BCM2835_GPIO_GPFSEL
v = bus_space_read_4(sc->sc_iot, sc->sc_ioh, BCM2835_GPIO_GPFSEL(regid));
bus_space_write_4(sc->sc_iot, sc->sc_ioh, BCM2835_GPIO_GPFSEL(regid), v);
v = bus_space_read_4(sc->sc_iot, sc->sc_ioh, BCM2835_GPIO_GPFSEL(regid));