xref: /freebsd/share/man/man4/intro.4 (revision 1a720cbec513210fa2e85c3882741ef2f6dc5f35)

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.Dd April 3, 2019
.Dt INTRO 4
.Os
.Sh NAME
.Nm intro
.Nd introduction to devices and device drivers
.Sh DESCRIPTION
This section contains information related to devices, device drivers
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 mechanism whereby the physical memory can be accessed using file
access semantics.
.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
.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 keyboard device is not likely to be useful.
.Pp
Aspects of the device abstraction have changed significantly in
.Fx
over the past two decades.
The section
.Sx Historical Notes
describes some of the more important differences.
.Ss Accessing Devices
Most of the devices in
.Fx
are accessed through
.Em device nodes ,
sometimes also called
.Em special files .
They are located within instances of the
.Xr devfs 4
filesystem, which is conventionally mounted on the directory
.Pa /dev
in the file system hierarchy
(see also
.Xr hier 7 ) .
.Pp
The
.Xr devfs 4
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.
.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
.Xr devfs.conf 5 ,
or dynamically according to rules defined in
.Xr devfs.rules 5
or set using the
.Xr devfs 8
command.
In the latter case, different rules may be used to make different sets
of devices visible within different instances of the
.Xr devfs 4
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.
.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 portions.
See also the files
.Pa /usr/src/sys/conf/NOTES
and
.Pa /usr/src/sys/${ARCH}/conf/NOTES .
.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).
.Ss Historical Notes
Prior to
.Fx 6.0 ,
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.
.Pp
Prior to
.Fx 5.0 ,
devices for disk and tape drives existed in two variants, known as
.Em block
and
.Em character
devices, or to use better terms, buffered and unbuffered
(raw)
devices.
The traditional names are reflected by the letters
.Dq Li b
and
.Dq Li c
as the file type identification in the output of
.Dq Li ls -l .
Raw devices were traditionally named with a prefix of
.Dq Li r ,
for example
.Pa /dev/rda0
would denote the raw version of the disk whose buffered device was
.Pa /dev/da0 .
.Em This is no longer the case ;
all disk devices are now
.Dq raw
in the traditional sense, even though they are not given
.Dq Li r
prefixes, and
.Dq buffered
devices no longer exist at all.
.Pp
Buffered devices were accessed through a buffer cache maintained by
the operating system; historically this was the system's primary disk
cache, but in
.Fx
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
.Xr write 2
calls are
.Em synchronous ,
not returning to the caller until the data has been handed off to the
device.
.Sh SEE ALSO
.Xr close 2 ,
.Xr ioctl 2 ,
.Xr mmap 2 ,
.Xr open 2 ,
.Xr read 2 ,
.Xr select 2 ,
.Xr socket 2 ,
.Xr write 2 ,
.Xr devfs 4 ,
.Xr hier 7 ,
.Xr config 8
.Sh HISTORY
This manual page first appeared in
.Fx 2.1 .
.Sh AUTHORS
.An -nosplit
This man page has been rewritten by
.An Andrew Gierth
from an earlier version written by
.An J\(:org Wunsch
with initial input by
.An David E. O'Brien .