REG_BCRM_EXPOSURE_TIME_RW
ret = alvium_read(alvium, REG_BCRM_EXPOSURE_TIME_RW, &exp, NULL);
ret = alvium_write_hshake(alvium, REG_BCRM_EXPOSURE_TIME_RW,
alvium_read(alvium, REG_BCRM_EXPOSURE_TIME_RW, &dft_exp, &ret);