TC_INIT(9)

NAME
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

•   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;
    	u_int64_t		tc_frequency;
    	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 null-terminated string.

tc_quality
    Used to determine if this timecounter is better than another timecounter - higher means better. If this field is negative, the counter is only used 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.

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

SEE ALSO
amdpm(4), elansc(4), gscpm(4), ichpcib(4), viapm(4), hz(9), microtime(9) Poul-Henning Kamp, Timecounter: Efficient and precise timekeeping in SMP kernels, The FreeBSD Project, http://phk.freebsd.dk/pubs/timecounter.pdf.

HISTORY
The timecounter interface first appeared in OpenBSD 3.6.