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:
· it runs at a fixed, known frequency; and
· it has sufficient bits to not roll over in less than approxi‐
mately 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;
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 time‐
counters 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 imple‐
mentation 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 arith‐
metics.
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 ALSOclock_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
