_UART_LSR_BI))            lsr = sw_uart_handle_rx(sw_uport, lsr);        sw_uart_modem_status(sw_uport);        #ifdef CONFIG_SW_UART_PTIME_MODE        if (iir == SW_UART_IIR_IID_THREMP)        #else        if (lsr & SW_UART_LSR_THRE)        #endif            sw_uart_handle_tx(sw_uport);    }    spin_unlock_irqrestore(&port->lock, flags);    return IRQ_HANDLED;}

我们要做的是发送操作,所以进入sw_uart_handle_tx：

static void sw_uart_handle_tx(struct sw_uart_port *sw_uport){    struct circ_buf *xmit = &sw_uport->port.state->xmit;    int count;    if (sw_uport->port.x_char) {                                                                                                                                       serial_out(&sw_uport->port, sw_uport->port.x_char, SW_UART_THR);        sw_uport->port.icount.tx++;        sw_uport->port.x_char = 0;#ifdef CONFIG_SW_UART_DUMP_DATA        sw_uport->dump_buff[sw_uport->dump_len++] = sw_uport->port.x_char;        SERIAL_DUMP(sw_uport, "Tx");#endif        return;    }    if (uart_circ_empty(xmit) || uart_tx_stopped(&sw_uport->port)) {        sw_uart_stop_tx(&sw_uport->port);        return;    }    count = sw_uport->port.fifosize / 2;    do {#ifdef CONFIG_SW_UART_DUMP_DATA        sw_uport->dump_buff[sw_uport->dump_len++] = xmit->buf[xmit->tail];#endif        serial_out(&sw_uport->port, xmit->buf[xmit->tail], SW_UART_THR);        xmit->tail = (xmit->tail + 1) & (UART_XMIT_SIZE - 1);        sw_uport->port.icount.tx++;        if (uart_circ_empty(xmit)) {            break;        }    } while (--count > 0);    SERIAL_DUMP(sw_uport, "Tx");    if (uart_circ_chars_pending(xmit) < WAKEUP_CHARS) {        spin_unlock(&sw_uport->port.lock);        uart_write_wakeup(&sw_uport->port);        spin_lock(&sw_uport->port.lock);    }    .......................... 

看到了，serial_out果然是取出circ_buf的buf数据，在do{}while语句中完成发送：

static inline void serial_out(struct uart_port *port, unsigned char value, int offs)                                                                           {    __raw_writeb(value, port->membase + offs);}

这样就把发送的数据写到了相应的寄存器中，硬件会自动完成数据发送操作

而上层read操作时候调用和write差不多，不同之处在于read是读取一个环形buf的数据，因为数据到了会产生中断，是中断服务函数自动接收数据并把它存储在buf中的；而前面写的时候是主动把数据写到buf去，所以先从中断服务函数中看是如何接收输入的：

static unsigned int sw_uart_handle_rx(struct sw_uart_port *sw_uport, unsigned int lsr){    struct tty_struct *tty = sw_uport->port.state->port.tty;    unsigned char ch = 0;    int max_count = 256;    char flag;    do {        if (likely(lsr & SW_UART_LSR_DR)) {            ch = serial_in(&sw_uport->port, SW_UART_RBR);       ..........................        if (uart_handle_sysrq_char(&sw_uport->port, ch))            goto ignore_char;        uart_insert_char(&sw_uport->port, lsr, SW_UART_LSR_OE, ch, flag);       ............................ 

从寄存器读取字符后赋值给ch, 接着uart_insert_char来处理该字符，其实就是把这个数据放到uart层去：

void uart_insert_char(struct uart_port *port, unsigned int status,         unsigned int overrun, unsigned int ch, unsigned int flag){    struct tty_struct *tty = port->state->port.tty;    if ((status & port->ignore_status_mask & ~overrun) == 0)        tty_insert_flip_char(tty, ch, flag);    /*     * Overrun is special.  Since it's reported immediately,     * it doesn't affect the current character.     */    if (status & ~port->ignore_status_mask & overrun)        tty_insert_flip_char(tty, 0, TTY_OVERRUN);}

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);}

当前的tty_buffer空间不够时调用tty_insert_flip_string_flags，在这个函数里会去查找下一个tty_buffer，并将数据放到下一个tty_buffer的char_buf_ptr里。
这里char_buf_ptr的数据是如何放到线路规程的read_buf中的呢？那是在tty open操作的时候，tty_init_dev -> initialize_tty_struct -> initialize_tty_struct -> tty_buffer_init:

void tty_buffer_init(struct tty_struct *tty)                                                                                                                   {    spin_lock_init(&tty->buf.lock);    tty->buf.head = NULL;    tty->buf.tail = NULL;    tty->buf.free = NULL;    tty->buf.memory_used = 0;    INIT_WORK(&tty->buf.work, flush_to_ldisc);}

可以看到初始化了工作队列的，而调用工作队列的时机是在这里操作完成后，继续sw_uart_handle_rx函数的tty_flip_buffer_push时候：

void tty_flip_buffer_push(struct tty_struct *tty){    unsigned long flags;    spin_lock_irqsave(&tty->buf.lock, flags);    if (tty->buf.tail != NULL)        tty->buf.tail->commit = tty->buf.tail->used;    spin_unlock_irqrestore(&tty->buf.lock, flags);    if (tty->low_latency)        flush_to_ldisc(&tty->buf.work);    else        schedule_work(&tty->buf.work);}EXPORT_SYMBOL(tty_flip_buffer_push); 

这里就有两种方法把数据上报给链路规程层，其实都差不多，这样数据就上报到了链路规程中，看看这个工作队列函数：

static void flush_to_ldisc(struct work_struct *work){    ..........................                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->ops->receive_buf(tty, char_buf,                            flag_buf, count);    ............................

链路规程的receive_buf函数：

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;    ................................        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;    ...........................        }        if (tty->ops->flush_chars)            tty->ops->flush_chars(tty);    }    n_tty_set_room(tty);

可以看到，很明显，memcpy将数据拷贝到了read_buf中。

现在，再回头从上层看read函数是如何读取数据的，流程也是tty核心 ->链路规程 :

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;     .......................................                c = tty->read_buf[tty->read_tail];    ...............................            uncopied = copy_from_read_buf(tty, &b, &nr);            uncopied += copy_from_read_buf(tty, &b, &nr);    .............................. 

static int copy_from_read_buf(struct tty_struct *tty,                      unsigned char __user **b,                      size_t *nr){    int retval;    size_t n;    unsigned long flags;    retval = 0;    spin_lock_irqsave(&tty->read_lock, flags);    n = min(tty->read_cnt, N_TTY_BUF_SIZE - tty->read_tail);    n = min(*nr, n);    spin_unlock_irqrestore(&tty->read_lock, flags);    if (n) {        retval = copy_to_user(*b, &tty->read_buf[tty->read_tail], n);                                                                                                  n -= retval;        tty_audit_add_data(tty, &tty->read_buf[tty->read_tail], n);        spin_lock_irqsave(&tty->read_lock, flags);        tty->read_tail = (tty->read_tail + n) & (N_TTY_BUF_SIZE-1);        tty->read_cnt -= n;        /* Turn single EOF into zero-length read */        if (L_EXTPROC(tty) && tty->icanon && n == 1) {            if (!tty->read_cnt && (*b)[n-1] == EOF_CHAR(tty))                n--;        }        spin_unlock_irqrestore(&tty->read_lock, flags);        *b += n;        *nr -= n;    }    return retval;} 

看到了copy_to_user函数，就是把read_buf的数据拷贝到了用户空间。

到这里，串口的读写流程就很清楚，一目了然了。