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/*
* Copyright (C) 2009 Intel Corporation.
* Author: Patrick Ohly <patrick.ohly@intel.com>
*
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation; either version 2 of the License, or
* (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program; if not, write to the Free Software
* Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
*/
#include <linux/timecompare.h>
#include <linux/module.h>
#include <linux/math64.h>
/*
* fixed point arithmetic scale factor for skew
*
* Usually one would measure skew in ppb (parts per billion, 1e9), but
* using a factor of 2 simplifies the math.
*/
#define TIMECOMPARE_SKEW_RESOLUTION (((s64)1)<<30)
ktime_t timecompare_transform(struct timecompare *sync,
u64 source_tstamp)
{
u64 nsec;
nsec = source_tstamp + sync->offset;
nsec += (s64)(source_tstamp - sync->last_update) * sync->skew /
TIMECOMPARE_SKEW_RESOLUTION;
return ns_to_ktime(nsec);
}
EXPORT_SYMBOL_GPL(timecompare_transform);
int timecompare_offset(struct timecompare *sync,
s64 *offset,
u64 *source_tstamp)
{
u64 start_source = 0, end_source = 0;
struct {
s64 offset;
s64 duration_target;
} buffer[10], sample, *samples;
int counter = 0, i;
int used;
int index;
int num_samples = sync->num_samples;
if (num_samples > sizeof(buffer)/sizeof(buffer[0])) {
samples = kmalloc(sizeof(*samples) * num_samples, GFP_ATOMIC);
if (!samples) {
samples = buffer;
num_samples = sizeof(buffer)/sizeof(buffer[0]);
}
} else {
samples = buffer;
}
/* run until we have enough valid samples, but do not try forever */
i = 0;
counter = 0;
while (1) {
u64 ts;
ktime_t start, end;
start = sync->target();
ts = timecounter_read(sync->source);
end = sync->target();
if (!i)
start_source = ts;
/* ignore negative durations */
sample.duration_target = ktime_to_ns(ktime_sub(end, start));
if (sample.duration_target >= 0) {
/*
* assume symetric delay to and from source:
* average target time corresponds to measured
* source time
*/
sample.offset =
(ktime_to_ns(end) + ktime_to_ns(start)) / 2 -
ts;
/* simple insertion sort based on duration */
index = counter - 1;
while (index >= 0) {
if (samples[index].duration_target <
sample.duration_target)
break;
samples[index + 1] = samples[index];
index--;
}
samples[index + 1] = sample;
counter++;
}
i++;
if (counter >= num_samples || i >= 100000) {
end_source = ts;
break;
}
}
*source_tstamp = (end_source + start_source) / 2;
/* remove outliers by only using 75% of the samples */
used = counter * 3 / 4;
if (!used)
used = counter;
if (used) {
/* calculate average */
s64 off = 0;
for (index = 0; index < used; index++)
off += samples[index].offset;
*offset = div_s64(off, used);
}
if (samples && samples != buffer)
kfree(samples);
return used;
}
EXPORT_SYMBOL_GPL(timecompare_offset);
void __timecompare_update(struct timecompare *sync,
u64 source_tstamp)
{
s64 offset;
u64 average_time;
if (!timecompare_offset(sync, &offset, &average_time))
return;
if (!sync->last_update) {
sync->last_update = average_time;
sync->offset = offset;
sync->skew = 0;
} else {
s64 delta_nsec = average_time - sync->last_update;
/* avoid division by negative or small deltas */
if (delta_nsec >= 10000) {
s64 delta_offset_nsec = offset - sync->offset;
s64 skew; /* delta_offset_nsec *
TIMECOMPARE_SKEW_RESOLUTION /
delta_nsec */
u64 divisor;
/* div_s64() is limited to 32 bit divisor */
skew = delta_offset_nsec * TIMECOMPARE_SKEW_RESOLUTION;
divisor = delta_nsec;
while (unlikely(divisor >= ((s64)1) << 32)) {
/* divide both by 2; beware, right shift
of negative value has undefined
behavior and can only be used for
the positive divisor */
skew = div_s64(skew, 2);
divisor >>= 1;
}
skew = div_s64(skew, divisor);
/*
* Calculate new overall skew as 4/16 the
* old value and 12/16 the new one. This is
* a rather arbitrary tradeoff between
* only using the latest measurement (0/16 and
* 16/16) and even more weight on past measurements.
*/
#define TIMECOMPARE_NEW_SKEW_PER_16 12
sync->skew =
div_s64((16 - TIMECOMPARE_NEW_SKEW_PER_16) *
sync->skew +
TIMECOMPARE_NEW_SKEW_PER_16 * skew,
16);
sync->last_update = average_time;
sync->offset = offset;
}
}
}
EXPORT_SYMBOL_GPL(__timecompare_update);
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