读usb驱动遇到的问题

本人正在研读prolific的usb转串口的驱动，在pl2303函数中首先是做了一些初始化操作，然后又submit了一次read urb和interrupt urb，代码如下：
587         dbg(\"%s - submitting read urb\", __FUNCTION__);
588         port->read_urb->dev = serial->dev;
589         result = usb_submit_urb (port->read_urb, GFP_KERNEL);
590         if (result) {
591                 dev_err(&port->dev, \"%s - failed submitting read urb, error %d\\n\", __FUNCTION__, result);
592                 pl2303_close (port, NULL);
593                 return -EPROTO;
594         }
595
596         dbg(\"%s - submitting interrupt urb\", __FUNCTION__);
597         port->interrupt_in_urb->dev = serial->dev;
598         result = usb_submit_urb (port->interrupt_in_urb, GFP_KERNEL);
599         if (result) {
600                 dev_err(&port->dev, \"%s - failed submitting interrupt urb, error %d\\n\", __FUNCTION__, result);
601                 pl2303_close (port, NULL);
602                 return -EPROTO;
603         }
不知道这段代码的目的何在，难得也是初始化，或者是用来测试状态的？
希望哪位大侠能够指点迷津，谢谢。

Pls. pay attention to read_urb,
Just send IN_TOKEN and INTERRUPT_TOKEN to client to get data.
You know USB client is a passive device.