177 lines
6.1 KiB
Groff
177 lines
6.1 KiB
Groff
.\"
|
|
.\" Copyright (c) 1996 David E. O'Brien, Joerg Wunsch
|
|
.\"
|
|
.\" All rights reserved.
|
|
.\"
|
|
.\" Redistribution and use in source and binary forms, with or without
|
|
.\" modification, are permitted provided that the following conditions
|
|
.\" are met:
|
|
.\" 1. Redistributions of source code must retain the above copyright
|
|
.\" notice, this list of conditions and the following disclaimer.
|
|
.\" 2. Redistributions in binary form must reproduce the above copyright
|
|
.\" notice, this list of conditions and the following disclaimer in the
|
|
.\" documentation and/or other materials provided with the distribution.
|
|
.\"
|
|
.\" THIS SOFTWARE IS PROVIDED BY THE DEVELOPERS ``AS IS'' AND ANY EXPRESS OR
|
|
.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
|
|
.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
|
|
.\" IN NO EVENT SHALL THE DEVELOPERS BE LIABLE FOR ANY DIRECT, INDIRECT,
|
|
.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
|
|
.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
|
|
.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
|
|
.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
|
|
.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
|
|
.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
|
|
.\"
|
|
.\" $Id: intro.4,v 1.2 1996/02/11 22:37:26 mpp Exp $
|
|
.\"
|
|
.Dd January 20, 1996
|
|
.Dt INTRO 4
|
|
.Os FreeBSD 2.1
|
|
.Sh NAME
|
|
.Nm intro
|
|
.Nd introduction to devices and device drivers
|
|
.Sh DESCRIPTION
|
|
This section contains information related to devices, device driver
|
|
and miscellaneous hardware.
|
|
.Ss The device abstraction
|
|
Device is a term used mostly for hardware-related stuff that belongs
|
|
to the system, like disks, printers, or a graphics display with its
|
|
keyboard. There are also so-called
|
|
.Em pseudo-devices
|
|
where a device driver emulates the behaviour of a device in software
|
|
without any particular underlying hardware. A typical example for
|
|
the latter class is
|
|
.Pa /dev/mem ,
|
|
a loophole where the physical memory can be accessed using the regular
|
|
file access semantics.
|
|
.Pp
|
|
The device abstraction generally provides a common set of system calls
|
|
layered on top of them, which are dispatched to the corresponding
|
|
device driver by the upper layers of the kernel. The set of system
|
|
calls available for devices is chosen from
|
|
.Xr open 2 ,
|
|
.Xr close 2 ,
|
|
.Xr read 2 ,
|
|
.Xr write 2 ,
|
|
.Xr ioctl 2 ,
|
|
.Xr select 2 ,
|
|
and
|
|
.Xr mmap 2 .
|
|
Not all drivers implement all system calls, for example, calling
|
|
.Xr mmap 2
|
|
on a terminal devices is likely to be not useful at all.
|
|
.Ss Accessing Devices
|
|
Most of the devices in a unix-like operating system are accessed
|
|
through so-called
|
|
.Em device nodes ,
|
|
sometimes also called
|
|
.Em special files .
|
|
They are usually located under the directory
|
|
.Pa /dev
|
|
in the file system hierarchy
|
|
.Pq see also Xr hier 7 .
|
|
.Pp
|
|
Until
|
|
.Xr devfs 5
|
|
is fully functional, each device node must be created statically and
|
|
independently of the existence of the associated device driver,
|
|
usually by running
|
|
.Xr MAKEDEV 8 .
|
|
.Pp
|
|
Note that this could lead to an inconsistent state, where either there
|
|
are device nodes that do not have a configured driver associated with
|
|
them, or there may be drivers that have successfully probed for their
|
|
devices, but cannot be accessed since the corresponding device node is
|
|
still missing. In the first case, any attempt to reference the device
|
|
through the device node will result in an error, returned by the upper
|
|
layers of the kernel, usually
|
|
.Ql ENXIO .
|
|
In the second case, the device node needs to be created before the
|
|
driver and its device will be usable.
|
|
.Pp
|
|
Some devices come in two flavors:
|
|
.Em block
|
|
and
|
|
.Em character
|
|
devices, or by a better name, buffered and unbuffered
|
|
.Pq raw
|
|
devices. The traditional names are reflected by the letters
|
|
.Ql b
|
|
and
|
|
.Ql c
|
|
as the file type identification in the output of
|
|
.Ql ls -l .
|
|
Buffered devices are being accessed through the buffer cache of the
|
|
operating system, and they are solely intended to layer a file system
|
|
on top of them. They are normally implemented for disks and disk-like
|
|
devices only, for historical reasons also for tape devices.
|
|
.Pp
|
|
Raw devices are available for all drivers, including those that also
|
|
implement a buffered device. For the latter group of devices, the
|
|
differentiation is conventionally done by prepending the latter
|
|
.Ql r
|
|
to the path name of the device node, for example
|
|
.Pa /dev/rsd0
|
|
denotes the raw device for the first SCSI disk, while
|
|
.Pa /dev/sd0
|
|
is the corresponding device node for the buffered device.
|
|
.Pp
|
|
Unbuffered devices should be used for all actions that are not related
|
|
to file system operations, even if the device in question is a disk
|
|
device. This includes making backups of entire disk partitions, or
|
|
to
|
|
.Em raw
|
|
floppy disks
|
|
.Pq i.e. those used like tapes .
|
|
.Pp
|
|
Access restrictions to device nodes are usually subject of the regular
|
|
file permissions of the device node entry, instead of being implied
|
|
directly by the drivers in the kernel.
|
|
.Ss Drivers without device nodes
|
|
Drivers for network devices do not use device nodes in order to be
|
|
accessed. Their selection is based on other decisions inside the
|
|
kernel, and instead of calling
|
|
.Xr open 2 ,
|
|
use of a network device is generally introduced by using the system
|
|
call
|
|
.Xr socket 2 .
|
|
.Ss Configuring a driver into the kernel
|
|
For each kernel, there is a configuration file that is used as a base
|
|
to select the facilities and drivers for that kernel, and to tune
|
|
several options. See
|
|
.Xr config 8
|
|
for a detailed description of the files involved. The individual
|
|
manual pages in this section provide a sample line for the
|
|
configuration file in their synopsis portion. See also the sample
|
|
config file
|
|
.Pa /sys/i386/conf/LINT
|
|
.Po
|
|
for the
|
|
.Em i386
|
|
architecture
|
|
.Pc .
|
|
.Sh SEE ALSO
|
|
.Xr open 2 ,
|
|
.Xr close 2 ,
|
|
.Xr read 2 ,
|
|
.Xr write 2 ,
|
|
.Xr ioctl 2 ,
|
|
.Xr select 2 ,
|
|
.Xr mmap 2 ,
|
|
.Xr socket 2 ,
|
|
.Xr hier 7 ,
|
|
.Xr config 8 ,
|
|
.Xr MAKEDEV 8 ,
|
|
.Xr devfs 5 Pq not yet existent .
|
|
.Sh AUTHORS
|
|
This man page has been written by
|
|
.if t J\(:org Wunsch
|
|
.if n Joerg Wunsch
|
|
with initial input by David E. O'Brien.
|
|
.Sh HISTORY
|
|
.Nm intro
|
|
appeared in
|
|
.Fx 2.1 .
|
|
|