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TIMECOUNTER(9)             Kernel Developer's Manual            TIMECOUNTER(9)

NAME
     timecounter, tc_init - machine-independent binary timescale

SYNOPSIS
     #include <sys/timetc.h>

     void
     tc_init(struct timecounter *tc);

DESCRIPTION
     The timecounter interface is a machine-independent implementation of a
     binary timescale using whatever hardware support is at hand for tracking
     time.

     A timecounter is a binary counter which has two properties:

           ⊕   it runs at a fixed, known frequency; and

           ⊕   it has sufficient bits to not roll over in less than
               approximately max(2 msec, 2/HZ seconds) (the value 2 here is
               really 1 + delta, for some indeterminate value of delta).

     The interface between the hardware which implements a timecounter and the
     machine-independent code which uses this to keep track of time is a
     timecounter structure:

           struct timecounter {
                   timecounter_get_t       *tc_get_timecount;
                   timecounter_pps_t       *tc_poll_pps;
                   u_int                   tc_counter_mask;
                   uint64_t                tc_frequency;
                   const char              *tc_name;
                   int                     tc_quality;
                   void                    *tc_priv;
                   struct timecounter      *tc_next;
           }

     The fields of the timecounter structure are described below.

     u_int (*tc_get_timecount)(struct timecounter *)
             This function reads the counter.  It is not required to mask any
             unimplemented bits out, as long as they are constant.

     void (*tc_poll_pps)(struct timecounter *)
             This function is optional and can be set to NULL.  It will be
             called whenever the timecounter is rewound, and is intended to
             check for PPS events.  Normal hardware does not need it but
             timecounters which latch PPS in hardware do.

     tc_counter_mask
             This mask should mask off any unimplemented bits.

     tc_frequency
             Frequency of the counter in Hz.

     tc_name
             Name of the timecounter.  Can be any NUL-terminated string.

     tc_quality
             Used to determine if this timecounter is better than another
             timecounter - higher means better.  Negative means "only use at
             explicit request".

     tc_priv
             Pointer to the timecounter's private parts.

     tc_next
             For internal use.

     To register a new timecounter, the hardware device driver should fill a
     timecounter structure with appropriate values and call the tc_init()
     function, giving a pointer to the structure as a tc parameter.

TIMESTAMP FORMAT
     The timestamp format used in the machine independent timecounter
     implementation is a bintime structure:

           struct bintime {
                   time_t  sec;
                   uint64_t frac;
           }

     The sec field records the number of seconds as well as the tv_sec field
     in the traditional UNIX timeval and timespec structures, described in
     timeval(3).

     The frac field records fractional seconds represented in a fully 64 bit
     integer, i.e. it goes all the way from 0 through 0xFFFFFFFFFFFFFFFF per
     each second.  The effective resolution of the frac value depends on a
     frequency of the machine dependent timecounter source.

     The bintime format is a binary number, not a pseudo-decimal number, so it
     can be used as a simple binary counter without expensive 64 bit
     arithmetic.

CODE REFERENCES
     The timecounter framework is implemented in the file sys/kern/kern_tc.c.
     The bintime structure and related functions are defined in the file
     <sys/time.h>.

SEE ALSO
     clock_settime(2), ntp_adjtime(2), settimeofday(2), bintime(9),
     bintime_add(9), binuptime(9), hz(9), time_second(9)

     Poul-Henning Kamp, "Timecounters: Efficient and precise timekeeping in
     SMP kernels", Proceedings of EuroBSDCon 2002, Amsterdam,
     http://phk.freebsd.dk/pubs/timecounter.pdf, 15-17 November, 2002.

HISTORY
     The timecounter interface first appeared in FreeBSD, and was ported to
     NetBSD 4.0 by Frank Kardel and Simon Burge.

NetBSD 10.99                  September 18, 2021                  NetBSD 10.99