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<table class="head">
<tr>
<td class="head-ltitle">INTRO(4)</td>
<td class="head-vol">Device Drivers Manual</td>
<td class="head-rtitle">INTRO(4)</td>
</tr>
</table>
<div class="manual-text">
<section class="Sh">
<h1 class="Sh" id="NAME"><a class="permalink" href="#NAME">NAME</a></h1>
<p class="Pp"><code class="Nm">intro</code> —
<span class="Nd">introduction to devices and device drivers</span></p>
</section>
<section class="Sh">
<h1 class="Sh" id="DESCRIPTION"><a class="permalink" href="#DESCRIPTION">DESCRIPTION</a></h1>
<p class="Pp">This section contains information related to devices, device
drivers and miscellaneous hardware.</p>
<section class="Ss">
<h2 class="Ss" id="The_device_abstraction"><a class="permalink" href="#The_device_abstraction">The
device abstraction</a></h2>
<p class="Pp">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
<a class="permalink" href="#pseudo-devices"><i class="Em" id="pseudo-devices">pseudo-devices</i></a>
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 <span class="Pa">/dev/mem</span>, a mechanism whereby the physical memory
can be accessed using file access semantics.</p>
<p class="Pp">The device abstraction generally provides a common set of system
calls, 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 <a class="Xr">open(2)</a>, <a class="Xr">close(2)</a>,
<a class="Xr">read(2)</a>, <a class="Xr">write(2)</a>,
<a class="Xr">ioctl(2)</a>, <a class="Xr">select(2)</a>, and
<a class="Xr">mmap(2)</a>. Not all drivers implement all system calls; for
example, calling <a class="Xr">mmap(2)</a> on a keyboard device is not
likely to be useful.</p>
<p class="Pp">Aspects of the device abstraction have changed significantly in
<span class="Ux">FreeBSD</span> over the past two decades. The section
<a class="Sx" href="#Historical_Notes">Historical Notes</a> describes some
of the more important differences.</p>
</section>
<section class="Ss">
<h2 class="Ss" id="Accessing_Devices"><a class="permalink" href="#Accessing_Devices">Accessing
Devices</a></h2>
<p class="Pp">Most of the devices in <span class="Ux">FreeBSD</span> are
accessed through
<a class="permalink" href="#device"><i class="Em" id="device">device
nodes</i></a>, sometimes also called
<a class="permalink" href="#special"><i class="Em" id="special">special
files</i></a>. They are located within instances of the
<a class="Xr">devfs(4)</a> filesystem, which is conventionally mounted on
the directory <span class="Pa">/dev</span> in the file system hierarchy (see
also <a class="Xr">hier(7)</a>).</p>
<p class="Pp">The <a class="Xr">devfs(4)</a> filesystem creates or removes
device nodes automatically according to the physical hardware recognized as
present at any given time. For pseudo-devices, device nodes may be created
and removed dynamically as required, depending on the nature of the
device.</p>
<p class="Pp">Access restrictions to device nodes are usually subject to the
regular file permissions of the device node entry, instead of being enforced
directly by the drivers in the kernel. But since device nodes are not stored
persistently between reboots, those file permissions are set at boot time
from rules specified in <a class="Xr">devfs.conf(5)</a>, or dynamically
according to rules defined in <a class="Xr">devfs.rules(5)</a> or set using
the <a class="Xr">devfs(8)</a> command. In the latter case, different rules
may be used to make different sets of devices visible within different
instances of the <a class="Xr">devfs(4)</a> filesystem, which may be used,
for example, to prevent jailed subsystems from accessing unsafe devices.
Manual changes to device node permissions may still be made, but will not
persist.</p>
</section>
<section class="Ss">
<h2 class="Ss" id="Drivers_without_device_nodes"><a class="permalink" href="#Drivers_without_device_nodes">Drivers
without device nodes</a></h2>
<p class="Pp">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 <a class="Xr">open(2)</a>, use of a network device is
generally introduced by using the system call
<a class="Xr">socket(2)</a>.</p>
</section>
<section class="Ss">
<h2 class="Ss" id="Configuring_a_driver_into_the_kernel"><a class="permalink" href="#Configuring_a_driver_into_the_kernel">Configuring
a driver into the kernel</a></h2>
<p class="Pp">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 <a class="Xr">config(8)</a> 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 portions. See also
the files <span class="Pa">/usr/src/sys/conf/NOTES</span> and
<span class="Pa">/usr/src/sys/${ARCH}/conf/NOTES</span>.</p>
<p class="Pp">Drivers need not be statically compiled into the kernel; they may
also be loaded as modules, in which case any device nodes they provide will
appear only after the module is loaded (and has attached to suitable
hardware, if applicable).</p>
</section>
<section class="Ss">
<h2 class="Ss" id="Historical_Notes"><a class="permalink" href="#Historical_Notes">Historical
Notes</a></h2>
<p class="Pp">Prior to <span class="Ux">FreeBSD 6.0</span>, device nodes could
be created in the traditional way as persistent entries in the file system.
While such entries can still be created, they no longer function to access
devices.</p>
<p class="Pp" id="block">Prior to <span class="Ux">FreeBSD 5.0</span>, devices
for disk and tape drives existed in two variants, known as
<a class="permalink" href="#block"><i class="Em">block</i></a> and
<a class="permalink" href="#character"><i class="Em" id="character">character</i></a>
devices, or to use better terms, buffered and unbuffered (raw) devices. The
traditional names are reflected by the letters
“<code class="Li">b</code>” and
“<code class="Li">c</code>” as the file type identification in
the output of “<code class="Li">ls -l</code>”. Raw devices
were traditionally named with a prefix of
“<code class="Li">r</code>”, for example
<span class="Pa">/dev/rda0</span> would denote the raw version of the disk
whose buffered device was <span class="Pa">/dev/da0</span>.
<a class="permalink" href="#This"><i class="Em" id="This">This is no longer
the case</i></a>; all disk devices are now “raw” in the
traditional sense, even though they are not given
“<code class="Li">r</code>” prefixes, and
“buffered” devices no longer exist at all.</p>
<p class="Pp" id="synchronous">Buffered devices were accessed through a buffer
cache maintained by the operating system; historically this was the system's
primary disk cache, but in <span class="Ux">FreeBSD</span> this was rendered
obsolete by the introduction of unified virtual memory management. Buffered
devices could be read or written at any byte position, with the buffer
mechanism handling the reading and writing of disk blocks. In contrast, raw
disk devices can be read or written only at positions and lengths that are
multiples of the underlying device block size, and
<a class="Xr">write(2)</a> calls are
<a class="permalink" href="#synchronous"><i class="Em">synchronous</i></a>,
not returning to the caller until the data has been handed off to the
device.</p>
</section>
</section>
<section class="Sh">
<h1 class="Sh" id="SEE_ALSO"><a class="permalink" href="#SEE_ALSO">SEE
ALSO</a></h1>
<p class="Pp"><a class="Xr">close(2)</a>, <a class="Xr">ioctl(2)</a>,
<a class="Xr">mmap(2)</a>, <a class="Xr">open(2)</a>,
<a class="Xr">read(2)</a>, <a class="Xr">select(2)</a>,
<a class="Xr">socket(2)</a>, <a class="Xr">write(2)</a>,
<a class="Xr">devfs(4)</a>, <a class="Xr">hier(7)</a>,
<a class="Xr">config(8)</a></p>
</section>
<section class="Sh">
<h1 class="Sh" id="HISTORY"><a class="permalink" href="#HISTORY">HISTORY</a></h1>
<p class="Pp">This manual page first appeared in <span class="Ux">FreeBSD
2.1</span>.</p>
</section>
<section class="Sh">
<h1 class="Sh" id="AUTHORS"><a class="permalink" href="#AUTHORS">AUTHORS</a></h1>
<p class="Pp">This man page has been rewritten by <span class="An">Andrew
Gierth</span> from an earlier version written by
<span class="An">Jörg Wunsch</span> with initial input by
<span class="An">David E. O'Brien</span>.</p>
</section>
</div>
<table class="foot">
<tr>
<td class="foot-date">April 3, 2019</td>
<td class="foot-os">FreeBSD 15.0</td>
</tr>
</table>
|
