AHCI_REG_IS
ahci_write(ap->ap_sc, AHCI_REG_IS, 1 << i);
is = ahci_read(sc, AHCI_REG_IS);
ahci_write(sc, AHCI_REG_IS, is);
ahci_read(ap->ap_sc, AHCI_REG_IS),
ahci_write(sc, AHCI_REG_IS, 1 << port);