pmac_do_feature_call
extern long pmac_do_feature_call(unsigned int selector, ...);
pmac_do_feature_call(PMAC_FTR_SOUND_CHIP_ENABLE,
.feature_call = pmac_do_feature_call,
pmac_do_feature_call(PMAC_FTR_WRITE_GPIO, NULL, smu->doorbell, 4);
gpio = pmac_do_feature_call(PMAC_FTR_READ_GPIO, NULL, smu->doorbell);
gpio = pmac_do_feature_call(PMAC_FTR_READ_GPIO, NULL, smu->doorbell);