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.Dd August 12, 2015
.Dt TIMECOUNTERS 4
.Os
.Sh NAME
.Nm timecounters
.Nd kernel time counters subsystem
.Sh SYNOPSIS
The kernel uses several types of time-related devices, such as: real time clocks,
time counters and event timers.
Real time clocks are responsible for tracking real world time, mostly when the system
is down.
Time counters are responsible for tracking purposes, when the system is running.
Event timers are responsible for generating interrupts at a specified time or
periodically, to run different time-based events.
This page is about the second.
.Sh DESCRIPTION
Time counters are the lowest level of time tracking in the kernel.
They provide monotonically increasing timestamps with known width and
update frequency.
They can overflow, drift, etc and so in raw form can be used only in very limited
performance-critical places like the process scheduler.
.Pp
More usable time is created by scaling the values read from the selected
time counter and combining it with some offset, regularly updated by
.Fn tc_windup
on
.Fn hardclock
invocation.
.Pp
Different platforms provide different kinds of timer hardware.
The goal of the time counters subsystem is to provide a unified way to access
that hardware.
.Pp
Each driver implementing time counters registers them with the subsystem.
It is possible to see the list of present time counters, via the
.Va kern.timecounter
.Xr sysctl 8
variable:
.Bd -literal
kern.timecounter.choice: TSC-low(-100) HPET(950) i8254(0) ACPI-fast(900) dummy(-1000000)
kern.timecounter.tc.ACPI-fast.mask: 16777215
kern.timecounter.tc.ACPI-fast.counter: 13467909
kern.timecounter.tc.ACPI-fast.frequency: 3579545
kern.timecounter.tc.ACPI-fast.quality: 900
kern.timecounter.tc.i8254.mask: 65535
kern.timecounter.tc.i8254.counter: 62692
kern.timecounter.tc.i8254.frequency: 1193182
kern.timecounter.tc.i8254.quality: 0
kern.timecounter.tc.HPET.mask: 4294967295
kern.timecounter.tc.HPET.counter: 3013495652
kern.timecounter.tc.HPET.frequency: 14318180
kern.timecounter.tc.HPET.quality: 950
kern.timecounter.tc.TSC-low.mask: 4294967295
kern.timecounter.tc.TSC-low.counter: 4067509463
kern.timecounter.tc.TSC-low.frequency: 11458556
kern.timecounter.tc.TSC-low.quality: -100
.Ed
.Pp
The output nodes are defined as follows:
.Bl -inset
.It Va kern.timecounter.tc. Ns Ar X Ns Va .mask
is a bitmask, defining valid counter bits,
.It Va kern.timecounter.tc. Ns Ar X Ns Va .counter
is a present counter value,
.It Va kern.timecounter.tc. Ns Ar X Ns Va .frequency
is a counter update frequency,
.It Va kern.timecounter.tc. Ns Ar X Ns Va .quality
is an integral value, defining the quality of this time counter
compared to others.
A negative value means this time counter is broken and should not be used.
.El
.Pp
The time management code of the kernel automatically switches to a
higher-quality time counter when it registers, unless the
.Va kern.timecounter.hardware
sysctl has been used to choose a specific device.
.Pp
There is no way to unregister a time counter once it has registered
with the kernel.
If a dynamically loaded module contains a time counter you will not
be able to unload that module, even if the time counter it contains
is not the one currently in use.
.Sh SEE ALSO
.Xr attimer 4 ,
.Xr eventtimers 4 ,
.Xr ffclock 4 ,
.Xr hpet 4