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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@gmail.com>
Acked-by: John Stultz <john.stultz@linaro.org>
Tested-by: Bob Liu <lliubbo@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Richard Cochran 2012-10-31 06:27:25 +00:00 committed by David S. Miller
parent dd87b22f90
commit 65f8f9a1c1
3 changed files with 1 additions and 319 deletions
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125
include/linux/timecompare.h
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@ -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@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 */

2
kernel/time/Makefile
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@ -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

193
kernel/time/timecompare.c
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@ -1,193 +0,0 @@
/*
* 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/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);