usb hcd questions

1.I noticed there is a new root-hub polling mechanism,what is it?And
what is the old one?

2.struct usb_hcd has an element of unsigned poll_pending, what does
this variable mean?In particular, this method sets poll_pending:

void usb_hcd_poll_rh_status(struct usb_hcd *hcd){
...
		if (length > 0)
			usb_hcd_giveback_urb (hcd, urb);
		else
			hcd->poll_pending = 1;
}

Why for length<=0, we need to set hcd->poll_pending to 1?

Thanks
Jason

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