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88 lines
3.2 KiB
Groff
88 lines
3.2 KiB
Groff
.\" Copyright (c) 2010 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 May 26, 2014
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.Dt ATTIMER 4
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.Os
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.Sh NAME
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.Nm attimer
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.Nd i8254 Programmable Interval Timer (AT Timer) driver
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.Sh SYNOPSIS
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This driver is a mandatory part of x86 kernels.
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.Pp
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The following tunables are settable from the
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.Xr loader 8 :
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.Bl -ohang
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.It Va hint.attimer. Ns Ar X Ns Va .clock
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controls support for the event timer functionality.
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Setting this value to
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.Dv 0
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disables it.
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The default value is
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.Dv 1 .
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.It Va hint.attimer. Ns Ar X Ns Va .timecounter
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controls support for the time counter functionality.
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Setting this value to
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.Dv 0
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disables it.
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The default value is
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.Dv 1 .
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.It Va hw.i8254.freq
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allows overriding the default counter frequency.
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The same value is also available at run-time via the
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.Va machdep.i8254_freq
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sysctl.
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.El
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.Sh DESCRIPTION
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This driver uses i8254 Programmable Interval Timer (AT Timer) hardware
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to supply the kernel with one timecounter and one event timer, and to generate
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sound tones for the system speaker.
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This hardware includes three channels.
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Each channel includes a 16 bit counter which decreases with a known,
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platform-dependent frequency.
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Counters can operate in several different modes, including periodic and
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one-shot.
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The output of each channel has platform-defined wiring: one channel is wired
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to the interrupt controller and may be used as event timer, one channel is
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wired to the speaker and used to generate sound tones, and one timer is reserved
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for platform purposes.
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.Pp
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The
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.Nm
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driver uses a single hardware channel to provide both time counter and event
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timer functionality.
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To make this possible, the respective counter must be running in periodic mode.
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As a result, the one-shot event timer mode is supported only when time counter
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functionality is disabled.
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.Pp
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The event timer provided by the driver is irrelevant to CPU power states.
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.Sh SEE ALSO
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.Xr apic 4 ,
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.Xr atrtc 4 ,
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.Xr eventtimers 4 ,
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.Xr hpet 4 ,
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.Xr timecounters 4
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