XHCI_MAX_XFER
if (xhci_ring_alloc(sc, &xp->ring, XHCI_MAX_XFER, XHCI_XFER_RING_ALIGN))
struct usbd_xfer *pending_xfers[XHCI_MAX_XFER];
uint8_t trb_processed[XHCI_MAX_XFER];