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, 15-17 November, 2002, http://phk.freebsd.dk/pubs/timecounter.pdf.

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