ktime_sub_ns
*vblank_time = ktime_sub_ns(etime, delta_ns);
kt = ktime_sub_ns(kt, (count * prng32_1M * 2) >> 20);
kt = ktime_sub_ns(kt, (count * prng32_1M * 2) >> 20);
kt = ktime_sub_ns(kt, (count * prng32_1M * 2) >> 20);