193 lines
6.8 KiB
Groff
193 lines
6.8 KiB
Groff
.\"
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.\" Copyright (c) 2010 The FreeBSD Foundation
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.\" All rights reserved.
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.\"
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.\" This software was developed by Semihalf under sponsorship from
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.\" the FreeBSD Foundation.
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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 July 12, 2010
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.Dt FDT 4
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.Os
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.Sh NAME
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.Nm fdt
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.Nd Flattened Device Tree support
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.Sh SYNOPSIS
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.Cd "options FDT"
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.Cd "makeoptions FDT_DTS_FILE=<board name>.dts"
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.Cd "options FDT_DTB_STATIC"
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.Sh DESCRIPTION
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.Em Flattened Device Tree
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is a mechanism for describing computer hardware resources, which cannot be
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probed or self enumerated, in a uniform and portable way.
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The primary consumers of this technology are
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.Em embedded systems,
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where a lot of designs are based on similar chips, but have different
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assignment of pins, memory layout, addresses bindings, interrupts routing and
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other resources.
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.Pp
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Configuration data, which cannot be self discovered in run-time, has to be
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supplied from external source.
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The concept of a flattened device tree is a
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platform and architecture independent approach for resolving such problems.
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The idea is inherited from Open Firmware IEEE 1275 device-tree notion, and has
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been successfully adopted by the embedded industry.
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The scheme works in the following way:
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.Bl -bullet
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.It
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Hardware platform resources are
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.Em manually
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described in a human readable text source format, where all non
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self-enumerating information is gathered.
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.It
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This source description is converted
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.Em (compiled)
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into a binary object i.e. a flattened device tree
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.Em blob
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which is passed to the kernel at boot time.
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.It
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The kernel (driver) learns about hardware resources details and dependencies
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from this [externally supplied] blob, which eliminates the need for embedding
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any information about the underlying platform hardware resources in the kernel.
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.It
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The flattened device tree mechanism in principle does not depend on any
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particular first-stage bootloader or firmware features.
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The only overall
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requirement for the environment is to provide a complete device tree
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description to the kernel.
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.El
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.Pp
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The
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.Nm
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layer allows any platform code in the kernel to retrieve information about
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hardware resources from a unified origin, which brings advantages to the
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embedded applications (eliminates hard-coded configuration approach, enforces
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code to be data driven and extensible) leading to easier porting and
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maintenance.
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.Sh DEFINITIONS
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.Bl -tag -width Ar
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.It Va Device tree source (DTS)
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The device tree source is a text file which describes hardware resources of a
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computer system in a human-readable form, with certain hierarchical structure
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(a tree).
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The default location for DTS files in the
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.Fx
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source repository is
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.Pa sys/boot/fdt/dts
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directory.
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.It Va Device tree blob (DTB)
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The textual device tree description (DTS file) is first converted (compiled)
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into a binary object (the device tree blob) i.e. the DTB, which is handed over
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to the final consumer (typically kernel) for parsing and processing of its
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contents.
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.It Va Device tree compiler (DTC)
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A utility program executed on the host, which transforms (compiles) a textual
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description of a device tree (DTS) into a binary object (DTB).
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.It Va Device tree bindings
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While the device tree textual description and the binary object are media to
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convey the hardware configuration information, an actual meaning and
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interpretation of the contents are defined by the device tree
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.Pa bindings .
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They are certain conventions describing definitions (encoding) of particular
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nodes in a device tree and their properties, allowed values, ranges and so on.
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Such reference conventions were provided by the legacy Open Firmware bindings,
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further supplemented by the ePAPR specification.
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.El
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.Sh "BUILDING THE WORLD"
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In order for the system to support
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.Nm
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it is required that
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.Fx
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world be built with the
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.Pa WITH_FDT
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build knob supplied either via
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.Xr src.conf 5
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or command line defined with -D.
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.Pp
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This creates the user space
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.Pa dtc
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compiler and enables
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.Nm
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support in
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.Xr loader 8 .
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.Sh "BUILDING KERNEL"
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There is a couple of options for managing
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.Nm
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support at the
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.Fx
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kernel level.
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.Bl -tag -width Ar
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.It Va options FDT
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The primary option for enabling
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.Nm
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support in the kernel.
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It covers all low-level and infrastructure parts of
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.Nm
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kernel support, which primarily are the
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.Xr fdtbus 4
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and
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.Xr simplebus 4
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drivers, as well as helper routines and libraries.
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.It Va makeoptions FDT_DTS_FILE=<board name>.dts
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Specifies a preferred (default) device tree source (DTS) file for a given
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kernel.
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The indicated DTS file will be converted (compiled) into a binary form
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along with building the kernel itself.
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The DTS file name is relative to the default location of DTS sources i.e.
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.Pa sys/boot/fdt/dts .
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This makeoption is not mandatory unless FDT_DTB_STATIC is also defined (see
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below).
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.It Va options FDT_DTB_STATIC
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Typically, the device tree blob (DTB) is a stand-alone file, physically
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separate from the kernel, but this option lets statically embed a
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DTB file into a kernel image.
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Note that when this is specified the
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FDT_DTS_FILE makeoption becomes mandatory (as there needs to be a DTS file
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specified in order to embed it into the kernel image).
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.El
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.Sh SEE ALSO
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.Xr fdtbus 4 ,
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.Xr openfirm 4 ,
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.Xr simplebus 4
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.Sh STANDARDS
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IEEE Std 1275: IEEE Standard for Boot (Initialization Configuration) Firmware:
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Core Requirements and Practices
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.Pq Vt Open Firmware .
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.Pp
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Power.org Standard for Embedded Power Architecture Platform Requirements
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.Pq Vt ePAPR .
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.Sh HISTORY
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The
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.Nm
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support first appeared in
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.Fx 9.0 .
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.Sh AUTHORS
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The
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.Nm
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support was developed by Semihalf under sponsorship from the FreeBSD
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Foundation.
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This manual page was written by
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.An Rafal Jaworowski .
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