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with higher quality registers (presumably in a module that has just been loaded), do not undo the user's choice by switching to the new timecounter. Document that behavior, and also the fact that there is no way to unregister a timecounter (and thus no way to unload a module containing one).
114 lines
4.6 KiB
Groff
114 lines
4.6 KiB
Groff
.\" Copyright (c) 2011 Alexander Motin <mav@FreeBSD.org>
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.\" All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" $FreeBSD$
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.\"
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.Dd August 12, 2015
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.Dt TIMECOUNTERS 4
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.Os
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.Sh NAME
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.Nm timecounters
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.Nd kernel time counters subsystem
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.Sh SYNOPSIS
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The kernel uses several types of time-related devices, such as: real time clocks,
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time counters and event timers.
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Real time clocks are responsible for tracking real world time, mostly when the system
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is down.
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Time counters are responsible for tracking purposes, when the system is running.
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Event timers are responsible for generating interrupts at a specified time or
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periodically, to run different time-based events.
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This page is about the second.
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.Sh DESCRIPTION
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Time counters are the lowest level of time tracking in the kernel.
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They provide monotonically increasing timestamps with known width and
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update frequency.
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They can overflow, drift, etc and so in raw form can be used only in very limited
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performance-critical places like the process scheduler.
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.Pp
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More usable time is created by scaling the values read from the selected
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time counter and combining it with some offset, regularly updated by
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.Fn tc_windup
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on
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.Fn hardclock
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invocation.
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.Pp
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Different platforms provide different kinds of timer hardware.
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The goal of the time counters subsystem is to provide a unified way to access
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that hardware.
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.Pp
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Each driver implementing time counters registers them with the subsystem.
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It is possible to see the list of present time counters, via the
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.Va kern.timecounter
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.Xr sysctl 8
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variable:
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.Bd -literal
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kern.timecounter.choice: TSC-low(-100) HPET(950) i8254(0) ACPI-fast(900) dummy(-1000000)
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kern.timecounter.tc.ACPI-fast.mask: 16777215
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kern.timecounter.tc.ACPI-fast.counter: 13467909
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kern.timecounter.tc.ACPI-fast.frequency: 3579545
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kern.timecounter.tc.ACPI-fast.quality: 900
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kern.timecounter.tc.i8254.mask: 65535
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kern.timecounter.tc.i8254.counter: 62692
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kern.timecounter.tc.i8254.frequency: 1193182
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kern.timecounter.tc.i8254.quality: 0
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kern.timecounter.tc.HPET.mask: 4294967295
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kern.timecounter.tc.HPET.counter: 3013495652
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kern.timecounter.tc.HPET.frequency: 14318180
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kern.timecounter.tc.HPET.quality: 950
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kern.timecounter.tc.TSC-low.mask: 4294967295
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kern.timecounter.tc.TSC-low.counter: 4067509463
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kern.timecounter.tc.TSC-low.frequency: 11458556
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kern.timecounter.tc.TSC-low.quality: -100
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.Ed
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.Pp
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The output nodes are defined as follows:
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.Bl -inset
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.It Va kern.timecounter.tc. Ns Ar X Ns Va .mask
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is a bitmask, defining valid counter bits,
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.It Va kern.timecounter.tc. Ns Ar X Ns Va .counter
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is a present counter value,
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.It Va kern.timecounter.tc. Ns Ar X Ns Va .frequency
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is a counter update frequency,
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.It Va kern.timecounter.tc. Ns Ar X Ns Va .quality
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is an integral value, defining the quality of this time counter
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compared to others.
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A negative value means this time counter is broken and should not be used.
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.El
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.Pp
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The time management code of the kernel automatically switches to a
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higher-quality time counter when it registers, unless the
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.Va kern.timecounter.hardware
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sysctl has been used to choose a specific device.
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.Pp
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There is no way to unregister a time counter once it has registered
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with the kernel.
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If a dynamically loaded module contains a time counter you will not
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be able to unload that module, even if the time counter it contains
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is not the one currently in use.
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.Sh SEE ALSO
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.Xr attimer 4 ,
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.Xr eventtimers 4 ,
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.Xr ffclock 4 ,
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.Xr hpet 4
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