[LinuxPPS] [PATCHv2 1/6] ntp: add hardpps implementation
Rodolfo Giometti
giometti at enneenne.com
Mon Mar 1 09:14:59 CET 2010
On Wed, Feb 24, 2010 at 03:28:12PM +0300, Alexander Gordeev wrote:
> This commit adds hardpps() implementation based upon the original one
> from the NTPv4 reference kernel code from David Mills. However, it is
> highly optimized towards very fast syncronization and maximum stickness
> to PPS signal. The typical error is less then a microsecond.
> To make it sync faster I had to throw away exponential phase filter so
> that the full phase offset is corrected immediately. Then I also had to
> throw away median phase filter because it gives a bigger error itself
> if used without exponential filter.
> Maybe we will find an appropriate filtering scheme in the future but
> it's not necessary if the signal quality is ok.
>
> Signed-off-by: Alexander Gordeev <lasaine at lvk.cs.msu.su>
> ---
> drivers/pps/Kconfig | 1 +
> include/linux/timex.h | 1 +
> kernel/time/Kconfig | 7 +
> kernel/time/ntp.c | 420 +++++++++++++++++++++++++++++++++++++++++++++++--
> 4 files changed, 414 insertions(+), 15 deletions(-)
>
> diff --git a/drivers/pps/Kconfig b/drivers/pps/Kconfig
> index cc2eb8e..2bd4f65 100644
> --- a/drivers/pps/Kconfig
> +++ b/drivers/pps/Kconfig
> @@ -7,6 +7,7 @@ menu "PPS support"
> config PPS
> tristate "PPS support"
> depends on EXPERIMENTAL
> + select NTP_PPS
> ---help---
> PPS (Pulse Per Second) is a special pulse provided by some GPS
> antennae. Userland can use it to get a high-precision time
> diff --git a/include/linux/timex.h b/include/linux/timex.h
> index e6967d1..5a93cd3 100644
> --- a/include/linux/timex.h
> +++ b/include/linux/timex.h
> @@ -274,6 +274,7 @@ extern u64 tick_length;
> extern void second_overflow(void);
> extern void update_ntp_one_tick(void);
> extern int do_adjtimex(struct timex *);
> +extern void hardpps(const struct timespec *, const struct timespec *);
>
> /* Don't use! Compatibility define for existing users. */
> #define tickadj (500/HZ ? : 1)
> diff --git a/kernel/time/Kconfig b/kernel/time/Kconfig
> index 95ed429..2da4900 100644
> --- a/kernel/time/Kconfig
> +++ b/kernel/time/Kconfig
> @@ -27,3 +27,10 @@ config GENERIC_CLOCKEVENTS_BUILD
> default y
> depends on GENERIC_CLOCKEVENTS || GENERIC_CLOCKEVENTS_MIGR
>
> +config NTP_PPS
> + bool "PPS kernel consumer support"
> + depends on PPS
> + help
> + This option adds support for direct in-kernel time
> + syncronization using an external PPS signal.
> +
This patch is both PPS and NTP related but I suppose is better moving
this setting into drivers/pps directory since people whose want to use
thise supports can go into same place and find whetever they want...
> diff --git a/kernel/time/ntp.c b/kernel/time/ntp.c
> index 4800f93..6db097a 100644
> --- a/kernel/time/ntp.c
> +++ b/kernel/time/ntp.c
> @@ -14,6 +14,7 @@
> #include <linux/timex.h>
> #include <linux/time.h>
> #include <linux/mm.h>
> +#include <linux/module.h>
>
> /*
> * NTP timekeeping variables:
> @@ -74,6 +75,161 @@ long time_adjust;
> /* constant (boot-param configurable) NTP tick adjustment (upscaled) */
> static s64 ntp_tick_adj;
>
> +#ifdef CONFIG_NTP_PPS
> +
> +/*
> + * The following variables are used when a pulse-per-second (PPS) signal
> + * is available. They establish the engineering parameters of the clock
> + * discipline loop when controlled by the PPS signal.
> + */
> +#define PPS_VALID 120 /* PPS signal watchdog max (s) */
> +#define PPS_POPCORN 4 /* popcorn spike threshold (shift) */
> +#define PPS_INTMIN 2 /* min freq interval (s) (shift) */
> +#define PPS_INTMAX 8 /* max freq interval (s) (shift) */
> +#define PPS_INTCOUNT 4 /* number of consecutive good intervals to
> + increase pps_shift or consecutive bad
> + intervals to decrease it */
> +#define PPS_MAXWANDER 100000 /* max PPS freq wander (ns/s) */
> +
> +static int pps_valid; /* signal watchdog counter */
> +static long pps_tf[3]; /* phase median filter */
> +static long pps_jitter; /* current jitter (ns) */
> +static struct timespec pps_fbase; /* beginning of the last freq interval */
> +static int pps_shift; /* current interval duration (s) (shift) */
> +static int pps_intcnt; /* interval counter */
> +static s64 pps_freq; /* frequency offset (scaled ns/s) */
> +static long pps_stabil; /* current stability (scaled ns/s) */
> +
> +/*
> + * PPS signal quality monitors
> + */
> +static long pps_calcnt; /* calibration intervals */
> +static long pps_jitcnt; /* jitter limit exceeded */
> +static long pps_stbcnt; /* stability limit exceeded */
> +static long pps_errcnt; /* calibration errors */
> +
> +
> +/**
> + * pps_clear - Clears the PPS state variables
> + *
> + * Must be called while holding a write on the xtime_lock
> + */
> +static inline void pps_clear(void)
> +{
> + pps_shift = PPS_INTMIN;
> + pps_tf[0] = 0;
> + pps_tf[1] = 0;
> + pps_tf[2] = 0;
> + pps_fbase.tv_sec = pps_fbase.tv_nsec = 0;
> + pps_freq = 0;
> +}
> +
> +/* PPS kernel consumer compensates the whole phase error immediately.
> + * Otherwise, reduce the offset by a fixed factor times the time constant.
> + */
> +static inline s64 ntp_offset_chunk(s64 offset)
> +{
> + if (time_status & STA_PPSTIME && time_status & STA_PPSSIGNAL)
> + return offset;
> + else
> + return shift_right(offset, SHIFT_PLL + time_constant);
> +}
> +
> +/* Decrease pps_valid to indicate that another second has passed since
> + * the last PPS signal. When it reaches 0, indicate that PPS signal is
> + * missing.
> + *
> + * Must be called while holding a write on the xtime_lock
> + */
> +static inline void pps_dec_valid(void)
> +{
> + if (pps_valid > 0)
> + pps_valid--;
> + else
> + time_status &= ~(STA_PPSSIGNAL | STA_PPSJITTER |
> + STA_PPSWANDER | STA_PPSERROR);
> +}
> +
> +static inline void pps_reset_freq_interval(void)
> +{
> + /* the PPS calibration interval may end
> + surprisingly early */
> + pps_shift = PPS_INTMIN;
> + pps_intcnt = 0;
> +}
> +
> +static inline void pps_set_freq(s64 freq)
> +{
> + pps_freq = freq;
> +}
> +
> +static inline int is_error_status(int status)
> +{
> + return (time_status & (STA_UNSYNC|STA_CLOCKERR))
> + /* PPS signal lost when either PPS time or
> + * PPS frequency synchronization requested
> + */
> + || ((time_status & (STA_PPSFREQ|STA_PPSTIME))
> + && !(time_status & STA_PPSSIGNAL))
> + /* PPS jitter exceeded when
> + * PPS time synchronization requested */
> + || ((time_status & (STA_PPSTIME|STA_PPSJITTER))
> + == (STA_PPSTIME|STA_PPSJITTER))
> + /* PPS wander exceeded or calibration error when
> + * PPS frequency synchronization requested
> + */
> + || ((time_status & STA_PPSFREQ)
> + && (time_status & (STA_PPSWANDER|STA_PPSERROR)));
> +}
> +
> +static inline void pps_fill_timex(struct timex *txc)
> +{
> + txc->ppsfreq = shift_right((pps_freq >> PPM_SCALE_INV_SHIFT) *
> + PPM_SCALE_INV, NTP_SCALE_SHIFT);
> + txc->jitter = pps_jitter;
> + if (!(time_status & STA_NANO))
> + txc->jitter /= NSEC_PER_USEC;
> + txc->shift = pps_shift;
> + txc->stabil = pps_stabil;
> + txc->jitcnt = pps_jitcnt;
> + txc->calcnt = pps_calcnt;
> + txc->errcnt = pps_errcnt;
> + txc->stbcnt = pps_stbcnt;
> +}
> +
> +#else /* !CONFIG_NTP_PPS */
> +
> +static inline void pps_clear(void) {}
> +
> +static inline s64 ntp_offset_chunk(s64 offset)
> +{
> + return shift_right(offset, SHIFT_PLL + time_constant);
> +}
> +
> +static inline void pps_dec_valid(void) {}
> +static inline void pps_reset_freq_interval(void) {}
> +static inline void pps_set_freq(s64 freq) {}
> +
> +static inline int is_error_status(int status)
> +{
> + return status & (STA_UNSYNC|STA_CLOCKERR);
> +}
> +
> +static inline void pps_fill_timex(struct timex *txc)
> +{
> + /* PPS is not implemented, so these are zero */
> + txc->ppsfreq = 0;
> + txc->jitter = 0;
> + txc->shift = 0;
> + txc->stabil = 0;
> + txc->jitcnt = 0;
> + txc->calcnt = 0;
> + txc->errcnt = 0;
> + txc->stbcnt = 0;
> +}
> +
> +#endif /* CONFIG_NTP_PPS */
> +
> /*
> * NTP methods:
> */
> @@ -177,6 +333,9 @@ void ntp_clear(void)
>
> tick_length = tick_length_base;
> time_offset = 0;
> +
> + /* Clear PPS state variables */
> + pps_clear();
> }
>
> /*
> @@ -242,16 +401,16 @@ void second_overflow(void)
> time_status |= STA_UNSYNC;
> }
>
> - /*
> - * Compute the phase adjustment for the next second. The offset is
> - * reduced by a fixed factor times the time constant.
> - */
> + /* Compute the phase adjustment for the next second */
> tick_length = tick_length_base;
>
> - delta = shift_right(time_offset, SHIFT_PLL + time_constant);
> + delta = ntp_offset_chunk(time_offset);
> time_offset -= delta;
> tick_length += delta;
>
> + /* Check PPS signal */
> + pps_dec_valid();
> +
> if (!time_adjust)
> return;
>
> @@ -361,6 +520,8 @@ static inline void process_adj_status(struct timex *txc, struct timespec *ts)
> if ((time_status & STA_PLL) && !(txc->status & STA_PLL)) {
> time_state = TIME_OK;
> time_status = STA_UNSYNC;
> + /* restart PPS frequency calibration */
> + pps_reset_freq_interval();
> }
>
> /*
> @@ -410,6 +571,8 @@ static inline void process_adjtimex_modes(struct timex *txc, struct timespec *ts
> time_freq = txc->freq * PPM_SCALE;
> time_freq = min(time_freq, MAXFREQ_SCALED);
> time_freq = max(time_freq, -MAXFREQ_SCALED);
> + /* update pps_freq */
> + pps_set_freq(time_freq);
> }
>
> if (txc->modes & ADJ_MAXERROR)
> @@ -500,7 +663,8 @@ int do_adjtimex(struct timex *txc)
> }
>
> result = time_state; /* mostly `TIME_OK' */
> - if (time_status & (STA_UNSYNC|STA_CLOCKERR))
> + /* check for errors */
> + if (is_error_status(time_status))
> result = TIME_ERROR;
>
> txc->freq = shift_right((time_freq >> PPM_SCALE_INV_SHIFT) *
> @@ -514,15 +678,8 @@ int do_adjtimex(struct timex *txc)
> txc->tick = tick_usec;
> txc->tai = time_tai;
>
> - /* PPS is not implemented, so these are zero */
> - txc->ppsfreq = 0;
> - txc->jitter = 0;
> - txc->shift = 0;
> - txc->stabil = 0;
> - txc->jitcnt = 0;
> - txc->calcnt = 0;
> - txc->errcnt = 0;
> - txc->stbcnt = 0;
> + /* fill PPS status fields */
> + pps_fill_timex(txc);
>
> write_sequnlock_irq(&xtime_lock);
>
> @@ -536,6 +693,239 @@ int do_adjtimex(struct timex *txc)
> return result;
> }
>
> +#ifdef CONFIG_NTP_PPS
> +
> +struct pps_normtime {
> + __kernel_time_t sec; /* seconds */
> + long nsec; /* nanoseconds */
> +};
> +
> +/* normalize the timestamp so that nsec is in the
> + ( -NSEC_PER_SEC / 2, NSEC_PER_SEC / 2 ] interval */
> +static inline struct pps_normtime pps_normalize_ts(struct timespec ts)
> +{
> + struct pps_normtime norm = {
> + .sec = ts.tv_sec,
> + .nsec = ts.tv_nsec
> + };
> +
> + if (norm.nsec > (NSEC_PER_SEC >> 1)) {
> + norm.nsec -= NSEC_PER_SEC;
> + norm.sec++;
> + }
> +
> + return norm;
> +}
> +
> +/* get current phase correction and jitter */
> +static inline long pps_phase_filter_get(long *jitter)
> +{
> + *jitter = pps_tf[0] - pps_tf[1];
> + if (*jitter < 0)
> + *jitter = -*jitter;
> +
> + /* TODO: test various filters */
> + return pps_tf[0];
> +}
> +
> +/* add the sample to the phase filter */
> +static inline void pps_phase_filter_add(long err)
> +{
> + pps_tf[2] = pps_tf[1];
> + pps_tf[1] = pps_tf[0];
> + pps_tf[0] = err;
> +}
> +
> +/* decrease frequency calibration interval length.
> + * It is halved after four consecutive unstable intervals.
> + */
> +static inline void pps_dec_freq_interval(void)
> +{
> + if (--pps_intcnt <= -PPS_INTCOUNT) {
> + pps_intcnt = -PPS_INTCOUNT;
> + if (pps_shift > PPS_INTMIN) {
> + pps_shift--;
> + pps_intcnt = 0;
> + }
> + }
> +}
> +
> +/* increase frequency calibration interval length.
> + * It is doubled after four consecutive stable intervals.
> + */
> +static inline void pps_inc_freq_interval(void)
> +{
> + if (++pps_intcnt >= PPS_INTCOUNT) {
> + pps_intcnt = PPS_INTCOUNT;
> + if (pps_shift < PPS_INTMAX) {
> + pps_shift++;
> + pps_intcnt = 0;
> + }
> + }
> +}
> +
> +/* update clock frequency based on MONOTONIC_RAW clock PPS signal
> + * timestamps
> + *
> + * At the end of the calibration interval the difference between the
> + * first and last MONOTONIC_RAW clock timestamps divided by the length
> + * of the interval becomes the frequency update. If the interval was
> + * too long, the data are discarded.
> + * Returns the difference between old and new frequency values.
> + */
> +static long hardpps_update_freq(struct pps_normtime freq_norm)
> +{
> + long delta, delta_mod;
> + s64 ftemp;
> +
> + /* check if the frequency interval was too long */
> + if (freq_norm.sec > (2 << pps_shift)) {
> + time_status |= STA_PPSERROR;
> + pps_errcnt++;
> + pps_dec_freq_interval();
> + pr_err("hardpps: PPSERROR: interval too long - %ld s\n",
> + freq_norm.sec);
> + return 0;
> + }
> +
> + /* here the raw frequency offset and wander (stability) is
> + * calculated. If the wander is less than the wander threshold
> + * the interval is increased; otherwise it is decreased.
> + */
> + ftemp = div_s64(((s64)(-freq_norm.nsec)) << NTP_SCALE_SHIFT,
> + freq_norm.sec);
> + delta = shift_right(ftemp - pps_freq, NTP_SCALE_SHIFT);
> + pps_freq = ftemp;
> + if (delta > PPS_MAXWANDER || delta < -PPS_MAXWANDER) {
> + pr_warning("hardpps: PPSWANDER: change=%ld\n", delta);
> + time_status |= STA_PPSWANDER;
> + pps_stbcnt++;
> + pps_dec_freq_interval();
> + } else { /* good sample */
> + pps_inc_freq_interval();
> + }
> +
> + /* the stability metric is calculated as the average of recent
> + * frequency changes, but is used only for performance
> + * monitoring
> + */
> + delta_mod = delta;
> + if (delta_mod < 0)
> + delta_mod = -delta_mod;
> + pps_stabil += (div_s64(((s64)delta_mod) <<
> + (NTP_SCALE_SHIFT - SHIFT_USEC),
> + NSEC_PER_USEC) - pps_stabil) >> PPS_INTMIN;
> +
> + /* if enabled, the system clock frequency is updated */
> + if ((time_status & STA_PPSFREQ) != 0 &&
> + (time_status & STA_FREQHOLD) == 0) {
> + time_freq = pps_freq;
> + ntp_update_frequency();
> + }
> +
> + return delta;
> +}
> +
> +/* correct REALTIME clock phase error against PPS signal */
> +static void hardpps_update_phase(long error)
> +{
> + long correction = -error;
> + long jitter;
> +
> + /* add the sample to the median filter */
> + pps_phase_filter_add(correction);
> + correction = pps_phase_filter_get(&jitter);
> +
> + /* Nominal jitter is due to PPS signal noise. If it exceeds the
> + * threshold, the sample is discarded; otherwise, if so enabled,
> + * the time offset is updated.
> + */
> + if (jitter > (pps_jitter << PPS_POPCORN)) {
> + pr_warning("hardpps: PPSJITTER: jitter=%ld, limit=%ld\n",
> + jitter, (pps_jitter << PPS_POPCORN));
> + time_status |= STA_PPSJITTER;
> + pps_jitcnt++;
> + } else if (time_status & STA_PPSTIME) {
> + /* correct the time using the phase offset */
> + time_offset = div_s64(((s64)correction) << NTP_SCALE_SHIFT,
> + NTP_INTERVAL_FREQ);
> + /* cancel running adjtime() */
> + time_adjust = 0;
> + }
> + /* update jitter */
> + pps_jitter += (jitter - pps_jitter) >> PPS_INTMIN;
> +}
> +
> +/*
> + * hardpps() - discipline CPU clock oscillator to external PPS signal
> + *
> + * This routine is called at each PPS signal arrival in order to
> + * discipline the CPU clock oscillator to the PPS signal. It takes two
> + * parameters: REALTIME and MONOTONIC_RAW clock timestamps. The former
> + * is used to correct clock phase error and the latter is used to
> + * correct the frequency.
> + *
> + * This code is based on David Mills's reference nanokernel
> + * implementation. It was mostly rewritten but keeps the same idea.
> + */
> +void hardpps(const struct timespec *phase_ts, const struct timespec *raw_ts)
> +{
> + struct pps_normtime pts_norm, freq_norm;
> + unsigned long flags;
> +
> + pts_norm = pps_normalize_ts(*phase_ts);
> +
> + write_seqlock_irqsave(&xtime_lock, flags);
> +
> + /* clear the error bits, they will be set again if needed */
> + time_status &= ~(STA_PPSJITTER | STA_PPSWANDER | STA_PPSERROR);
> +
> + /* indicate signal presence */
> + time_status |= STA_PPSSIGNAL;
> + pps_valid = PPS_VALID;
> +
> + /* when called for the first time,
> + * just start the frequency interval */
> + if (unlikely(pps_fbase.tv_sec == 0)) {
> + pps_fbase = *raw_ts;
> + write_sequnlock_irq(&xtime_lock);
> + return;
> + }
> +
> + /* ok, now we have a base for frequency calculation */
> + freq_norm = pps_normalize_ts(timespec_sub(*raw_ts, pps_fbase));
> +
> + /* check that the signal is in the range
> + * [1s - MAXFREQ us, 1s + MAXFREQ us], otherwise reject it */
> + if ((freq_norm.sec == 0) ||
> + (freq_norm.nsec > MAXFREQ * freq_norm.sec) ||
> + (freq_norm.nsec < -MAXFREQ * freq_norm.sec)) {
> + time_status |= STA_PPSJITTER;
> + /* restart the frequency calibration interval */
> + pps_fbase = *raw_ts;
> + write_sequnlock_irqrestore(&xtime_lock, flags);
> + pr_err("hardpps: PPSJITTER: bad pulse\n");
> + return;
> + }
> +
> + /* signal is ok */
> +
> + /* check if the current frequency interval is finished */
> + if (freq_norm.sec >= (1 << pps_shift)) {
> + pps_calcnt++;
> + /* restart the frequency calibration interval */
> + pps_fbase = *raw_ts;
> + hardpps_update_freq(freq_norm);
> + }
> +
> + hardpps_update_phase(pts_norm.nsec);
> +
> + write_sequnlock_irqrestore(&xtime_lock, flags);
> +}
> +EXPORT_SYMBOL(hardpps);
> +
> +#endif /* CONFIG_NTP_PPS */
> +
> static int __init ntp_tick_adj_setup(char *str)
> {
> ntp_tick_adj = simple_strtol(str, NULL, 0);
> --
> 1.6.6.1
>
--
GNU/Linux Solutions e-mail: giometti at enneenne.com
Linux Device Driver giometti at linux.it
Embedded Systems phone: +39 349 2432127
UNIX programming skype: rodolfo.giometti
Freelance ICT Italia - Consulente ICT Italia - www.consulenti-ict.it
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