XHCI_ERSTSZ
xhci_rt_write_4(sc, XHCI_ERSTSZ(0), XHCI_EVENT_RING_SEGMENTS);
xhci_rt_write_4(sc, XHCI_ERSTSZ(0), 0);
sc->sc_regs.erstsz0 = xhci_rt_read_4(sc, XHCI_ERSTSZ(0));
xhci_rt_write_4(sc, XHCI_ERSTSZ(0), sc->sc_regs.erstsz0);