What does USB_DT_HUB_NONVAR_SIZE mean?

Sorry I cannot find this in usb 2.0 spec.
But inside the code,drivers/usb/core/hub.c:
    103 /* USB 2.0 spec Section 11.24.4.5 */
    104 static int get_hub_descriptor(struct usb_device *hdev, void
*data, int size)
    105 {
    106         int i, ret;
    107
    108         for (i = 0; i < 3; i++) {
    109                 ret = usb_control_msg(hdev, usb_rcvctrlpipe(hdev, 0),
    110                         USB_REQ_GET_DESCRIPTOR, USB_DIR_IN | USB_RT_HUB,
    111                         USB_DT_HUB << 8, 0, data, size,
    112                         HZ * USB_CTRL_GET_TIMEOUT);
    113                 if (ret >= (USB_DT_HUB_NONVAR_SIZE + 2))
    114                         return ret;
    115         }
    116         return -EINVAL;
    117 }
what does this macro mean? And why here we loop three times?

Thanks
Jason Xiao

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