UHCI_FRNUM
next = (UREAD2(sc, UHCI_FRNUM) + 3) % UHCI_VFRAMELIST_COUNT;
#define UHCI_CURFRAME(sc) (UREAD2(sc, UHCI_FRNUM) & UHCI_FRNUM_MASK)
UWRITE2(sc, UHCI_FRNUM, 0);
UWRITE2(sc, UHCI_FRNUM, sc->sc_saved_frnum);
sc->sc_saved_frnum = UREAD2(sc, UHCI_FRNUM);
UREAD2(sc, UHCI_INTR), UREAD2(sc, UHCI_FRNUM));