aboutsummaryrefslogtreecommitdiffstats
path: root/kernel/time
diff options
context:
space:
mode:
authorPatrick Ohly <patrick.ohly@intel.com>2009-02-12 00:03:35 -0500
committerDavid S. Miller <davem@davemloft.net>2009-02-16 01:43:32 -0500
commita75244c3d519fcb490ca2bf3f123c98017f1e8d0 (patch)
treedee5b6a3589cfaae698a11297667e9e0c8b01ef5 /kernel/time
parenta038a353c3de4040d8445ec568acebdac144436f (diff)
timecompare: generic infrastructure to map between two time bases
Mapping from a struct timecounter to a time returned by functions like ktime_get_real() is implemented. This is sufficient to use this code in a network device driver which wants to support hardware time stamping and transformation of hardware time stamps to system time. The interface could have been made more versatile by not depending on a time counter, but this wasn't done to avoid writing glue code elsewhere. The method implemented here is the one used and analyzed under the name "assisted PTP" in the LCI PTP paper: http://www.linuxclustersinstitute.org/conferences/archive/2008/PDF/Ohly_92221.pdf Acked-by: John Stultz <johnstul@us.ibm.com> Signed-off-by: Patrick Ohly <patrick.ohly@intel.com> Signed-off-by: David S. Miller <davem@davemloft.net>
Diffstat (limited to 'kernel/time')
-rw-r--r--kernel/time/Makefile2
-rw-r--r--kernel/time/timecompare.c191
2 files changed, 192 insertions, 1 deletions
diff --git a/kernel/time/Makefile b/kernel/time/Makefile
index 905b0b50792d..0b0a6366c9d4 100644
--- a/kernel/time/Makefile
+++ b/kernel/time/Makefile
@@ -1,4 +1,4 @@
1obj-y += timekeeping.o ntp.o clocksource.o jiffies.o timer_list.o 1obj-y += timekeeping.o ntp.o clocksource.o jiffies.o timer_list.o timecompare.o
2 2
3obj-$(CONFIG_GENERIC_CLOCKEVENTS_BUILD) += clockevents.o 3obj-$(CONFIG_GENERIC_CLOCKEVENTS_BUILD) += clockevents.o
4obj-$(CONFIG_GENERIC_CLOCKEVENTS) += tick-common.o 4obj-$(CONFIG_GENERIC_CLOCKEVENTS) += tick-common.o
diff --git a/kernel/time/timecompare.c b/kernel/time/timecompare.c
new file mode 100644
index 000000000000..71e7f1a19156
--- /dev/null
+++ b/kernel/time/timecompare.c
@@ -0,0 +1,191 @@
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);