176 lines
6.0 KiB
Groff
176 lines
6.0 KiB
Groff
.\"
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.\" Copyright (c) 1996 David E. O'Brien, Joerg Wunsch
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.\"
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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 DEVELOPERS ``AS IS'' AND ANY EXPRESS OR
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.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
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.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
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.\" IN NO EVENT SHALL THE DEVELOPERS BE LIABLE FOR ANY DIRECT, INDIRECT,
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.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
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.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
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.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
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.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
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.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
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.\"
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.\" $Id: intro.4,v 1.8 1997/02/22 13:24:33 peter Exp $
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.\"
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.Dd January 20, 1996
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.Dt INTRO 4
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.Os FreeBSD 2.1
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.Sh NAME
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.Nm intro
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.Nd introduction to devices and device drivers
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.Sh DESCRIPTION
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This section contains information related to devices, device driver
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and miscellaneous hardware.
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.Ss The device abstraction
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Device is a term used mostly for hardware-related stuff that belongs
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to the system, like disks, printers, or a graphics display with its
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keyboard. There are also so-called
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.Em pseudo-devices
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where a device driver emulates the behaviour of a device in software
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without any particular underlying hardware. A typical example for
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the latter class is
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.Pa /dev/mem ,
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a loophole where the physical memory can be accessed using the regular
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file access semantics.
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.Pp
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The device abstraction generally provides a common set of system calls
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layered on top of them, which are dispatched to the corresponding
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device driver by the upper layers of the kernel. The set of system
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calls available for devices is chosen from
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.Xr open 2 ,
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.Xr close 2 ,
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.Xr read 2 ,
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.Xr write 2 ,
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.Xr ioctl 2 ,
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.Xr select 2 ,
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and
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.Xr mmap 2 .
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Not all drivers implement all system calls, for example, calling
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.Xr mmap 2
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on a terminal devices is likely to be not useful at all.
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.Ss Accessing Devices
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Most of the devices in a unix-like operating system are accessed
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through so-called
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.Em device nodes ,
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sometimes also called
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.Em special files .
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They are usually located under the directory
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.Pa /dev
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in the file system hierarchy
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.Pq see also Xr hier 7 .
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.Pp
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Until
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.Xr devfs 5
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is fully functional, each device node must be created statically and
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independently of the existence of the associated device driver,
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usually by running
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.Xr MAKEDEV 8 .
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.Pp
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Note that this could lead to an inconsistent state, where either there
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are device nodes that do not have a configured driver associated with
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them, or there may be drivers that have successfully probed for their
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devices, but cannot be accessed since the corresponding device node is
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still missing. In the first case, any attempt to reference the device
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through the device node will result in an error, returned by the upper
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layers of the kernel, usually
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.Ql ENXIO .
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In the second case, the device node needs to be created before the
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driver and its device will be usable.
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.Pp
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Some devices come in two flavors:
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.Em block
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and
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.Em character
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devices, or by a better name, buffered and unbuffered
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.Pq raw
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devices. The traditional names are reflected by the letters
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.Ql b
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and
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.Ql c
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as the file type identification in the output of
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.Ql ls -l .
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Buffered devices are being accessed through the buffer cache of the
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operating system, and they are solely intended to layer a file system
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on top of them. They are normally implemented for disks and disk-like
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devices only, for historical reasons also for tape devices.
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.Pp
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Raw devices are available for all drivers, including those that also
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implement a buffered device. For the latter group of devices, the
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differentiation is conventionally done by prepending the latter
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.Ql r
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to the path name of the device node, for example
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.Pa /dev/rsd0
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denotes the raw device for the first SCSI disk, while
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.Pa /dev/sd0
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is the corresponding device node for the buffered device.
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.Pp
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Unbuffered devices should be used for all actions that are not related
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to file system operations, even if the device in question is a disk
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device. This includes making backups of entire disk partitions, or
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to
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.Em raw
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floppy disks
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.Pq i.e. those used like tapes .
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.Pp
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Access restrictions to device nodes are usually subject of the regular
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file permissions of the device node entry, instead of being implied
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directly by the drivers in the kernel.
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.Ss Drivers without device nodes
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Drivers for network devices do not use device nodes in order to be
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accessed. Their selection is based on other decisions inside the
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kernel, and instead of calling
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.Xr open 2 ,
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use of a network device is generally introduced by using the system
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call
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.Xr socket 2 .
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.Ss Configuring a driver into the kernel
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For each kernel, there is a configuration file that is used as a base
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to select the facilities and drivers for that kernel, and to tune
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several options. See
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.Xr config 8
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for a detailed description of the files involved. The individual
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manual pages in this section provide a sample line for the
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configuration file in their synopsis portion. See also the sample
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config file
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.Pa /sys/i386/conf/LINT
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.Po
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for the
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.Em i386
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architecture
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.Pc .
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.Sh SEE ALSO
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.Xr close 2 ,
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.Xr ioctl 2 ,
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.Xr mmap 2 ,
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.Xr open 2 ,
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.Xr read 2 ,
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.Xr select 2 ,
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.Xr socket 2 ,
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.Xr write 2 ,
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.Xr devfs 5 ,
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.Xr hier 7 ,
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.Xr config 8 ,
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.Xr MAKEDEV 8
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.Sh AUTHORS
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This man page has been written by
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.if t J\(:org Wunsch
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.if n Joerg Wunsch
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with initial input by David E. O'Brien.
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.Sh HISTORY
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.Nm intro
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appeared in
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.Fx 2.1 .
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