FreeBSD/Linux Kernel Cross Reference
sys/kern/kern_ntptime.c

     /***********************************************************************
      *                                                                     *
      * Copyright (c) David L. Mills 1993-2001                              *
      *                                                                     *
      * Permission to use, copy, modify, and distribute this software and   *
      * its documentation for any purpose and without fee is hereby         *
      * granted, provided that the above copyright notice appears in all    *
      * copies and that both the copyright notice and this permission       *
      * notice appear in supporting documentation, and that the name        *
     * University of Delaware not be used in advertising or publicity      *
     * pertaining to distribution of the software without specific,        *
     * written prior permission. The University of Delaware makes no       *
     * representations about the suitability this software for any         *
     * purpose. It is provided "as is" without express or implied          *
     * warranty.                                                           *
     *                                                                     *
     **********************************************************************/
    
    /*
     * Adapted from the original sources for FreeBSD and timecounters by:
     * Poul-Henning Kamp <phk@FreeBSD.org>.
     *
     * The 32bit version of the "LP" macros seems a bit past its "sell by" 
     * date so I have retained only the 64bit version and included it directly
     * in this file.
     *
     * Only minor changes done to interface with the timecounters over in
     * sys/kern/kern_clock.c.   Some of the comments below may be (even more)
     * confusing and/or plain wrong in that context.
     *
     * $FreeBSD: src/sys/kern/kern_ntptime.c,v 1.32.2.2 2001/04/22 11:19:46 jhay Exp $
     */
    
    #include "opt_ntp.h"
    
    #include <sys/param.h>
    #include <sys/systm.h>
    #include <sys/sysproto.h>
    #include <sys/kernel.h>
    #include <sys/proc.h>
    #include <sys/priv.h>
    #include <sys/time.h>
    #include <sys/timex.h>
    #include <sys/timepps.h>
    #include <sys/sysctl.h>
    
    #include <sys/thread2.h>
    #include <sys/mplock2.h>
    
    /*
     * Single-precision macros for 64-bit machines
     */
    typedef long long l_fp;
    #define L_ADD(v, u)     ((v) += (u))
    #define L_SUB(v, u)     ((v) -= (u))
    #define L_ADDHI(v, a)   ((v) += (long long)(a) << 32)
    #define L_NEG(v)        ((v) = -(v))
    #define L_RSHIFT(v, n) \
            do { \
                    if ((v) < 0) \
                            (v) = -(-(v) >> (n)); \
                    else \
                            (v) = (v) >> (n); \
            } while (0)
    #define L_MPY(v, a)     ((v) *= (a))
    #define L_CLR(v)        ((v) = 0)
    #define L_ISNEG(v)      ((v) < 0)
    #define L_LINT(v, a)    ((v) = (long long)(a) << 32)
    #define L_GINT(v)       ((v) < 0 ? -(-(v) >> 32) : (v) >> 32)
    
    /*
     * Generic NTP kernel interface
     *
     * These routines constitute the Network Time Protocol (NTP) interfaces
     * for user and daemon application programs. The ntp_gettime() routine
     * provides the time, maximum error (synch distance) and estimated error
     * (dispersion) to client user application programs. The ntp_adjtime()
     * routine is used by the NTP daemon to adjust the system clock to an
     * externally derived time. The time offset and related variables set by
     * this routine are used by other routines in this module to adjust the
     * phase and frequency of the clock discipline loop which controls the
     * system clock.
     *
     * When the kernel time is reckoned directly in nanoseconds (NTP_NANO
     * defined), the time at each tick interrupt is derived directly from
     * the kernel time variable. When the kernel time is reckoned in
     * microseconds, (NTP_NANO undefined), the time is derived from the
     * kernel time variable together with a variable representing the
     * leftover nanoseconds at the last tick interrupt. In either case, the
     * current nanosecond time is reckoned from these values plus an
     * interpolated value derived by the clock routines in another
     * architecture-specific module. The interpolation can use either a
     * dedicated counter or a processor cycle counter (PCC) implemented in
     * some architectures.
     *
     * Note that all routines must run at priority splclock or higher.
     */
    /*
     * Phase/frequency-lock loop (PLL/FLL) definitions
    *
    * The nanosecond clock discipline uses two variable types, time
    * variables and frequency variables. Both types are represented as 64-
    * bit fixed-point quantities with the decimal point between two 32-bit
    * halves. On a 32-bit machine, each half is represented as a single
    * word and mathematical operations are done using multiple-precision
    * arithmetic. On a 64-bit machine, ordinary computer arithmetic is
    * used.
    *
    * A time variable is a signed 64-bit fixed-point number in ns and
    * fraction. It represents the remaining time offset to be amortized
    * over succeeding tick interrupts. The maximum time offset is about
    * 0.5 s and the resolution is about 2.3e-10 ns.
    *
    *                      1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
    *  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    * |s s s|                       ns                                |
    * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    * |                        fraction                               |
    * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    *
    * A frequency variable is a signed 64-bit fixed-point number in ns/s
    * and fraction. It represents the ns and fraction to be added to the
    * kernel time variable at each second. The maximum frequency offset is
    * about +-500000 ns/s and the resolution is about 2.3e-10 ns/s.
    *
    *                      1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
    *  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    * |s s s s s s s s s s s s s|            ns/s                     |
    * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    * |                        fraction                               |
    * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    */
   /*
    * The following variables establish the state of the PLL/FLL and the
    * residual time and frequency offset of the local clock.
    */
   #define SHIFT_PLL       4               /* PLL loop gain (shift) */
   #define SHIFT_FLL       2               /* FLL loop gain (shift) */
   
   static int time_state = TIME_OK;        /* clock state */
   static int time_status = STA_UNSYNC;    /* clock status bits */
   static long time_tai;                   /* TAI offset (s) */
   static long time_monitor;               /* last time offset scaled (ns) */
   static long time_constant;              /* poll interval (shift) (s) */
   static long time_precision = 1;         /* clock precision (ns) */
   static long time_maxerror = MAXPHASE / 1000; /* maximum error (us) */
   static long time_esterror = MAXPHASE / 1000; /* estimated error (us) */
   static time_t time_reftime;             /* time at last adjustment (s) */
   static long time_tick;                  /* nanoseconds per tick (ns) */
   static l_fp time_offset;                /* time offset (ns) */
   static l_fp time_freq;                  /* frequency offset (ns/s) */
   static l_fp time_adj;                   /* tick adjust (ns/s) */
   
   #ifdef PPS_SYNC
   /*
    * The following variables are used when a pulse-per-second (PPS) signal
    * is available and connected via a modem control lead. They establish
    * the engineering parameters of the clock discipline loop when
    * controlled by the PPS signal.
    */
   #define PPS_FAVG        2               /* min freq avg interval (s) (shift) */
   #define PPS_FAVGDEF     8               /* default freq avg int (s) (shift) */
   #define PPS_FAVGMAX     15              /* max freq avg interval (s) (shift) */
   #define PPS_PAVG        4               /* phase avg interval (s) (shift) */
   #define PPS_VALID       120             /* PPS signal watchdog max (s) */
   #define PPS_MAXWANDER   100000          /* max PPS wander (ns/s) */
   #define PPS_POPCORN     2               /* popcorn spike threshold (shift) */
   
   static struct timespec pps_tf[3];       /* phase median filter */
   static l_fp pps_freq;                   /* scaled frequency offset (ns/s) */
   static long pps_fcount;                 /* frequency accumulator */
   static long pps_jitter;                 /* nominal jitter (ns) */
   static long pps_stabil;                 /* nominal stability (scaled ns/s) */
   static long pps_lastsec;                /* time at last calibration (s) */
   static int pps_valid;                   /* signal watchdog counter */
   static int pps_shift = PPS_FAVG;        /* interval duration (s) (shift) */
   static int pps_shiftmax = PPS_FAVGDEF;  /* max interval duration (s) (shift) */
   static int pps_intcnt;                  /* wander counter */
   
   /*
    * 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 */
   #endif /* PPS_SYNC */
   /*
    * End of phase/frequency-lock loop (PLL/FLL) definitions
    */
   
   static void ntp_init(void);
   static void hardupdate(long offset);
   
   /*
    * ntp_gettime() - NTP user application interface
    *
    * See the timex.h header file for synopsis and API description. Note
    * that the TAI offset is returned in the ntvtimeval.tai structure
    * member.
    */
   static int
   ntp_sysctl(SYSCTL_HANDLER_ARGS)
   {
           struct ntptimeval ntv;  /* temporary structure */
           struct timespec atv;    /* nanosecond time */
   
           nanotime(&atv);
           ntv.time.tv_sec = atv.tv_sec;
           ntv.time.tv_nsec = atv.tv_nsec;
           ntv.maxerror = time_maxerror;
           ntv.esterror = time_esterror;
           ntv.tai = time_tai;
           ntv.time_state = time_state;
   
           /*
            * Status word error decode. If any of these conditions occur,
            * an error is returned, instead of the status word. Most
            * applications will care only about the fact the system clock
            * may not be trusted, not about the details.
            *
            * Hardware or software error
            */
           if ((time_status & (STA_UNSYNC | STA_CLOCKERR)) ||
   
           /*
            * PPS signal lost when either time or frequency synchronization
            * requested
            */
               (time_status & (STA_PPSFREQ | STA_PPSTIME) &&
               !(time_status & STA_PPSSIGNAL)) ||
   
           /*
            * PPS jitter exceeded when time synchronization requested
            */
               (time_status & STA_PPSTIME &&
               time_status & STA_PPSJITTER) ||
   
           /*
            * PPS wander exceeded or calibration error when frequency
            * synchronization requested
            */
               (time_status & STA_PPSFREQ &&
               time_status & (STA_PPSWANDER | STA_PPSERROR)))
                   ntv.time_state = TIME_ERROR;
           return (sysctl_handle_opaque(oidp, &ntv, sizeof ntv, req));
   }
   
   SYSCTL_NODE(_kern, OID_AUTO, ntp_pll, CTLFLAG_RW, 0, "");
   SYSCTL_PROC(_kern_ntp_pll, OID_AUTO, gettime, CTLTYPE_OPAQUE|CTLFLAG_RD,
           0, sizeof(struct ntptimeval) , ntp_sysctl, "S,ntptimeval", "");
   
   #ifdef PPS_SYNC
   SYSCTL_INT(_kern_ntp_pll, OID_AUTO, pps_shiftmax, CTLFLAG_RW, &pps_shiftmax, 0, "");
   SYSCTL_INT(_kern_ntp_pll, OID_AUTO, pps_shift, CTLFLAG_RW, &pps_shift, 0, "");
   SYSCTL_INT(_kern_ntp_pll, OID_AUTO, time_monitor, CTLFLAG_RD, &time_monitor, 0, "");
   
   SYSCTL_OPAQUE(_kern_ntp_pll, OID_AUTO, pps_freq, CTLFLAG_RD, &pps_freq, sizeof(pps_freq), "I", "");
   SYSCTL_OPAQUE(_kern_ntp_pll, OID_AUTO, time_freq, CTLFLAG_RD, &time_freq, sizeof(time_freq), "I", "");
   #endif
   /*
    * ntp_adjtime() - NTP daemon application interface
    *
    * See the timex.h header file for synopsis and API description. Note
    * that the timex.constant structure member has a dual purpose to set
    * the time constant and to set the TAI offset.
    *
    * MPALMOSTSAFE
    */
   int
   sys_ntp_adjtime(struct ntp_adjtime_args *uap)
   {
           struct thread *td = curthread;
           struct timex ntv;       /* temporary structure */
           long freq;              /* frequency ns/s) */
           int modes;              /* mode bits from structure */
           int error;
   
           error = copyin((caddr_t)uap->tp, (caddr_t)&ntv, sizeof(ntv));
           if (error)
                   return(error);
   
           /*
            * Update selected clock variables - only the superuser can
            * change anything. Note that there is no error checking here on
            * the assumption the superuser should know what it is doing.
            * Note that either the time constant or TAI offset are loaded
            * from the ntv.constant member, depending on the mode bits. If
            * the STA_PLL bit in the status word is cleared, the state and
            * status words are reset to the initial values at boot.
            */
           modes = ntv.modes;
           if (modes)
                   error = priv_check(td, PRIV_NTP_ADJTIME);
           if (error)
                   return (error);
   
           get_mplock();
           crit_enter();
           if (modes & MOD_MAXERROR)
                   time_maxerror = ntv.maxerror;
           if (modes & MOD_ESTERROR)
                   time_esterror = ntv.esterror;
           if (modes & MOD_STATUS) {
                   if (time_status & STA_PLL && !(ntv.status & STA_PLL)) {
                           time_state = TIME_OK;
                           time_status = STA_UNSYNC;
   #ifdef PPS_SYNC
                           pps_shift = PPS_FAVG;
   #endif /* PPS_SYNC */
                   }
                   time_status &= STA_RONLY;
                   time_status |= ntv.status & ~STA_RONLY;
           }
           if (modes & MOD_TIMECONST) {
                   if (ntv.constant < 0)
                           time_constant = 0;
                   else if (ntv.constant > MAXTC)
                           time_constant = MAXTC;
                   else
                           time_constant = ntv.constant;
           }
           if (modes & MOD_TAI) {
                   if (ntv.constant > 0) /* XXX zero & negative numbers ? */
                           time_tai = ntv.constant;
           }
   #ifdef PPS_SYNC
           if (modes & MOD_PPSMAX) {
                   if (ntv.shift < PPS_FAVG)
                           pps_shiftmax = PPS_FAVG;
                   else if (ntv.shift > PPS_FAVGMAX)
                           pps_shiftmax = PPS_FAVGMAX;
                   else
                           pps_shiftmax = ntv.shift;
           }
   #endif /* PPS_SYNC */
           if (modes & MOD_NANO)
                   time_status |= STA_NANO;
           if (modes & MOD_MICRO)
                   time_status &= ~STA_NANO;
           if (modes & MOD_CLKB)
                   time_status |= STA_CLK;
           if (modes & MOD_CLKA)
                   time_status &= ~STA_CLK;
           if (modes & MOD_OFFSET) {
                   if (time_status & STA_NANO)
                           hardupdate(ntv.offset);
                   else
                           hardupdate(ntv.offset * 1000);
           }
           /*
            * Note: the userland specified frequency is in seconds per second
            * times 65536e+6.  Multiply by a thousand and divide by 65336 to
            * get nanoseconds.
            */
           if (modes & MOD_FREQUENCY) {
                   freq = (ntv.freq * 1000LL) >> 16;
                   if (freq > MAXFREQ)
                           L_LINT(time_freq, MAXFREQ);
                   else if (freq < -MAXFREQ)
                           L_LINT(time_freq, -MAXFREQ);
                   else
                           L_LINT(time_freq, freq);
   #ifdef PPS_SYNC
                   pps_freq = time_freq;
   #endif /* PPS_SYNC */
           }
   
           /*
            * Retrieve all clock variables. Note that the TAI offset is
            * returned only by ntp_gettime();
            */
           if (time_status & STA_NANO)
                   ntv.offset = time_monitor;
           else
                   ntv.offset = time_monitor / 1000; /* XXX rounding ? */
           ntv.freq = L_GINT((time_freq / 1000LL) << 16);
           ntv.maxerror = time_maxerror;
           ntv.esterror = time_esterror;
           ntv.status = time_status;
           ntv.constant = time_constant;
           if (time_status & STA_NANO)
                   ntv.precision = time_precision;
           else
                   ntv.precision = time_precision / 1000;
           ntv.tolerance = MAXFREQ * SCALE_PPM;
   #ifdef PPS_SYNC
           ntv.shift = pps_shift;
           ntv.ppsfreq = L_GINT((pps_freq / 1000LL) << 16);
           if (time_status & STA_NANO)
                   ntv.jitter = pps_jitter;
           else
                   ntv.jitter = pps_jitter / 1000;
           ntv.stabil = pps_stabil;
           ntv.calcnt = pps_calcnt;
           ntv.errcnt = pps_errcnt;
           ntv.jitcnt = pps_jitcnt;
           ntv.stbcnt = pps_stbcnt;
   #endif /* PPS_SYNC */
           crit_exit();
           rel_mplock();
   
           error = copyout((caddr_t)&ntv, (caddr_t)uap->tp, sizeof(ntv));
           if (error)
                   return (error);
   
           /*
            * Status word error decode. See comments in
            * ntp_gettime() routine.
            */
           if ((time_status & (STA_UNSYNC | STA_CLOCKERR)) ||
               (time_status & (STA_PPSFREQ | STA_PPSTIME) &&
               !(time_status & STA_PPSSIGNAL)) ||
               (time_status & STA_PPSTIME &&
               time_status & STA_PPSJITTER) ||
               (time_status & STA_PPSFREQ &&
               time_status & (STA_PPSWANDER | STA_PPSERROR))) {
                   uap->sysmsg_result = TIME_ERROR;
           } else {
                   uap->sysmsg_result = time_state;
           }
           return (error);
   }
   
   /*
    * second_overflow() - called after ntp_tick_adjust()
    *
    * This routine is ordinarily called from hardclock() whenever the seconds
    * hand rolls over.  It returns leap seconds to add or drop, and sets nsec_adj
    * to the total adjustment to make over the next second in (ns << 32).
    *
    * This routine is only called by cpu #0.
    */
   int
   ntp_update_second(time_t newsec, int64_t *nsec_adj)
   {
           l_fp ftemp;             /* 32/64-bit temporary */
           int  adjsec = 0;
   
           /*
            * On rollover of the second both the nanosecond and microsecond
            * clocks are updated and the state machine cranked as
            * necessary. The phase adjustment to be used for the next
            * second is calculated and the maximum error is increased by
            * the tolerance.
            */
           time_maxerror += MAXFREQ / 1000;
   
           /*
            * Leap second processing. If in leap-insert state at
            * the end of the day, the system clock is set back one
            * second; if in leap-delete state, the system clock is
            * set ahead one second. The nano_time() routine or
            * external clock driver will insure that reported time
            * is always monotonic.
            */
           switch (time_state) {
   
                   /*
                    * No warning.
                    */
                   case TIME_OK:
                   if (time_status & STA_INS)
                           time_state = TIME_INS;
                   else if (time_status & STA_DEL)
                           time_state = TIME_DEL;
                   break;
   
                   /*
                    * Insert second 23:59:60 following second
                    * 23:59:59.
                    */
                   case TIME_INS:
                   if (!(time_status & STA_INS))
                           time_state = TIME_OK;
                   else if ((newsec) % 86400 == 0) {
                           --adjsec;
                           time_state = TIME_OOP;
                   }
                   break;
   
                   /*
                    * Delete second 23:59:59.
                    */
                   case TIME_DEL:
                   if (!(time_status & STA_DEL))
                           time_state = TIME_OK;
                   else if (((newsec) + 1) % 86400 == 0) {
                           ++adjsec;
                           time_tai--;
                           time_state = TIME_WAIT;
                   }
                   break;
   
                   /*
                    * Insert second in progress.
                    */
                   case TIME_OOP:
                           time_tai++;
                           time_state = TIME_WAIT;
                   break;
   
                   /*
                    * Wait for status bits to clear.
                    */
                   case TIME_WAIT:
                   if (!(time_status & (STA_INS | STA_DEL)))
                           time_state = TIME_OK;
           }
   
           /*
            * time_offset represents the total time adjustment we wish to
            * make (over no particular period of time).  time_freq represents
            * the frequency compensation we wish to apply.
            *
            * time_adj represents the total adjustment we wish to make over
            * one full second.  hardclock usually applies this adjustment in
            * time_adj / hz jumps, hz times a second.
            */
           ftemp = time_offset;
   #ifdef PPS_SYNC
           /* XXX even if PPS signal dies we should finish adjustment ? */
           if ((time_status & STA_PPSTIME) && (time_status & STA_PPSSIGNAL))
                   L_RSHIFT(ftemp, pps_shift);
           else
                   L_RSHIFT(ftemp, SHIFT_PLL + time_constant);
   #else
                   L_RSHIFT(ftemp, SHIFT_PLL + time_constant);
   #endif /* PPS_SYNC */
           time_adj = ftemp;               /* adjustment for part of the offset */
           L_SUB(time_offset, ftemp);
           L_ADD(time_adj, time_freq);     /* add frequency correction */
           *nsec_adj = time_adj;
   #ifdef PPS_SYNC
           if (pps_valid > 0)
                   pps_valid--;
           else
                   time_status &= ~STA_PPSSIGNAL;
   #endif /* PPS_SYNC */
           return(adjsec);
   }
   
   /*
    * ntp_init() - initialize variables and structures
    *
    * This routine must be called after the kernel variables hz and tick
    * are set or changed and before the next tick interrupt. In this
    * particular implementation, these values are assumed set elsewhere in
    * the kernel. The design allows the clock frequency and tick interval
    * to be changed while the system is running. So, this routine should
    * probably be integrated with the code that does that.
    */
   static void
   ntp_init(void)
   {
   
           /*
            * The following variable must be initialized any time the
            * kernel variable hz is changed.
            */
           time_tick = NANOSECOND / hz;
   
           /*
            * The following variables are initialized only at startup. Only
            * those structures not cleared by the compiler need to be
            * initialized, and these only in the simulator. In the actual
            * kernel, any nonzero values here will quickly evaporate.
            */
           L_CLR(time_offset);
           L_CLR(time_freq);
   #ifdef PPS_SYNC
           pps_tf[0].tv_sec = pps_tf[0].tv_nsec = 0;
           pps_tf[1].tv_sec = pps_tf[1].tv_nsec = 0;
           pps_tf[2].tv_sec = pps_tf[2].tv_nsec = 0;
           pps_fcount = 0;
           L_CLR(pps_freq);
   #endif /* PPS_SYNC */      
   }
   
   SYSINIT(ntpclocks, SI_BOOT2_CLOCKS, SI_ORDER_FIRST, ntp_init, NULL)
   
   /*
    * hardupdate() - local clock update
    *
    * This routine is called by ntp_adjtime() to update the local clock
    * phase and frequency. The implementation is of an adaptive-parameter,
    * hybrid phase/frequency-lock loop (PLL/FLL). The routine computes new
    * time and frequency offset estimates for each call. If the kernel PPS
    * discipline code is configured (PPS_SYNC), the PPS signal itself
    * determines the new time offset, instead of the calling argument.
    * Presumably, calls to ntp_adjtime() occur only when the caller
    * believes the local clock is valid within some bound (+-128 ms with
    * NTP). If the caller's time is far different than the PPS time, an
    * argument will ensue, and it's not clear who will lose.
    *
    * For uncompensated quartz crystal oscillators and nominal update
    * intervals less than 256 s, operation should be in phase-lock mode,
    * where the loop is disciplined to phase. For update intervals greater
    * than 1024 s, operation should be in frequency-lock mode, where the
    * loop is disciplined to frequency. Between 256 s and 1024 s, the mode
    * is selected by the STA_MODE status bit.
    */
   static void
   hardupdate(long offset)
   {
           long mtemp;
           l_fp ftemp;
   
           /*
            * Select how the phase is to be controlled and from which
            * source. If the PPS signal is present and enabled to
            * discipline the time, the PPS offset is used; otherwise, the
            * argument offset is used.
            */
           if (!(time_status & STA_PLL))
                   return;
           if (!((time_status & STA_PPSTIME) && (time_status & STA_PPSSIGNAL))) {
                   if (offset > MAXPHASE)
                           time_monitor = MAXPHASE;
                   else if (offset < -MAXPHASE)
                           time_monitor = -MAXPHASE;
                   else
                           time_monitor = offset;
                   L_LINT(time_offset, time_monitor);
           }
   
           /*
            * Select how the frequency is to be controlled and in which
            * mode (PLL or FLL). If the PPS signal is present and enabled
            * to discipline the frequency, the PPS frequency is used;
            * otherwise, the argument offset is used to compute it.
            */
           if ((time_status & STA_PPSFREQ) && time_status & STA_PPSSIGNAL) {
                   time_reftime = time_uptime;
                   return;
           }
           if ((time_status & STA_FREQHOLD) || time_reftime == 0)
                   time_reftime = time_uptime;
           mtemp = time_uptime - time_reftime;
           L_LINT(ftemp, time_monitor);
           L_RSHIFT(ftemp, (SHIFT_PLL + 2 + time_constant) << 1);
           L_MPY(ftemp, mtemp);
           L_ADD(time_freq, ftemp);
           time_status &= ~STA_MODE;
           if (mtemp >= MINSEC && (time_status & STA_FLL || mtemp > MAXSEC)) {
                   L_LINT(ftemp, (time_monitor << 4) / mtemp);
                   L_RSHIFT(ftemp, SHIFT_FLL + 4);
                   L_ADD(time_freq, ftemp);
                   time_status |= STA_MODE;
           }
           time_reftime = time_uptime;
           if (L_GINT(time_freq) > MAXFREQ)
                   L_LINT(time_freq, MAXFREQ);
           else if (L_GINT(time_freq) < -MAXFREQ)
                   L_LINT(time_freq, -MAXFREQ);
   }
   
   #ifdef PPS_SYNC
   /*
    * hardpps() - discipline CPU clock oscillator to external PPS signal
    *
    * This routine is called at each PPS interrupt in order to discipline
    * the CPU clock oscillator to the PPS signal. There are two independent
    * first-order feedback loops, one for the phase, the other for the
    * frequency. The phase loop measures and grooms the PPS phase offset
    * and leaves it in a handy spot for the seconds overflow routine. The
    * frequency loop averages successive PPS phase differences and
    * calculates the PPS frequency offset, which is also processed by the
    * seconds overflow routine. The code requires the caller to capture the
    * time and architecture-dependent hardware counter values in
    * nanoseconds at the on-time PPS signal transition.
    *
    * Note that, on some Unix systems this routine runs at an interrupt
    * priority level higher than the timer interrupt routine hardclock().
    * Therefore, the variables used are distinct from the hardclock()
    * variables, except for the actual time and frequency variables, which
    * are determined by this routine and updated atomically.
    */
   void
   hardpps(struct timespec *tsp, long nsec)
   {
           long u_sec, u_nsec, v_nsec; /* temps */
           l_fp ftemp;
   
           /*
            * The signal is first processed by a range gate and frequency
            * discriminator. The range gate rejects noise spikes outside
            * the range +-500 us. The frequency discriminator rejects input
            * signals with apparent frequency outside the range 1 +-500
            * PPM. If two hits occur in the same second, we ignore the
            * later hit; if not and a hit occurs outside the range gate,
            * keep the later hit for later comparison, but do not process
            * it.
            */
           time_status |= STA_PPSSIGNAL | STA_PPSJITTER;
           time_status &= ~(STA_PPSWANDER | STA_PPSERROR);
           pps_valid = PPS_VALID;
           u_sec = tsp->tv_sec;
           u_nsec = tsp->tv_nsec;
           if (u_nsec >= (NANOSECOND >> 1)) {
                   u_nsec -= NANOSECOND;
                   u_sec++;
           }
           v_nsec = u_nsec - pps_tf[0].tv_nsec;
           if (u_sec == pps_tf[0].tv_sec && v_nsec < NANOSECOND -
               MAXFREQ)
                   return;
           pps_tf[2] = pps_tf[1];
           pps_tf[1] = pps_tf[0];
           pps_tf[0].tv_sec = u_sec;
           pps_tf[0].tv_nsec = u_nsec;
   
           /*
            * Compute the difference between the current and previous
            * counter values. If the difference exceeds 0.5 s, assume it
            * has wrapped around, so correct 1.0 s. If the result exceeds
            * the tick interval, the sample point has crossed a tick
            * boundary during the last second, so correct the tick. Very
            * intricate.
            */
           u_nsec = nsec;
           if (u_nsec > (NANOSECOND >> 1))
                   u_nsec -= NANOSECOND;
           else if (u_nsec < -(NANOSECOND >> 1))
                   u_nsec += NANOSECOND;
           pps_fcount += u_nsec;
           if (v_nsec > MAXFREQ || v_nsec < -MAXFREQ)
                   return;
           time_status &= ~STA_PPSJITTER;
   
           /*
            * A three-stage median filter is used to help denoise the PPS
            * time. The median sample becomes the time offset estimate; the
            * difference between the other two samples becomes the time
            * dispersion (jitter) estimate.
            */
           if (pps_tf[0].tv_nsec > pps_tf[1].tv_nsec) {
                   if (pps_tf[1].tv_nsec > pps_tf[2].tv_nsec) {
                           v_nsec = pps_tf[1].tv_nsec;     /* 0 1 2 */
                           u_nsec = pps_tf[0].tv_nsec - pps_tf[2].tv_nsec;
                   } else if (pps_tf[2].tv_nsec > pps_tf[0].tv_nsec) {
                           v_nsec = pps_tf[0].tv_nsec;     /* 2 0 1 */
                           u_nsec = pps_tf[2].tv_nsec - pps_tf[1].tv_nsec;
                   } else {
                           v_nsec = pps_tf[2].tv_nsec;     /* 0 2 1 */
                           u_nsec = pps_tf[0].tv_nsec - pps_tf[1].tv_nsec;
                   }
           } else {
                   if (pps_tf[1].tv_nsec < pps_tf[2].tv_nsec) {
                           v_nsec = pps_tf[1].tv_nsec;     /* 2 1 0 */
                           u_nsec = pps_tf[2].tv_nsec - pps_tf[0].tv_nsec;
                   } else if (pps_tf[2].tv_nsec < pps_tf[0].tv_nsec) {
                           v_nsec = pps_tf[0].tv_nsec;     /* 1 0 2 */
                           u_nsec = pps_tf[1].tv_nsec - pps_tf[2].tv_nsec;
                   } else {
                           v_nsec = pps_tf[2].tv_nsec;     /* 1 2 0 */
                           u_nsec = pps_tf[1].tv_nsec - pps_tf[0].tv_nsec;
                   }
           }
   
           /*
            * Nominal jitter is due to PPS signal noise and interrupt
            * latency. If it exceeds the popcorn threshold, the sample is
            * discarded. otherwise, if so enabled, the time offset is
            * updated. We can tolerate a modest loss of data here without
            * much degrading time accuracy.
            */
           if (u_nsec > (pps_jitter << PPS_POPCORN)) {
                   time_status |= STA_PPSJITTER;
                   pps_jitcnt++;
           } else if (time_status & STA_PPSTIME) {
                   time_monitor = -v_nsec;
                   L_LINT(time_offset, time_monitor);
           }
           pps_jitter += (u_nsec - pps_jitter) >> PPS_FAVG;
           u_sec = pps_tf[0].tv_sec - pps_lastsec;
           if (u_sec < (1 << pps_shift))
                   return;
   
           /*
            * At the end of the calibration interval the difference between
            * the first and last counter values becomes the scaled
            * frequency. It will later be divided by the length of the
            * interval to determine the frequency update. If the frequency
            * exceeds a sanity threshold, or if the actual calibration
            * interval is not equal to the expected length, the data are
            * discarded. We can tolerate a modest loss of data here without
            * much degrading frequency accuracy.
            */
           pps_calcnt++;
           v_nsec = -pps_fcount;
           pps_lastsec = pps_tf[0].tv_sec;
           pps_fcount = 0;
           u_nsec = MAXFREQ << pps_shift;
           if (v_nsec > u_nsec || v_nsec < -u_nsec || u_sec != (1 <<
               pps_shift)) {
                   time_status |= STA_PPSERROR;
                   pps_errcnt++;
                   return;
           }
   
           /*
            * Here the raw frequency offset and wander (stability) is
            * calculated. If the wander is less than the wander threshold
            * for four consecutive averaging intervals, the interval is
            * doubled; if it is greater than the threshold for four
            * consecutive intervals, the interval is halved. The scaled
            * frequency offset is converted to frequency offset. The
            * stability metric is calculated as the average of recent
            * frequency changes, but is used only for performance
            * monitoring.
            */
           L_LINT(ftemp, v_nsec);
           L_RSHIFT(ftemp, pps_shift);
           L_SUB(ftemp, pps_freq);
           u_nsec = L_GINT(ftemp);
           if (u_nsec > PPS_MAXWANDER) {
                   L_LINT(ftemp, PPS_MAXWANDER);
                   pps_intcnt--;
                   time_status |= STA_PPSWANDER;
                   pps_stbcnt++;
           } else if (u_nsec < -PPS_MAXWANDER) {
                   L_LINT(ftemp, -PPS_MAXWANDER);
                   pps_intcnt--;
                   time_status |= STA_PPSWANDER;
                   pps_stbcnt++;
           } else {
                   pps_intcnt++;
           }
           if (pps_intcnt >= 4) {
                   pps_intcnt = 4;
                   if (pps_shift < pps_shiftmax) {
                           pps_shift++;
                           pps_intcnt = 0;
                   }
           } else if (pps_intcnt <= -4 || pps_shift > pps_shiftmax) {
                   pps_intcnt = -4;
                   if (pps_shift > PPS_FAVG) {
                           pps_shift--;
                           pps_intcnt = 0;
                   }
           }
           if (u_nsec < 0)
                   u_nsec = -u_nsec;
           pps_stabil += (u_nsec * SCALE_PPM - pps_stabil) >> PPS_FAVG;
   
           /*
            * The PPS frequency is recalculated and clamped to the maximum
            * MAXFREQ. If enabled, the system clock frequency is updated as
            * well.
            */
           L_ADD(pps_freq, ftemp);
           u_nsec = L_GINT(pps_freq);
           if (u_nsec > MAXFREQ)
                   L_LINT(pps_freq, MAXFREQ);
           else if (u_nsec < -MAXFREQ)
                   L_LINT(pps_freq, -MAXFREQ);
           if (time_status & STA_PPSFREQ)
                   time_freq = pps_freq;
   }
   #endif /* PPS_SYNC */

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