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
  32 ktime_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 }
  43 EXPORT_SYMBOL(timecompare_transform);
  44
  45 int 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(end) + ktime_to_ns(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 }
 134 EXPORT_SYMBOL(timecompare_offset);
 135
 136 void __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 }
 191 EXPORT_SYMBOL(__timecompare_update);