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NetBSD 6.1.5 - man page for timecounter (netbsd section 9)

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TIMECOUNTER(9)			  BSD 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:

	   o   it runs at a fixed, known frequency; and

	   o   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-indepen-
     dent 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;
		   u_int64_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 struc-
     ture with appropriate values and call the tc_init() function, giving a pointer to the struc-
     ture 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 resolu-
     tion 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 arithmetics.

CODE REFERENCES
     The timecounter framework is implemented in the file sys/kern/kern_tc.c.  The bintime struc-
     ture 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.

BSD					   June 8, 2010 				      BSD
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