LXR linux/kernel/time/timecompare.c

   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/math64.h>
  23
  24/*
  25 * fixed point arithmetic scale factor for skew
  26 *
  27 * Usually one would measure skew in ppb (parts per billion, 1e9), but
  28 * using a factor of 2 simplifies the math.
  29 */
  30#define TIMECOMPARE_SKEW_RESOLUTION (((s64)1)<<30)
  31
  32ktime_t timecompare_transform(struct timecompare *sync,
  33                              u64 source_tstamp)
  34{
  35        u64 nsec;
  36
  37        nsec = source_tstamp + sync->offset;
  38        nsec += (s64)(source_tstamp - sync->last_update) * sync->skew /
  39                TIMECOMPARE_SKEW_RESOLUTION;
  40
  41        return ns_to_ktime(nsec);
  42}
  43EXPORT_SYMBOL(timecompare_transform);
  44
  45int timecompare_offset(struct timecompare *sync,
  46                       s64 *offset,
  47                       u64 *source_tstamp)
  48{
  49        u64 start_source = 0, end_source = 0;
  50        struct {
  51                s64 offset;
  52                s64 duration_target;
  53        } buffer[10], sample, *samples;
  54        int counter = 0, i;
  55        int used;
  56        int index;
  57        int num_samples = sync->num_samples;
  58
  59        if (num_samples > sizeof(buffer)/sizeof(buffer[0])) {
  60                samples = kmalloc(sizeof(*samples) * num_samples, GFP_ATOMIC);
  61                if (!samples) {
  62                        samples = buffer;
  63                        num_samples = sizeof(buffer)/sizeof(buffer[0]);
  64                }
  65        } else {
  66                samples = buffer;
  67        }
  68
  69        /* run until we have enough valid samples, but do not try forever */
  70        i = 0;
  71        counter = 0;
  72        while (1) {
  73                u64 ts;
  74                ktime_t start, end;
  75
  76                start = sync->target();
  77                ts = timecounter_read(sync->source);
  78                end = sync->target();
  79
  80                if (!i)
  81                        start_source = ts;
  82
  83                /* ignore negative durations */
  84                sample.duration_target = ktime_to_ns(ktime_sub(end, start));
  85                if (sample.duration_target >= 0) {
  86                        /*
  87                         * assume symetric delay to and from source:
  88                         * average target time corresponds to measured
  89                         * source time
  90                         */
  91                        sample.offset =
  92                                ktime_to_ns(ktime_add(end, start)) / 2 -
  93                                ts;
  94
  95                        /* simple insertion sort based on duration */
  96                        index = counter - 1;
  97                        while (index >= 0) {
  98                                if (samples[index].duration_target <
  99                                    sample.duration_target)
 100                                        break;
 101                                samples[index + 1] = samples[index];
 102                                index--;
 103                        }
 104                        samples[index + 1] = sample;
 105                        counter++;
 106                }
 107
 108                i++;
 109                if (counter >= num_samples || i >= 100000) {
 110                        end_source = ts;
 111                        break;
 112                }
 113        }
 114
 115        *source_tstamp = (end_source + start_source) / 2;
 116
 117        /* remove outliers by only using 75% of the samples */
 118        used = counter * 3 / 4;
 119        if (!used)
 120                used = counter;
 121        if (used) {
 122                /* calculate average */
 123                s64 off = 0;
 124                for (index = 0; index < used; index++)
 125                        off += samples[index].offset;
 126                *offset = div_s64(off, used);
 127        }
 128
 129        if (samples && samples != buffer)
 130                kfree(samples);
 131
 132        return used;
 133}
 134EXPORT_SYMBOL(timecompare_offset);
 135
 136void __timecompare_update(struct timecompare *sync,
 137                          u64 source_tstamp)
 138{
 139        s64 offset;
 140        u64 average_time;
 141
 142        if (!timecompare_offset(sync, &offset, &average_time))
 143                return;
 144
 145        if (!sync->last_update) {
 146                sync->last_update = average_time;
 147                sync->offset = offset;
 148                sync->skew = 0;
 149        } else {
 150                s64 delta_nsec = average_time - sync->last_update;
 151
 152                /* avoid division by negative or small deltas */
 153                if (delta_nsec >= 10000) {
 154                        s64 delta_offset_nsec = offset - sync->offset;
 155                        s64 skew; /* delta_offset_nsec *
 156                                     TIMECOMPARE_SKEW_RESOLUTION /
 157                                     delta_nsec */
 158                        u64 divisor;
 159
 160                        /* div_s64() is limited to 32 bit divisor */
 161                        skew = delta_offset_nsec * TIMECOMPARE_SKEW_RESOLUTION;
 162                        divisor = delta_nsec;
 163                        while (unlikely(divisor >= ((s64)1) << 32)) {
 164                                /* divide both by 2; beware, right shift
 165                                   of negative value has undefined
 166                                   behavior and can only be used for
 167                                   the positive divisor */
 168                                skew = div_s64(skew, 2);
 169                                divisor >>= 1;
 170                        }
 171                        skew = div_s64(skew, divisor);
 172
 173                        /*
 174                         * Calculate new overall skew as 4/16 the
 175                         * old value and 12/16 the new one. This is
 176                         * a rather arbitrary tradeoff between
 177                         * only using the latest measurement (0/16 and
 178                         * 16/16) and even more weight on past measurements.
 179                         */
 180#define TIMECOMPARE_NEW_SKEW_PER_16 12
 181                        sync->skew =
 182                                div_s64((16 - TIMECOMPARE_NEW_SKEW_PER_16) *
 183                                        sync->skew +
 184                                        TIMECOMPARE_NEW_SKEW_PER_16 * skew,
 185                                        16);
 186                        sync->last_update = average_time;
 187                        sync->offset = offset;
 188                }
 189        }
 190}
 191EXPORT_SYMBOL(__timecompare_update);
 192