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/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
  34 ktime_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 }
  45 EXPORT_SYMBOL_GPL(timecompare_transform);
  46
  47 int 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 }
 136 EXPORT_SYMBOL_GPL(timecompare_offset);
 137
 138 void __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 }
 193 EXPORT_SYMBOL_GPL(__timecompare_update);