SMPL_INTVL_US
sleep_usec = 3 * SMPL_INTVL_US + SLEEP_SLACK_US - ktime_to_us(ktime);
#define SMPL_INTVL_MS (SMPL_INTVL_US / USEC_PER_MSEC)
#define CALIBRATION_TIME_US (10 * SMPL_INTVL_US)