Mastering Linux Kernel Development

Tracking and maintaining time

Timekeeping helper routines timekeeping_get_ns() and timekeeping_get_ns() help get the correction factor (delta t) between universal time and terrestrial time in nanoseconds:

static inline u64 timekeeping_delta_to_ns(struct tk_read_base *tkr, u64 delta)
{
        u64 nsec;

        nsec = delta * tkr->mult + tkr->xtime_nsec;
        nsec >>= tkr->shift;

        /* If arch requires, add in get_arch_timeoffset() */
        return nsec + arch_gettimeoffset();
}

static inline u64 timekeeping_get_ns(struct tk_read_base *tkr)
{
        u64 delta;

        delta = timekeeping_get_delta(tkr);
        return timekeeping_delta_to_ns(tkr, delta);
}

The routine logarithmic_accumulation() updates mono, raw, and xtime timelines; it accumulates shifted intervals of cycles into a shifted interval ...