.\"	$OpenBSD: tc_init.9,v 1.7 2013/06/04 19:27:13 schwarze Exp $
.\"
.\" Copyright (c) 2004 Alexander Yurchenko <grange@openbsd.org>
.\"
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.Dd $Mdocdate: June 4 2013 $
.Dt TC_INIT 9
.Os
.Sh NAME
.Nm tc_init
.Nd machine-independent binary timescale
.Sh SYNOPSIS
.In sys/timetc.h
.Ft void
.Fn tc_init "struct timecounter *tc"
.Sh DESCRIPTION
The timecounter interface is a machine-independent implementation
of a binary timescale using whatever hardware support is at hand
for tracking time.
.Pp
A timecounter is a binary counter which has two properties:
.Bl -bullet -offset indent
.It
it runs at a fixed, known frequency
.It
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)
.El
.Pp
The interface between the hardware which implements a timecounter and the
machine-independent code which uses this to keep track of time is a
.Va timecounter
structure:
.Bd -literal -offset indent
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;
}
.Ed
.Pp
The fields of the
.Va timecounter
structure are described below.
.Bl -tag -width indent
.It Fn "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.
.It Fn "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.
.It Va tc_counter_mask
This mask should mask off any unimplemented bits.
.It Va tc_frequency
Frequency of the counter in Hz.
.It Va tc_name
Name of the timecounter.
Can be any null-terminated string.
.It Va tc_quality
Used to determine if this timecounter is better than another timecounter \-
higher means better.
Negative means ``only use at explicit request''.
.It Va tc_priv
Pointer to the timecounter's private parts.
.It Va tc_next
For internal use.
.El
.Pp
To register a new timecounter,
the hardware device driver should fill a
.Va timecounter
structure with appropriate values and call the
.Fn tc_init
function, giving a pointer to the structure as a
.Fa tc
parameter.
.Sh CODE REFERENCES
The timecounter framework is implemented in the file
.Pa sys/kern/kern_tc.c .
.Sh SEE ALSO
.Xr amdpm 4 ,
.Xr elansc 4 ,
.Xr gscpm 4 ,
.Xr ichpcib 4 ,
.Xr viapm 4 ,
.Xr hz 9 ,
.Xr microtime 9
.Rs
.%A Poul-Henning Kamp
.%T Timecounter: Efficient and precise timekeeping in SMP kernels
.%J The FreeBSD Project
.%U http://phk.freebsd.dk/pubs/timecounter.pdf
.Re
.Sh HISTORY
The timecounter interface first appeared in
.Ox 3.6 .