projects/linux·blob v3.6 · refs

193 lines · 4.8 KiB

1/*
2 * Copyright (C) 2009 Intel Corporation.
3 * Author: Patrick Ohly <[email protected]>
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);