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34 .Nd introduction to devices and device drivers
36 This section contains information related to devices, device drivers
37 and miscellaneous hardware.
38 .Ss The device abstraction
39 Device is a term used mostly for hardware-related stuff that belongs
40 to the system, like disks, printers, or a graphics display with its
42 There are also so-called
44 where a device driver emulates the behaviour of a device in software
45 without any particular underlying hardware.
49 a loophole where the physical memory can be accessed using the regular
50 file access semantics.
52 The device abstraction generally provides a common set of system calls
53 layered on top of them, which are dispatched to the corresponding
54 device driver by the upper layers of the kernel.
56 calls available for devices is chosen from
65 Not all drivers implement all system calls, for example, calling
67 on terminal devices is likely to be not useful at all.
69 Most of the devices in a unix-like operating system are accessed
74 They are usually located under the directory
76 in the file system hierarchy
80 Each device node must be created statically and
81 independently of the existence of the associated device driver,
85 Note that this could lead to an inconsistent state, where either there
86 are device nodes that do not have a configured driver associated with
87 them, or there may be drivers that have successfully probed for their
88 devices, but cannot be accessed since the corresponding device node is
90 In the first case, any attempt to reference the device
91 through the device node will result in an error, returned by the upper
92 layers of the kernel, usually
94 In the second case, the device node needs to be created before the
95 driver and its device will be usable.
97 Some devices come in two flavors:
101 devices, or to use better terms, buffered and unbuffered
104 The traditional names are reflected by the letters
108 as the file type identification in the output of
110 Buffered devices are being accessed through the buffer cache of the
111 operating system, and they are solely intended to layer a file system
113 They are normally implemented for disks and disk-like
114 devices only and, for historical reasons, for tape devices.
116 Raw devices are available for all drivers, including those that also
117 implement a buffered device.
118 For the latter group of devices, the
119 differentiation is conventionally done by prepending the letter
121 to the path name of the device node, for example
123 denotes the raw device for the first SCSI disk, while
125 is the corresponding device node for the buffered device.
127 Unbuffered devices should be used for all actions that are not related
128 to file system operations, even if the device in question is a disk
130 This includes making backups of entire disk partitions, or
134 (i.e. those used like tapes).
136 Access restrictions to device nodes are usually subject to the regular
137 file permissions of the device node entry, instead of being enforced
138 directly by the drivers in the kernel.
139 .Ss Drivers without device nodes
140 Drivers for network devices do not use device nodes in order to be
142 Their selection is based on other decisions inside the
143 kernel, and instead of calling
145 use of a network device is generally introduced by using the system
148 .Ss Configuring a driver into the kernel
149 For each kernel, there is a configuration file that is used as a base
150 to select the facilities and drivers for that kernel, and to tune
154 for a detailed description of the files involved.
155 The individual manual pages in this section provide a sample line for the
156 configuration file in their synopsis portion. See also the sample
177 manual page first appeared in
181 This man page was written by
183 with initial input by
184 .An David E. O'Brien .