void __init
sched_clock_register(u64 (*read)(void), int bits, unsigned long rate)
{
    u64 res, wrap, new_mask, new_epoch, cyc, ns;
    u32 new_mult, new_shift;
    unsigned long r, flags;
    char r_unit;
    struct clock_read_data rd;

    if (cd.rate > rate)
        return;

    /* Cannot register a sched_clock with interrupts on */
    local_irq_save(flags);

    /* Calculate the mult/shift to convert counter ticks to ns. */
    clocks_calc_mult_shift(&new_mult, &new_shift, rate, NSEC_PER_SEC, 3600);

    new_mask = CLOCKSOURCE_MASK(bits);
    cd.rate = rate;

    /* Calculate how many nanosecs until we risk wrapping */
    wrap = clocks_calc_max_nsecs(new_mult, new_shift, 0, new_mask, NULL);
    cd.wrap_kt = ns_to_ktime(wrap);

    rd = cd.read_data[0];

    /* Update epoch for new counter and update 'epoch_ns' from old counter*/
    new_epoch = read();
    cyc = cd.actual_read_sched_clock();
    ns = rd.epoch_ns + cyc_to_ns((cyc - rd.epoch_cyc) & rd.sched_clock_mask, rd.mult, rd.shift);
    cd.actual_read_sched_clock = read;

    rd.read_sched_clock    = read;
    rd.sched_clock_mask    = new_mask;
    rd.mult            = new_mult;
    rd.shift        = new_shift;
    rd.epoch_cyc        = new_epoch;
    rd.epoch_ns        = ns;

    update_clock_read_data(&rd);

    if (sched_clock_timer.function != NULL) {
        /* update timeout for clock wrap */
        hrtimer_start(&sched_clock_timer, cd.wrap_kt,
                  HRTIMER_MODE_REL_HARD);
    }

    r = rate;
    if (r >= 4000000) {
        r /= 1000000;
        r_unit = 'M';
    } else {
        if (r >= 1000) {
            r /= 1000;
            r_unit = 'k';
        } else {
            r_unit = ' ';
        }
    }

    /* Calculate the ns resolution of this counter */
    res = cyc_to_ns(1ULL, new_mult, new_shift);

    pr_info("sched_clock: %u bits at %lu%cHz, resolution %lluns, wraps every %lluns\n",
        bits, r, r_unit, res, wrap);

    /* Enable IRQ time accounting if we have a fast enough sched_clock() */
    if (irqtime > 0 || (irqtime == -1 && rate >= 1000000))
        enable_sched_clock_irqtime();

    local_irq_restore(flags);

    pr_debug("Registered %pS as sched_clock source\n", read);
}

 

 

[    0.219204] clocksource: arm,sp804: mask: 0xffffffff max_cycles: 0xffffffff, max_idle_ns: 39817925974 ns
[    0.229087] sched_clock: 32 bits at 48MHz, resolution 20ns, wraps every 44739242997ns