.\"    $OpenBSD: tc_init.9,v 1.12 2023/04/02 00:02:26 cheloha Exp $
.\"
.\" Copyright (c) 2004 Alexander Yurchenko <grange@openbsd.org>
.\" Copyright (c) 2023 Scott Cheloha <cheloha@openbsd.org>
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.Dd $Mdocdate: April 2 2023 $
.Dt TC_INIT 9
.Os
.Sh NAME
.Nm tc_init
.Nd timecounting subsystem
.Sh SYNOPSIS
.In sys/timetc.h
.Ft void
.Fn tc_init "struct timecounter *tc"
.Sh DESCRIPTION
The
.Sy timecounting
subsystem implements a uniform interface to timekeeping hardware,
measures the passage of time,
and implements the kernel's software clocks
.Po see
.Xr microtime 9
for details
.Pc .
.Pp
A hardware clock is suitable for counting time if it meets the following
requirements:
.Bl -enum -offset indent
.It
It is a binary counter.
.It
It advances at a fixed, known frequency.
.It
Its count is synchronized between all CPUs on the system.
.It
It continues counting when it rolls over.
.It
If
.Xr hz 9
is less than or equal to one millisecond,
the counter does not roll over in less than two milliseconds.
If
.Xr hz 9
exceeds one millisecond,
the counter does not roll over in less than
.Pq 2 / Va hz
seconds.
.El
.Pp
Hardware clocks are described with a
.Va timecounter
structure:
.Bd -literal -offset indent
struct timecounter {
	u_int (*tc_get_timecount)(struct timecounter *);
	u_int tc_counter_mask;
	u_int64_t tc_frequency;
	char *tc_name;
	int tc_quality;
	void *tc_priv;
	u_int tc_user;
};
.Ed
.Bl -tag -width indent
.It Ft u_int Fn (*tc_get_timecount) "struct timecounter *"
Reads the hardware clock and returns its count.
Any unimplemented bits only need to be masked if they are not constant.
If the counter is larger than 32 bits,
this function must return a 32-bit subset.
The subsystem requires an upward count;
downward counts must be inverted before they are returned.
.It Va tc_counter_mask
The mask of implemented bits.
Used to discard unimplemented bits from
.Fn tc_get_timecount .
.It Va tc_frequency
The counter's fixed frequency.
.It Va tc_name
The counter's unique name.
A
.Dv NUL Ns -terminated string.
.It Va tc_quality
A relative quality metric used to compare counters.
Higher values indicate a better counter.
A negative value indicates that the counter is non-monotonic
or otherwise deficient.
The system will only use negative-quality counters if requested.
.It Va tc_priv
May point to anything the driver needs during
.Fn tc_get_timecount .
.It Va tc_user
If non-zero,
a unique value identifying the userspace implementation of
.Fn tc_get_timecount .
.El
.Pp
To register a timecounter,
a device driver initializes the above-described fields of a
.Va timecounter
structure and calls
.Fn tc_init
with a pointer to that structure as argument.
.Sh CONTEXT
.Fn tc_init
may only be called during autoconf.
.Sh CODE REFERENCES
.Pa sys/kern/kern_tc.c
.Sh SEE ALSO
.Xr amdpm 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
.%D 2002
.%U https://papers.freebsd.org/2002/phk-timecounters.files/timecounter.pdf
.Re
.Sh HISTORY
The timecounting subsystem first appeared in
.Fx 3.0 .
It was ported to
.Ox 3.6 .
.Sh AUTHORS
.An Poul-Henning Kamp