Date:	Mon, 09 Feb 2009 11:27:47 -0800
From:	John Stultz <johnstul@...ibm.com>
To:	Patrick Ohly <patrick.ohly@...el.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	David Miller <davem@...emloft.net>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH NET-NEXT 02/10] time sync: generic infrastructure	to
	map between time stamps generated by a time counter and system time

On Mon, 2009-02-09 at 18:02 +0100, Patrick Ohly wrote:
> On Thu, 2009-02-05 at 10:21 +0000, Patrick Ohly wrote:
> > On Wed, 2009-02-04 at 21:44 +0200, john stultz wrote:
> > > On Wed, 2009-02-04 at 14:01 +0100, Patrick Ohly wrote:
> > > I sort of object to the name clocksync, as you're not really doing
> > > anything to sync clocks in the code. One, "clock" is an way overloaded
> > > term in the kernel. Two, you're really seem to be just providing deltas
> > > and skew rates between notions of time. I want to avoid someone thinking
> > > "Oh, NTP must use this code". 
> > > 
> > > So maybe something like timecompare.c? 
> > 
> > Fine with me.
> 
> As there were no other comments I renamed the file, functions and struct
> accordingly. As I said in my mail, I prefer "struct timecompare" over
> "struct time_comparator". I also used "timecompare_transform()".
> 
> Is this revision of the patch okay? How should the two patches get
> included in the main kernel - via netdev-next-2.6?
> 
> Bye, Patrick

Small comment below, but otherwise it looks ok to me. I usually push
patches through Andrew, so I'd probably go that way. But I'd leave it to
Dave if he's comfortable pushing them to Linus.

Acked-by: John Stultz <johnstul@...ibm.com>




> -------------------------------------------------
> 
> Subject: [PATCH NET-NEXT 02/10] timecompare: generic infrastructure to map between two time bases
> 
> Mapping from a struct timecounter to a time returned by functions like
> ktime_get_real() is implemented. This is sufficient to use this code
> in a network device driver which wants to support hardware time
> stamping and transformation of hardware time stamps to system time.
> 
> The interface could have been made more versatile by not depending on
> a time counter, but this wasn't done to avoid writing glue code
> elsewhere.
> 
> The method implemented here is the one used and analyzed under the name
> "assisted PTP" in the LCI PTP paper:
> http://www.linuxclustersinstitute.org/conferences/archive/2008/PDF/Ohly_92221.pdf
> ---
>  include/linux/timecompare.h |  125 +++++++++++++++++++++++++++
>  kernel/time/Makefile        |    2 +-
>  kernel/time/timecompare.c   |  194 +++++++++++++++++++++++++++++++++++++++++++
>  3 files changed, 320 insertions(+), 1 deletions(-)
>  create mode 100644 include/linux/timecompare.h
>  create mode 100644 kernel/time/timecompare.c
> 
> diff --git a/include/linux/timecompare.h b/include/linux/timecompare.h
> new file mode 100644
> index 0000000..f88c454
> --- /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@...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 905b0b5..0b0a636 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 0000000..1e94abc
> --- /dev/null
> +++ b/kernel/time/timecompare.c
> @@ -0,0 +1,194 @@
> +/*
> + * 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/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) {

You may also want to checking the bounds on the duration_target. If
preemption hits and the values are too out of whack, the symetric delay
assumption below might be quite invalid.

I guess the outliers removal probably covers this as well, but seems
some sanity checking might be good.

> +			/*
> +			 * 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;
> +
> +	printk(KERN_DEBUG
> +		"average offset: %lld\n", offset);
> +
> +	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);
> -- 
> 1.6.1.2
> 

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