1 The Linux Kernel Device Model
2
3 Patrick Mochel <mochel@digitalimplant.org>
4
5 Drafted 26 August 2002
6 Updated 31 January 2006
7
8
9 Overview
10 ~~~~~~~~
11
12 The Linux Kernel Driver Model is a unification of all the disparate driver
13 models that were previously used in the kernel. It is intended to augment the
14 bus-specific drivers for bridges and devices by consolidating a set of data
15 and operations into globally accessible data structures.
16
17 Traditional driver models implemented some sort of tree-like structure
18 (sometimes just a list) for the devices they control. There wasn't any
19 uniformity across the different bus types.
20
21 The current driver model provides a comon, uniform data model for describing
22 a bus and the devices that can appear under the bus. The unified bus
23 model includes a set of common attributes which all busses carry, and a set
24 of common callbacks, such as device discovery during bus probing, bus
25 shutdown, bus power management, etc.
26
27 The common device and bridge interface reflects the goals of the modern
28 computer: namely the ability to do seamless device "plug and play", power
29 management, and hot plug. In particular, the model dictated by Intel and
30 Microsoft (namely ACPI) ensures that almost every device on almost any bus
31 on an x86-compatible system can work within this paradigm. Of course,
32 not every bus is able to support all such operations, although most
33 buses support a most of those operations.
34
35
36 Downstream Access
37 ~~~~~~~~~~~~~~~~~
38
39 Common data fields have been moved out of individual bus layers into a common
40 data structure. These fields must still be accessed by the bus layers,
41 and sometimes by the device-specific drivers.
42
43 Other bus layers are encouraged to do what has been done for the PCI layer.
44 struct pci_dev now looks like this:
45
46 struct pci_dev {
47 ...
48
49 struct device dev;
50 };
51
52 Note first that it is statically allocated. This means only one allocation on
53 device discovery. Note also that it is at the _end_ of struct pci_dev. This is
54 to make people think about what they're doing when switching between the bus
55 driver and the global driver; and to prevent against mindless casts between
56 the two.
57
58 The PCI bus layer freely accesses the fields of struct device. It knows about
59 the structure of struct pci_dev, and it should know the structure of struct
60 device. Individual PCI device drivers that have been converted the the current
61 driver model generally do not and should not touch the fields of struct device,
62 unless there is a strong compelling reason to do so.
63
64 This abstraction is prevention of unnecessary pain during transitional phases.
65 If the name of the field changes or is removed, then every downstream driver
66 will break. On the other hand, if only the bus layer (and not the device
67 layer) accesses struct device, it is only that layer that needs to change.
68
69
70 User Interface
71 ~~~~~~~~~~~~~~
72
73 By virtue of having a complete hierarchical view of all the devices in the
74 system, exporting a complete hierarchical view to userspace becomes relatively
75 easy. This has been accomplished by implementing a special purpose virtual
76 file system named sysfs. It is hence possible for the user to mount the
77 whole sysfs filesystem anywhere in userspace.
78
79 This can be done permanently by providing the following entry into the
80 /etc/fstab (under the provision that the mount point does exist, of course):
81
82 none /sys sysfs defaults 0 0
83
84 Or by hand on the command line:
85
86 # mount -t sysfs sysfs /sys
87
88 Whenever a device is inserted into the tree, a directory is created for it.
89 This directory may be populated at each layer of discovery - the global layer,
90 the bus layer, or the device layer.
91
92 The global layer currently creates two files - 'name' and 'power'. The
93 former only reports the name of the device. The latter reports the
94 current power state of the device. It will also be used to set the current
95 power state.
96
97 The bus layer may also create files for the devices it finds while probing the
98 bus. For example, the PCI layer currently creates 'irq' and 'resource' files
99 for each PCI device.
100
101 A device-specific driver may also export files in its directory to expose
102 device-specific data or tunable interfaces.
103
104 More information about the sysfs directory layout can be found in
105 the other documents in this directory and in the file
106 Documentation/filesystems/sysfs.txt.
107
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