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diff --git a/include/linux/timecompare.h b/include/linux/timecompare.h
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+/*
+ * 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 */