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;
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 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.
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 arithmetics.
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,
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.