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
 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);