What does struct usb_tt * tt used for?

  I saw there is a struct usb_tt *tt as an element of struct
usb_device, and there also is a struct usb_tt tt as an element of
struct usb_hub, I am confused by these two "tt". I thought only hubs
can have tt, so what does the pointer tt within struct usb_deivce used
for normal usb device?In other words, why do we prepare a pointer tt
for struct usb_device?

In particular, inside hub_port_init(),

	/* Set up TT records, if needed  */
	if (hdev->tt) {
		udev->tt = hdev->tt;
		udev->ttport = hdev->ttport;
	} else if (udev->speed != USB_SPEED_HIGH
			&& hdev->speed == USB_SPEED_HIGH) {
		udev->tt = &hub->tt;
		udev->ttport = port1;
	}

I don't understand this entire "if statement",say, if hdev->tt is
true, this means it is a high speed hub, so what? else if the device
is full/low speed and the hub is high speed, still so what?

Regards
Jason

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