linux/kernel/time/timecompare.c
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   1/*
   2 * Copyright (C) 2009 Intel Corporation.
   3 * Author: Patrick Ohly <patrick.ohly@intel.com>
   4 *
   5 * This program is free software; you can redistribute it and/or modify
   6 * it under the terms of the GNU General Public License as published by
   7 * the Free Software Foundation; either version 2 of the License, or
   8 * (at your option) any later version.
   9 *
  10 * This program is distributed in the hope that it will be useful,
  11 * but WITHOUT ANY WARRANTY; without even the implied warranty of
  12 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  13 * GNU General Public License for more details.
  14 *
  15 * You should have received a copy of the GNU General Public License
  16 * along with this program; if not, write to the Free Software
  17 * Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
  18 */
  19
  20#include <linux/timecompare.h>
  21#include <linux/module.h>
  22#include <linux/slab.h>
  23#include <linux/math64.h>
  24#include <linux/kernel.h>
  25
  26/*
  27 * fixed point arithmetic scale factor for skew
  28 *
  29 * Usually one would measure skew in ppb (parts per billion, 1e9), but
  30 * using a factor of 2 simplifies the math.
  31 */
  32#define TIMECOMPARE_SKEW_RESOLUTION (((s64)1)<<30)
  33
  34ktime_t timecompare_transform(struct timecompare *sync,
  35                              u64 source_tstamp)
  36{
  37        u64 nsec;
  38
  39        nsec = source_tstamp + sync->offset;
  40        nsec += (s64)(source_tstamp - sync->last_update) * sync->skew /
  41                TIMECOMPARE_SKEW_RESOLUTION;
  42
  43        return ns_to_ktime(nsec);
  44}
  45EXPORT_SYMBOL_GPL(timecompare_transform);
  46
  47int timecompare_offset(struct timecompare *sync,
  48                       s64 *offset,
  49                       u64 *source_tstamp)
  50{
  51        u64 start_source = 0, end_source = 0;
  52        struct {
  53                s64 offset;
  54                s64 duration_target;
  55        } buffer[10], sample, *samples;
  56        int counter = 0, i;
  57        int used;
  58        int index;
  59        int num_samples = sync->num_samples;
  60
  61        if (num_samples > ARRAY_SIZE(buffer)) {
  62                samples = kmalloc(sizeof(*samples) * num_samples, GFP_ATOMIC);
  63                if (!samples) {
  64                        samples = buffer;
  65                        num_samples = ARRAY_SIZE(buffer);
  66                }
  67        } else {
  68                samples = buffer;
  69        }
  70
  71        /* run until we have enough valid samples, but do not try forever */
  72        i = 0;
  73        counter = 0;
  74        while (1) {
  75                u64 ts;
  76                ktime_t start, end;
  77
  78                start = sync->target();
  79                ts = timecounter_read(sync->source);
  80                end = sync->target();
  81
  82                if (!i)
  83                        start_source = ts;
  84
  85                /* ignore negative durations */
  86                sample.duration_target = ktime_to_ns(ktime_sub(end, start));
  87                if (sample.duration_target >= 0) {
  88                        /*
  89                         * assume symetric delay to and from source:
  90                         * average target time corresponds to measured
  91                         * source time
  92                         */
  93                        sample.offset =
  94                                (ktime_to_ns(end) + ktime_to_ns(start)) / 2 -
  95                                ts;
  96
  97                        /* simple insertion sort based on duration */
  98                        index = counter - 1;
  99                        while (index >= 0) {
 100                                if (samples[index].duration_target <
 101                                    sample.duration_target)
 102                                        break;
 103                                samples[index + 1] = samples[index];
 104                                index--;
 105                        }
 106                        samples[index + 1] = sample;
 107                        counter++;
 108                }
 109
 110                i++;
 111                if (counter >= num_samples || i >= 100000) {
 112                        end_source = ts;
 113                        break;
 114                }
 115        }
 116
 117        *source_tstamp = (end_source + start_source) / 2;
 118
 119        /* remove outliers by only using 75% of the samples */
 120        used = counter * 3 / 4;
 121        if (!used)
 122                used = counter;
 123        if (used) {
 124                /* calculate average */
 125                s64 off = 0;
 126                for (index = 0; index < used; index++)
 127                        off += samples[index].offset;
 128                *offset = div_s64(off, used);
 129        }
 130
 131        if (samples && samples != buffer)
 132                kfree(samples);
 133
 134        return used;
 135}
 136EXPORT_SYMBOL_GPL(timecompare_offset);
 137
 138void __timecompare_update(struct timecompare *sync,
 139                          u64 source_tstamp)
 140{
 141        s64 offset;
 142        u64 average_time;
 143
 144        if (!timecompare_offset(sync, &offset, &average_time))
 145                return;
 146
 147        if (!sync->last_update) {
 148                sync->last_update = average_time;
 149                sync->offset = offset;
 150                sync->skew = 0;
 151        } else {
 152                s64 delta_nsec = average_time - sync->last_update;
 153
 154                /* avoid division by negative or small deltas */
 155                if (delta_nsec >= 10000) {
 156                        s64 delta_offset_nsec = offset - sync->offset;
 157                        s64 skew; /* delta_offset_nsec *
 158                                     TIMECOMPARE_SKEW_RESOLUTION /
 159                                     delta_nsec */
 160                        u64 divisor;
 161
 162                        /* div_s64() is limited to 32 bit divisor */
 163                        skew = delta_offset_nsec * TIMECOMPARE_SKEW_RESOLUTION;
 164                        divisor = delta_nsec;
 165                        while (unlikely(divisor >= ((s64)1) << 32)) {
 166                                /* divide both by 2; beware, right shift
 167                                   of negative value has undefined
 168                                   behavior and can only be used for
 169                                   the positive divisor */
 170                                skew = div_s64(skew, 2);
 171                                divisor >>= 1;
 172                        }
 173                        skew = div_s64(skew, divisor);
 174
 175                        /*
 176                         * Calculate new overall skew as 4/16 the
 177                         * old value and 12/16 the new one. This is
 178                         * a rather arbitrary tradeoff between
 179                         * only using the latest measurement (0/16 and
 180                         * 16/16) and even more weight on past measurements.
 181                         */
 182#define TIMECOMPARE_NEW_SKEW_PER_16 12
 183                        sync->skew =
 184                                div_s64((16 - TIMECOMPARE_NEW_SKEW_PER_16) *
 185                                        sync->skew +
 186                                        TIMECOMPARE_NEW_SKEW_PER_16 * skew,
 187                                        16);
 188                        sync->last_update = average_time;
 189                        sync->offset = offset;
 190                }
 191        }
 192}
 193EXPORT_SYMBOL_GPL(__timecompare_update);
 194
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