BCM2835_FSEL_MASK
enum bcm2835_fsel status = (val >> FSEL_SHIFT(pin)) & BCM2835_FSEL_MASK;
cur = (val >> FSEL_SHIFT(pin)) & BCM2835_FSEL_MASK;
val &= ~(BCM2835_FSEL_MASK << FSEL_SHIFT(pin));
val &= ~(BCM2835_FSEL_MASK << FSEL_SHIFT(pin));