linux设备模型之uart驱动架构分析

 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：

[!--empirenews.page--]

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 %dn",

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) calledn", 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;[!--empirenews.page--]

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;[!--empirenews.page--]

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。