timecompare.c

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/*
 * Copyright (C) 2009 Intel Corporation.
 * Author: Patrick Ohly <[email protected]>
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
 */

#include <linux/timecompare.h>
#include <linux/module.h>
#include <linux/slab.h>
#include <linux/math64.h>
#include <linux/kernel.h>

/*
 * fixed point arithmetic scale factor for skew
 *
 * Usually one would measure skew in ppb (parts per billion, 1e9), but
 * using a factor of 2 simplifies the math.
 */
#define TIMECOMPARE_SKEW_RESOLUTION (((s64)1)<<30)

ktime_t timecompare_transform(struct timecompare *sync,
                  u64 source_tstamp)
{
    u64 nsec;

    nsec = source_tstamp + sync->offset;
    nsec += (s64)(source_tstamp - sync->last_update) * sync->skew /
        TIMECOMPARE_SKEW_RESOLUTION;

    return ns_to_ktime(nsec);
}
EXPORT_SYMBOL_GPL(timecompare_transform);

int timecompare_offset(struct timecompare *sync,
               s64 *offset,
               u64 *source_tstamp)
{
    u64 start_source = 0, end_source = 0;
    struct {
        s64 offset;
        s64 duration_target;
    } buffer[10], sample, *samples;
    int counter = 0, i;
    int used;
    int index;
    int num_samples = sync->num_samples;

    if (num_samples > ARRAY_SIZE(buffer)) {
        samples = kmalloc(sizeof(*samples) * num_samples, GFP_ATOMIC);
        if (!samples) {
            samples = buffer;
            num_samples = ARRAY_SIZE(buffer);
        }
    } else {
        samples = buffer;
    }

    /* run until we have enough valid samples, but do not try forever */
    i = 0;
    counter = 0;
    while (1) {
        u64 ts;
        ktime_t start, end;

        start = sync->target();
        ts = timecounter_read(sync->source);
        end = sync->target();

        if (!i)
            start_source = ts;

        /* ignore negative durations */
        sample.duration_target = ktime_to_ns(ktime_sub(end, start));
        if (sample.duration_target >= 0) {
            /*
             * assume symetric delay to and from source:
             * average target time corresponds to measured
             * source time
             */
            sample.offset =
                (ktime_to_ns(end) + ktime_to_ns(start)) / 2 -
                ts;

            /* simple insertion sort based on duration */
            index = counter - 1;
            while (index >= 0) {
                if (samples[index].duration_target <
                    sample.duration_target)
                    break;
                samples[index + 1] = samples[index];
                index--;
            }
            samples[index + 1] = sample;
            counter++;
        }

        i++;
        if (counter >= num_samples || i >= 100000) {
            end_source = ts;
            break;
        }
    }

    *source_tstamp = (end_source + start_source) / 2;

    /* remove outliers by only using 75% of the samples */
    used = counter * 3 / 4;
    if (!used)
        used = counter;
    if (used) {
        /* calculate average */
        s64 off = 0;
        for (index = 0; index < used; index++)
            off += samples[index].offset;
        *offset = div_s64(off, used);
    }

    if (samples && samples != buffer)
        kfree(samples);

    return used;
}
EXPORT_SYMBOL_GPL(timecompare_offset);

void __timecompare_update(struct timecompare *sync,
              u64 source_tstamp)
{
    s64 offset;
    u64 average_time;

    if (!timecompare_offset(sync, &offset, &average_time))
        return;

    if (!sync->last_update) {
        sync->last_update = average_time;
        sync->offset = offset;
        sync->skew = 0;
    } else {
        s64 delta_nsec = average_time - sync->last_update;

        /* avoid division by negative or small deltas */
        if (delta_nsec >= 10000) {
            s64 delta_offset_nsec = offset - sync->offset;
            s64 skew; /* delta_offset_nsec *
                     TIMECOMPARE_SKEW_RESOLUTION /
                     delta_nsec */
            u64 divisor;

            /* div_s64() is limited to 32 bit divisor */
            skew = delta_offset_nsec * TIMECOMPARE_SKEW_RESOLUTION;
            divisor = delta_nsec;
            while (unlikely(divisor >= ((s64)1) << 32)) {
                /* divide both by 2; beware, right shift
                   of negative value has undefined
                   behavior and can only be used for
                   the positive divisor */
                skew = div_s64(skew, 2);
                divisor >>= 1;
            }
            skew = div_s64(skew, divisor);

            /*
             * Calculate new overall skew as 4/16 the
             * old value and 12/16 the new one. This is
             * a rather arbitrary tradeoff between
             * only using the latest measurement (0/16 and
             * 16/16) and even more weight on past measurements.
             */
#define TIMECOMPARE_NEW_SKEW_PER_16 12
            sync->skew =
                div_s64((16 - TIMECOMPARE_NEW_SKEW_PER_16) *
                    sync->skew +
                    TIMECOMPARE_NEW_SKEW_PER_16 * skew,
                    16);
            sync->last_update = average_time;
            sync->offset = offset;
        }
    }
}
EXPORT_SYMBOL_GPL(__timecompare_update);