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); 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); for (i = 0; i < drv->nr; i++) { struct uart_state *state = drv->state + i; state->close_delay = 500; state->closing_wait = 30000; 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 (!(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); 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); 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); retval = -ENODEV; if (line >= tty->driver->num) goto fail; state = uart_get(drv, line); if (IS_ERR(state)) { retval = PTR_ERR(state); goto fail; } tty->driver_data = state; tty->low_latency = (state->port->flags & UPF_LOW_LATENCY) ? 1 : 0; tty->alt_speed = 0; state->info->tty = tty; if (tty_hung_up_p(filp)) { retval = -EAGAIN; state->count--; mutex_unlock(&state->mutex); goto fail; } if (state->count == 1) uart_change_pm(state, 0); retval = uart_startup(state, 0); if (retval == 0) retval = uart_block_til_ready(filp, state); mutex_unlock(&state->mutex); 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); 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_bit(TTY_IO_ERROR, &info->tty->flags); if (port->type == PORT_UNKNOWN) return 0; 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) { uart_change_speed(state, NULL); 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; 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。 |