Merge branch 'master' of /home/davem/src/GIT/linux-2.6/
[linux-2.6] / 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(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 }
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);