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33 .Nd introduction to devices and device drivers
35 This section contains information related to devices, device driver
36 and miscellaneous hardware.
37 .Ss The device abstraction
38 Device is a term used mostly for hardware-related stuff that belongs
39 to the system, like disks, printers, or a graphics display with its
40 keyboard. There are also so-called
42 where a device driver emulates the behaviour of a device in software
43 without any particular underlying hardware. A typical example for
46 a loophole where the physical memory can be accessed using the regular
47 file access semantics.
49 The device abstraction generally provides a common set of system calls
50 layered on top of them, which are dispatched to the corresponding
51 device driver by the upper layers of the kernel. The set of system
52 calls available for devices is chosen from
61 Not all drivers implement all system calls, for example, calling
63 on a terminal devices is likely to be not useful at all.
65 Most of the devices in a unix-like operating system are accessed
70 They are usually located under the directory
72 in the file system hierarchy
73 .Pq see also Xr hier 7 .
77 is fully functional, each device node must be created statically and
78 independently of the existence of the associated device driver,
82 Note that this could lead to an inconsistent state, where either there
83 are device nodes that do not have a configured driver associated with
84 them, or there may be drivers that have successfully probed for their
85 devices, but cannot be accessed since the corresponding device node is
86 still missing. In the first case, any attempt to reference the device
87 through the device node will result in an error, returned by the upper
88 layers of the kernel, usually
90 In the second case, the device node needs to be created before the
91 driver and its device will be usable.
93 Some devices come in two flavors:
97 devices, or by a better name, buffered and unbuffered
99 devices. The traditional names are reflected by the letters
103 as the file type identification in the output of
105 Buffered devices are being accessed through the buffer cache of the
106 operating system, and they are solely intended to layer a file system
107 on top of them. They are normally implemented for disks and disk-like
108 devices only, for historical reasons also for tape devices.
110 Raw devices are available for all drivers, including those that also
111 implement a buffered device. For the latter group of devices, the
112 differentiation is conventionally done by prepending the letter
114 to the path name of the device node, for example
116 denotes the raw device for the first SCSI disk, while
118 is the corresponding device node for the buffered device.
120 Unbuffered devices should be used for all actions that are not related
121 to file system operations, even if the device in question is a disk
122 device. This includes making backups of entire disk partitions, or
126 .Pq i.e. those used like tapes .
128 Access restrictions to device nodes are usually subject of the regular
129 file permissions of the device node entry, instead of being implied
130 directly by the drivers in the kernel.
131 .Ss Drivers without device nodes
132 Drivers for network devices do not use device nodes in order to be
133 accessed. Their selection is based on other decisions inside the
134 kernel, and instead of calling
136 use of a network device is generally introduced by using the system
139 .Ss Configuring a driver into the kernel
140 For each kernel, there is a configuration file that is used as a base
141 to select the facilities and drivers for that kernel, and to tune
144 for a detailed description of the files involved. The individual
145 manual pages in this section provide a sample line for the
146 configuration file in their synopsis portion. See also the sample
148 .Pa /sys/i386/conf/LINT
168 This man page has been written by
171 with initial input by
172 .An David E. O'Brien .