diff --git a/include/linux/timecompare.h b/include/linux/timecompare.h
new file mode 100644
index 0000000000000000000000000000000000000000..546e2234e4b33d49cafd995c38e9f08f19cd6907
--- /dev/null
+++ b/include/linux/timecompare.h
@@ -0,0 +1,125 @@
+/*
+ * Utility code which helps transforming between two different time
+ * bases, called "source" and "target" time in this code.
+ *
+ * Source time has to be provided via the timecounter API while target
+ * time is accessed via a function callback whose prototype
+ * intentionally matches ktime_get() and ktime_get_real(). These
+ * interfaces where chosen like this so that the code serves its
+ * initial purpose without additional glue code.
+ *
+ * This purpose is synchronizing a hardware clock in a NIC with system
+ * time, in order to implement the Precision Time Protocol (PTP,
+ * IEEE1588) with more accurate hardware assisted time stamping. In
+ * that context only synchronization against system time (=
+ * ktime_get_real()) is currently needed. But this utility code might
+ * become useful in other situations, which is why it was written as
+ * general purpose utility code.
+ *
+ * The source timecounter is assumed to return monotonically
+ * increasing time (but this code does its best to compensate if that
+ * is not the case) whereas target time may jump.
+ *
+ * The target time corresponding to a source time is determined by
+ * reading target time, reading source time, reading target time
+ * again, then assuming that average target time corresponds to source
+ * time. In other words, the assumption is that reading the source
+ * time is slow and involves equal time for sending the request and
+ * receiving the reply, whereas reading target time is assumed to be
+ * fast.
+ *
+ * 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 and conditions of the GNU General Public License,
+ * version 2, as published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope 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.,
+ * 51 Franklin St - Fifth Floor, Boston, MA 02110-1301 USA.
+ */
+#ifndef _LINUX_TIMECOMPARE_H
+#define _LINUX_TIMECOMPARE_H
+
+#include <linux/clocksource.h>
+#include <linux/ktime.h>
+
+/**
+ * struct timecompare - stores state and configuration for the two clocks
+ *
+ * Initialize to zero, then set source/target/num_samples.
+ *
+ * Transformation between source time and target time is done with:
+ * target_time = source_time + offset +
+ * (source_time - last_update) * skew /
+ * TIMECOMPARE_SKEW_RESOLUTION
+ *
+ * @source: used to get source time stamps via timecounter_read()
+ * @target: function returning target time (for example, ktime_get
+ * for monotonic time, or ktime_get_real for wall clock)
+ * @num_samples: number of times that source time and target time are to
+ * be compared when determining their offset
+ * @offset: (target time - source time) at the time of the last update
+ * @skew: average (target time - source time) / delta source time *
+ * TIMECOMPARE_SKEW_RESOLUTION
+ * @last_update: last source time stamp when time offset was measured
+ */
+struct timecompare {
+ struct timecounter *source;
+ ktime_t (*target)(void);
+ int num_samples;
+
+ s64 offset;
+ s64 skew;
+ u64 last_update;
+};
+
+/**
+ * timecompare_transform - transform source time stamp into target time base
+ * @sync: context for time sync
+ * @source_tstamp: the result of timecounter_read() or
+ * timecounter_cyc2time()
+ */
+extern ktime_t timecompare_transform(struct timecompare *sync,
+ u64 source_tstamp);
+
+/**
+ * timecompare_offset - measure current (target time - source time) offset
+ * @sync: context for time sync
+ * @offset: average offset during sample period returned here
+ * @source_tstamp: average source time during sample period returned here
+ *
+ * Returns number of samples used. Might be zero (= no result) in the
+ * unlikely case that target time was monotonically decreasing for all
+ * samples (= broken).
+ */
+extern int timecompare_offset(struct timecompare *sync,
+ s64 *offset,
+ u64 *source_tstamp);
+
+extern void __timecompare_update(struct timecompare *sync,
+ u64 source_tstamp);
+
+/**
+ * timecompare_update - update offset and skew by measuring current offset
+ * @sync: context for time sync
+ * @source_tstamp: the result of timecounter_read() or
+ * timecounter_cyc2time(), pass zero to force update
+ *
+ * Updates are only done at most once per second.
+ */
+static inline void timecompare_update(struct timecompare *sync,
+ u64 source_tstamp)
+{
+ if (!source_tstamp ||
+ (s64)(source_tstamp - sync->last_update) >= NSEC_PER_SEC)
+ __timecompare_update(sync, source_tstamp);
+}
+
+#endif /* _LINUX_TIMECOMPARE_H */
diff --git a/kernel/time/Makefile b/kernel/time/Makefile
index 905b0b50792da7a9da1f3bbab3428604ec6ea9fb..0b0a6366c9d482b8cabbbdc3e290087e2622c0bf 100644
--- a/kernel/time/Makefile
+++ b/kernel/time/Makefile
@@ -1,4 +1,4 @@
-obj-y += timekeeping.o ntp.o clocksource.o jiffies.o timer_list.o
+obj-y += timekeeping.o ntp.o clocksource.o jiffies.o timer_list.o timecompare.o
obj-$(CONFIG_GENERIC_CLOCKEVENTS_BUILD) += clockevents.o
obj-$(CONFIG_GENERIC_CLOCKEVENTS) += tick-common.o
diff --git a/kernel/time/timecompare.c b/kernel/time/timecompare.c
new file mode 100644
index 0000000000000000000000000000000000000000..71e7f1a19156183b70fe7a85da2ae8fa0329989b
--- /dev/null
+++ b/kernel/time/timecompare.c
@@ -0,0 +1,191 @@
+/*
+ * 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(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(ktime_add(end, 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(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(__timecompare_update);