root_hub 和中断传输

老早听人讲，usb hub有一个而且只有一个INT_POINT;
所以按照操作流程，你想访问它，必须这样来：
 
usb_submit_urb()-->usb_hcd_submit_urb()-->rh_queue_status()-->rh_timer_func()-->usb_hcd_poll_rh_status()
1:在usb_submit_urb()中有：
if (urb->interval > (1024 * 8))
    urb->interval = 1024 * 8;
2:在usb_hcd_submit_urb()中有:
if (is_root_hub(urb->dev))
   status = rh_urb_enqueue(hcd, urb);
3:在rh_urb_enqueue中有
    if (usb_endpoint_xfer_int(&urb->ep->desc))
        return rh_queue_status (hcd, urb);
4：在rh_queue_status()

/*-------------------------------------------------------------------------*/

static int rh_queue_status (struct usb_hcd *hcd, struct urb *urb)
{
    int retval;
    unsigned long    flags;
    unsigned    len = 1 + (urb->dev->maxchild / 8);

    spin_lock_irqsave (&hcd_root_hub_lock, flags);
    if (hcd->status_urb || urb->transfer_buffer_length < len) {
        dev_dbg (hcd->self.controller, "not queuing rh status urb\n");
        retval = -EINVAL;
        goto done;
    }

    retval = usb_hcd_link_urb_to_ep(hcd, urb);
    if (retval)
        goto done;

    hcd->status_urb = urb;
    urb->hcpriv = hcd; /* indicate it

靠，搞不明白了：中断传输呢？？

百度，再百度，在网上找到这么一句话,
Root Hub interrupt transfers are polled using a timer if the driver requests it;
也就是讲，roothub的中断传输，其实用个定时器，去访问HC的register；快晕菜了；

/*-------------------------------------------------------------------------*/

/*
 * Root Hub interrupt transfers are polled using a timer if the
 * driver requests it; otherwise the driver is responsible for
 * calling usb_hcd_poll_rh_status() when an event occurs.
 *
 * Completions are called in_interrupt(), but they may or may not
 * be in_irq().
 */
void usb_hcd_poll_rh_status(struct usb_hcd *hcd)
{
    struct urb *urb;
    int length;
    unsigned long    flags;
    char        buffer[6]; /* Any root hubs with > 31 ports? */

    if (unlikely(!hcd->rh_registered))
        return;
    if (!hcd->uses_new_polling && !hcd->status_urb)
        return;

    length = hcd->driver->hub_status_data(hcd, buffer);
    if (length > 0) {

        /* try to complete the status urb */
        spin_lock_irqsave(&hcd_root_hub_lock, flags);
        urb = hcd->status_urb;
        if (urb) {
            hcd->poll_pending = 0;
            hcd->status_urb = NULL;
            urb->actual_length = length;
            memcpy(urb->transfer_buffer, buffer, length);

            usb_hcd_unlink_urb_from_ep(hcd, urb);
            spin_unlock(&hcd_root_hub_lock);
            usb_hcd_giveback_urb(hcd, urb, 0);
            spin_lock(&hcd_root_hub_lock);
        } else {
            length = 0;
            hcd->poll_pending = 1;
        }
        spin_unlock_irqrestore(&hcd_root_hub_lock, flags);
    }

    /* The USB 2.0 spec says 256 ms. This is close enough and won

总计：usb的IN事务在此处变成了一个定时器的工作了，毕竟HC和roothub在出生的时候就绑在一起了；
现在回头看看，hub驱动的探测过程中hub_probe()中启动hub_configure(hub, endpoint)也就是唤醒timer；

至于hub_irq的调用：找了很久，没有整明白，可能又和ehci和系统之间的中断有关联了，避免问题太复杂，我这里引用fudan_abc的话：
我们曾经在hub_configure中讲过中断传输,当时调用了usb_fill_int_urb()函数,并且把hub_irq作为一个参数传递了进去,最终把urb->complete赋值为hub_irq.然后,主机控制器会定期询问hub,每当hub端口上有一个设备插入或者拔除时,它就会向主机控制器打小报告.。。。
(具体来说,从硬件的角度看,就是hub会向host controller返回一些信息,或者说data,这个Data被称作”Hub and Port Status Change Bitmap”,而从软件角度来看,host controller的驱动程序接下来会在处理好这个过程的urb之后,调用该urb的complete函数,对于hub来说,这个函数就是hub_irq().