Message-Id: <e100d86136270dae62ff546e3a6c5ef10631c68c.1348851539.git.richardcochran@gmail.com>
Date:	Fri, 28 Sep 2012 19:20:28 +0200
From:	Richard Cochran <richardcochran@...il.com>
To:	<netdev@...r.kernel.org>
Cc:	<device-drivers-devel@...ckfin.uclinux.org>,
	<uclinux-dist-devel@...ckfin.uclinux.org>,
	David Miller <davem@...emloft.net>,
	Jacob Keller <jacob.e.keller@...el.com>,
	Jeff Kirsher <jeffrey.t.kirsher@...el.com>,
	John Ronciak <john.ronciak@...el.com>,
	John Stultz <john.stultz@...aro.org>,
	Mike Frysinger <vapier@...too.org>,
	Miroslav Lichvar <mlichvar@...hat.com>,
	Patrick Ohly <patrick.ohly@...el.com>,
	Sonic Zhang <sonic.zhang@...log.com>
Subject: [PATCH RFC net-next 4/4] time: remove the timecompare code.

This patch removes the timecompare code from the kernel. The top five
reasons to do this are:

1. There are no more users of this code.
2. The original idea was a bit weak.
3. The original author has disappeared.
4. The code was not general purpose but tuned to a particular hardware,
5. There are better ways to accomplish clock synchronization.

Signed-off-by: Richard Cochran <richardcochran@...il.com>
---
 include/linux/timecompare.h |  125 ----------------------------
 kernel/time/Makefile        |    2 +-
 kernel/time/timecompare.c   |  193 -------------------------------------------
 3 files changed, 1 insertions(+), 319 deletions(-)
 delete mode 100644 include/linux/timecompare.h
 delete mode 100644 kernel/time/timecompare.c

diff --git a/include/linux/timecompare.h b/include/linux/timecompare.h
deleted file mode 100644
index 546e223..0000000
--- a/include/linux/timecompare.h
+++ /dev/null
@@ -1,125 +0,0 @@
-/*
- * 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@...el.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 e2fd74b..ff7d9d2 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 timecompare.o
+obj-y += timekeeping.o ntp.o clocksource.o jiffies.o timer_list.o
 obj-y += timeconv.o posix-clock.o alarmtimer.o
 
 obj-$(CONFIG_GENERIC_CLOCKEVENTS_BUILD)		+= clockevents.o
diff --git a/kernel/time/timecompare.c b/kernel/time/timecompare.c
deleted file mode 100644
index a9ae369..0000000
--- a/kernel/time/timecompare.c
+++ /dev/null
@@ -1,193 +0,0 @@
-/*
- * Copyright (C) 2009 Intel Corporation.
- * Author: Patrick Ohly <patrick.ohly@...el.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/slab.h>
-#include <linux/math64.h>
-#include <linux/kernel.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 > ARRAY_SIZE(buffer)) {
-		samples = kmalloc(sizeof(*samples) * num_samples, GFP_ATOMIC);
-		if (!samples) {
-			samples = buffer;
-			num_samples = ARRAY_SIZE(buffer);
-		}
-	} 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);
-- 
1.7.2.5

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