GLXIIC_SMB_SDA
bus_read_1(sc->smb_res, GLXIIC_SMB_SDA);
bus_write_1(sc->smb_res, GLXIIC_SMB_SDA, data);
data = bus_read_1(sc->smb_res, GLXIIC_SMB_SDA);
bus_write_1(sc->smb_res, GLXIIC_SMB_SDA, slave);
bus_write_1(sc->smb_res, GLXIIC_SMB_SDA, *sc->data++);
*sc->data++ = bus_read_1(sc->smb_res, GLXIIC_SMB_SDA);