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Message-ID: <CAA_GA1fP79GzVkvLSp5a9NNTEUhyw1h0UTYYq09iUqdsj4iVQg@mail.gmail.com>
Date: Thu, 1 Nov 2012 17:41:00 +0800
From: Bob Liu <lliubbo@...il.com>
To: Richard Cochran <richardcochran@...il.com>
Cc: netdev@...r.kernel.org, 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>,
Sonic Zhang <sonic.zhang@...log.com>
Subject: Re: [PATCH V2 net-next 4/4] time: remove the timecompare code.
On Thu, Nov 1, 2012 at 12:27 AM, Richard Cochran
<richardcochran@...il.com> wrote:
> 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>
Tested-by: Bob Liu <lliubbo@...il.com>
> Acked-by: John Stultz <john.stultz@...aro.org>
> ---
> 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
>
--
Regards,
--Bob
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