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linux设备模型之uart驱动架构分析
 

发布于2013-2-20

 

一:前言

接着前面的终端控制台分析,接下来分析serial的驱动。在linux中,serial也对应着终端,通常被称为串口终端。在shell上,我们看到的/dev/ttyS*就是串口终端所对应的设备节点。

在分析具体的serial驱动之前。有必要先分析uart驱动架构。uart是Universal Asynchronous Receiver and Transmitter的缩写。翻译成中文即为”通用异步收发器”。它是串口设备驱动的封装层。

二:uart驱动架构概貌

如下图所示:

上图中红色部份标识即为uart部份的操作。

从上图可以看到,uart设备是继tty_driver的又一层封装。实际上uart_driver就是对应tty_driver.在它的操作函数中,将操作转入uart_port.

在写操作的时候,先将数据放入一个叫做circ_buf的环形缓存区。然后uart_port从缓存区中取数据,将其写入到串口设备中。

当uart_port从serial设备接收到数据时,会将设备放入对应line discipline的缓存区中。

这样。用户在编写串口驱动的时候,只先要注册一个uart_driver.它的主要作用是定义设备节点号。然后将对设备的各项操作封装在uart_port.驱动工程师没必要关心上层的流程,只需按硬件规范将uart_port中的接口函数完成就可以了。

三:uart驱动中重要的数据结构及其关联

我们可以自己考虑下,基于上面的架构代码应该要怎么写。首先考虑以下几点:

1: 一个uart_driver通常会注册一段设备号。即在用户空间会看到uart_driver对应有多个设备节点。例如:

/dev/ttyS0 /dev/ttyS1

每个设备节点是对应一个具体硬件的,从上面的架构来看,每个设备文件应该对应一个uart_port.

也就是说:uart_device怎么同多个uart_port关系起来?怎么去区分操作的是哪一个设备文件?

2:每个uart_port对应一个circ_buf,所以uart_port必须要和这个缓存区关系起来

回忆tty驱动架构中。tty_driver有一个叫成员指向一个数组,即tty->ttys.每个设备文件对应设数组中的一项。而这个数组所代码的数据结构为tty_struct. 相应的tty_struct会将tty_driver和ldisc关联起来。

那在uart驱动中,是否也可用相同的方式来处理呢?

将uart驱动常用的数据结构表示如下:

结合上面提出的疑问。可以很清楚的看懂这些结构的设计。

四:uart_driver的注册操作

Uart_driver注册对应的函数为: uart_register_driver()代码如下:

    int uart_register_driver(struct uart_driver *drv)
    {
    struct tty_driver *normal = NULL;
    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);
    retval = -ENOMEM;
    if (!drv->state)
    goto out;
    normal  = alloc_tty_driver(drv->nr);
    if (!normal)
    goto out;
    drv->tty_driver = normal;
    normal->owner      = drv->owner;
    normal->driver_name    = drv->driver_name;
    normal->name       = drv->dev_name;
    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;
    state->close_delay     = 500;    /* .5 seconds */
    state->closing_wait    = 30000;  /* 30 seconds */
    mutex_init(&state->mutex);
    }
    retval = tty_register_driver(normal);
    out:
    if (retval < 0) {
    put_tty_driver(normal);
    kfree(drv->state);
    }
    return retval;
    }

从上面代码可以看出。uart_driver中很多数据结构其实就是tty_driver中的。将数据转换为tty_driver之后,注册tty_driver.然后初始化uart_driver->state的存储空间。

这样,就会注册uart_driver->nr个设备节点。主设备号为uart_driver-> major. 开始的次设备号为uart_driver-> minor.

值得注意的是。在这里将tty_driver的操作集统一设为了uart_ops.其次,在tty_driver-> driver_state保存了这个uart_driver.这样做是为了在用户空间对设备文件的操作时,很容易转到对应的uart_driver.

另外:tty_driver的flags成员值为: TTY_DRIVER_REAL_RAW | TTY_DRIVER_DYNAMIC_DEV.里面包含有TTY_DRIVER_DYNAMIC_DEV标志。结合之前对tty的分析。如果包含有这个标志,是不会在初始化的时候去注册device.也就是说在/dev/下没有动态生成结点(如果是/dev下静态创建了这个结点就另当别论了^_^)。

流程图如下:

五: uart_add_one_port()操作

在前面提到。在对uart设备文件过程中。会将操作转换到对应的port上,这个port跟uart_driver是怎么关联起来的呢?这就是uart_add_ont_port()的主要工作了。

顾名思义,这个函数是在uart_driver增加一个port.代码如下:

    int uart_add_one_port(struct uart_driver *drv, struct uart_port *port)
    {
    struct uart_state *state;
    int ret = 0;
    struct device *tty_dev;
    BUG_ON(in_interrupt());
    if (port->line >= drv->nr)
    return -EINVAL;
    state = drv->state + port->line;
    mutex_lock(&port_mutex);
    mutex_lock(&state->mutex);
    if (state->port) {
    ret = -EINVAL;
    goto out;
    }
    state->port = port;
    state->pm_state = -1;
    port->cons = drv->cons;
    port->info = state->info;
    /*
    * If this port is a console, then the spinlock is already
    * initialised.
    */
    if (!(uart_console(port) && (port->cons->flags & CON_ENABLED))) {
    spin_lock_init(&port->lock);
    lockdep_set_class(&port->lock, &port_lock_key);
    }
    uart_configure_port(drv, state, port);
    /*
    * 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, port->line, port->dev);
    if (likely(!IS_ERR(tty_dev))) {
    device_can_wakeup(tty_dev) = 1;
    device_set_wakeup_enable(tty_dev, 0);
    } else
    printk(KERN_ERR "Cannot register tty device on line %d\n",
    port->line);
    /*
    * Ensure UPF_DEAD is not set.
    */
    port->flags &= ~UPF_DEAD;
    out:
    mutex_unlock(&state->mutex);
    mutex_unlock(&port_mutex);
    return ret;
    } 

首先这个函数不能在中断环境中使用。 Uart_port->line就是对uart设备文件序号。它对应的也就是uart_driver->state数组中的uart_port->line项。

它主要初始化对应uart_driver->state项。接着调用uart_configure_port()进行port的自动配置。然后注册tty_device.如果用户空间运行了udev或者已经配置好了hotplug.就会在/dev下自动生成设备文件了。

操作流程图如下所示:

六:设备节点的open操作

在用户空间执行open操作的时候,就会执行uart_ops->open. Uart_ops的定义如下:

    static const struct tty_operations uart_ops = {
    .open         = uart_open,
    .close        = uart_close,
    .write        = uart_write,
    .put_char = uart_put_char,
    .flush_chars  = uart_flush_chars,
    .write_room   = uart_write_room,
    .chars_in_buffer= uart_chars_in_buffer,
    .flush_buffer = uart_flush_buffer,
    .ioctl        = uart_ioctl,
    .throttle = uart_throttle,
    .unthrottle   = uart_unthrottle,
    .send_xchar   = uart_send_xchar,
    .set_termios  = uart_set_termios,
    .stop         = uart_stop,
    .start        = uart_start,
    .hangup       = uart_hangup,
    .break_ctl    = uart_break_ctl,
    .wait_until_sent= uart_wait_until_sent,
    #ifdef CONFIG_PROC_FS
    .read_proc    = uart_read_proc,
    #endif
    .tiocmget = uart_tiocmget,
    .tiocmset = uart_tiocmset,
    };

对应open的操作接口为uart_open.代码如下:

    static int uart_open(struct tty_struct *tty, struct file *filp)
    {
    struct uart_driver *drv = (struct uart_driver *)tty->driver->driver_state;
    struct uart_state *state;
    int retval, line = tty->index;
    BUG_ON(!kernel_locked());
    pr_debug("uart_open(%d) called\n", line);
    /*
    * tty->driver->num won't change, so we won't fail here with
    * tty->driver_data set to something non-NULL (and therefore
    * we won't get caught by uart_close())。
    */
    retval = -ENODEV;
    if (line >= tty->driver->num)
    goto fail;
    /*
    * We take the semaphore inside uart_get to guarantee that we won't
    * be re-entered while allocating the info structure, or while we
    * request any IRQs that the driver may need.  This also has the nice
    * side-effect that it delays the action of uart_hangup, so we can
    * guarantee that info->tty will always contain something reasonable.
    */
    state = uart_get(drv, line);
    if (IS_ERR(state)) {
    retval = PTR_ERR(state);
    goto fail;
    }
    /*
    * Once we set tty->driver_data here, we are guaranteed that
    * uart_close() will decrement the driver module use count.
    * Any failures from here onwards should not touch the count.
    */
    tty->driver_data = state;
    tty->low_latency = (state->port->flags & UPF_LOW_LATENCY) ? 1 : 0;
    tty->alt_speed = 0;
    state->info->tty = tty;
    /*
    * If the port is in the middle of closing, bail out now.
    */
    if (tty_hung_up_p(filp)) {
    retval = -EAGAIN;
    state->count--;
    mutex_unlock(&state->mutex);
    goto fail;
    }
    /*
    * Make sure the device is in D0 state.
    */
    if (state->count == 1)
    uart_change_pm(state, 0);
    /*
    * Start up the serial port.
    */
    retval = uart_startup(state, 0);
    /*
    * If we succeeded, wait until the port is ready.
    */
    if (retval == 0)
    retval = uart_block_til_ready(filp, state);
    mutex_unlock(&state->mutex);
    /*
    * If this is the first open to succeed, adjust things to suit.
    */
    if (retval == 0 && !(state->info->flags & UIF_NORMAL_ACTIVE)) {
    state->info->flags |= UIF_NORMAL_ACTIVE;
    uart_update_termios(state);
    }
    fail:
    return retval;
    }
	    int ret = 0;
    state = drv->state + line;
    if (mutex_lock_interruptible(&state->mutex)) {
    ret = -ERESTARTSYS;
    goto err;
    }
    state->count++;
    if (!state->port || state->port->flags & UPF_DEAD) {
    ret = -ENXIO;
    goto err_unlock;
    }
    if (!state->info) {
    state->info = kzalloc(sizeof(struct uart_info), GFP_KERNEL);
    if (state->info) {
    init_waitqueue_head(&state->info->open_wait);
    init_waitqueue_head(&state->info->delta_msr_wait);
    /*
    * Link the info into the other structures.
    */
    state->port->info = state->info;
    tasklet_init(&state->info->tlet, uart_tasklet_action,
    (unsigned long)state);
    } else {
    ret = -ENOMEM;
    goto err_unlock;
    }
    }
    return state;
    err_unlock:
    state->count--;
    mutex_unlock(&state->mutex);
    err:
    return ERR_PTR(ret);
    } 

从代码中可以看出。这里注要是操作是初始化state->info.注意port->info就是state->info的一个副本。即port直接通过port->info可以找到它要操作的缓存区。

uart_startup()代码如下:

    static int uart_startup(struct uart_state *state, int init_hw)
    {
    struct uart_info *info = state->info;
    struct uart_port *port = state->port;
    unsigned long page;
    int retval = 0;
    if (info->flags & UIF_INITIALIZED)
    return 0;
    /*
    * Set the TTY IO error marker - we will only clear this
    * once we have successfully opened the port.  Also set
    * up the tty->alt_speed kludge
    */
    set_bit(TTY_IO_ERROR, &info->tty->flags);
    if (port->type == PORT_UNKNOWN)
    return 0;
    /*
    * Initialise and allocate the transmit and temporary
    * buffer.
    */
    if (!info->xmit.buf) {
    page = get_zeroed_page(GFP_KERNEL);
    if (!page)
    return -ENOMEM;
    info->xmit.buf = (unsigned char *) page;
    uart_circ_clear(&info->xmit);
    }
    retval = port->ops->startup(port);
    if (retval == 0) {
    if (init_hw) {
    /*
    * Initialise the hardware port settings.
    */
    uart_change_speed(state, NULL);
    /*
    * Setup the RTS and DTR signals once the
    * port is open and ready to respond.
    */
    if (info->tty->termios->c_cflag & CBAUD)
    uart_set_mctrl(port, TIOCM_RTS | TIOCM_DTR);
    }
    if (info->flags & UIF_CTS_FLOW) {
    spin_lock_irq(&port->lock);
    if (!(port->ops->get_mctrl(port) & TIOCM_CTS))
    info->tty->hw_stopped = 1;
    spin_unlock_irq(&port->lock);
    }
    info->flags |= UIF_INITIALIZED;
    clear_bit(TTY_IO_ERROR, &info->tty->flags);
    }
    if (retval && capable(CAP_SYS_ADMIN))
    retval = 0;
    return retval;
    } 

在这里,注要完成对环形缓冲,即info->xmit的初始化。然后调用port->ops->startup( )将这个port带入到工作状态。其它的是一个可调参数的设置,就不详细讲解了。

七:设备节点的write操作

Write操作对应的操作接口为uart_write( )。代码如下:

    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;
    /*
    * This means you called this function _after_ the port was
    * closed.  No cookie for you.
    */
    if (!state || !state->info) {
    WARN_ON(1);
    return -EL3HLT;
    }
    port = state->port;
    circ = &state->info->xmit;
    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;
    } 

Uart_start()代码如下:

    static void uart_start(struct tty_struct *tty)
    {
    struct uart_state *state = tty->driver_data;
    struct uart_port *port = state->port;
    unsigned long flags;
    spin_lock_irqsave(&port->lock, flags);
    __uart_start(tty);
    spin_unlock_irqrestore(&port->lock, flags);
    }
    static void __uart_start(struct tty_struct *tty)
    {
    struct uart_state *state = tty->driver_data;
    struct uart_port *port = state->port;
    if (!uart_circ_empty(&state->info->xmit) && state->info->xmit.buf &&
    !tty->stopped && !tty->hw_stopped)
    port->ops->start_tx(port);
    } 

显然,对于write操作而言,它就是将数据copy到环形缓存区。然后调用port->ops->start_tx()将数据写到硬件寄存器。

八:Read操作

Uart的read操作同Tty的read操作相同,即都是调用ldsic->read()读取read_buf中的内容。有对这部份内容不太清楚的,参阅《 linux设备模型之tty驱动架构》.

九:小结

本小节是分析serial驱动的基础。在理解了tty驱动架构之后,再来理解uart驱动架构应该不是很难。随着我们在linux设备驱动分析的深入,越来越深刻的体会到,linux的设备驱动架构很多都是相通的。只要深刻理解了一种驱动架构。举一反三。也就很容易分析出其它架构的驱动了。

 
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