1=============================
2The Linux Kernel Device Model
3=============================
4
5Patrick Mochel	<mochel@digitalimplant.org>
6
7Drafted 26 August 2002
8Updated 31 January 2006
9
10
11Overview
12~~~~~~~~
13
14The Linux Kernel Driver Model is a unification of all the disparate driver
15models that were previously used in the kernel. It is intended to augment the
16bus-specific drivers for bridges and devices by consolidating a set of data
17and operations into globally accessible data structures.
18
19Traditional driver models implemented some sort of tree-like structure
20(sometimes just a list) for the devices they control. There wasn't any
21uniformity across the different bus types.
22
23The current driver model provides a common, uniform data model for describing
24a bus and the devices that can appear under the bus. The unified bus
25model includes a set of common attributes which all busses carry, and a set
26of common callbacks, such as device discovery during bus probing, bus
27shutdown, bus power management, etc.
28
29The common device and bridge interface reflects the goals of the modern
30computer: namely the ability to do seamless device "plug and play", power
31management, and hot plug. In particular, the model dictated by Intel and
32Microsoft (namely ACPI) ensures that almost every device on almost any bus
33on an x86-compatible system can work within this paradigm.  Of course,
34not every bus is able to support all such operations, although most
35buses support most of those operations.
36
37
38Downstream Access
39~~~~~~~~~~~~~~~~~
40
41Common data fields have been moved out of individual bus layers into a common
42data structure. These fields must still be accessed by the bus layers,
43and sometimes by the device-specific drivers.
44
45Other bus layers are encouraged to do what has been done for the PCI layer.
46struct pci_dev now looks like this::
47
48  struct pci_dev {
49	...
50
51	struct device dev;     /* Generic device interface */
52	...
53  };
54
55Note first that the struct device dev within the struct pci_dev is
56statically allocated. This means only one allocation on device discovery.
57
58Note also that that struct device dev is not necessarily defined at the
59front of the pci_dev structure.  This is to make people think about what
60they're doing when switching between the bus driver and the global driver,
61and to discourage meaningless and incorrect casts between the two.
62
63The PCI bus layer freely accesses the fields of struct device. It knows about
64the structure of struct pci_dev, and it should know the structure of struct
65device. Individual PCI device drivers that have been converted to the current
66driver model generally do not and should not touch the fields of struct device,
67unless there is a compelling reason to do so.
68
69The above abstraction prevents unnecessary pain during transitional phases.
70If it were not done this way, then when a field was renamed or removed, every
71downstream driver would break.  On the other hand, if only the bus layer
72(and not the device layer) accesses the struct device, it is only the bus
73layer that needs to change.
74
75
76User Interface
77~~~~~~~~~~~~~~
78
79By virtue of having a complete hierarchical view of all the devices in the
80system, exporting a complete hierarchical view to userspace becomes relatively
81easy. This has been accomplished by implementing a special purpose virtual
82file system named sysfs.
83
84Almost all mainstream Linux distros mount this filesystem automatically; you
85can see some variation of the following in the output of the "mount" command::
86
87  $ mount
88  ...
89  none on /sys type sysfs (rw,noexec,nosuid,nodev)
90  ...
91  $
92
93The auto-mounting of sysfs is typically accomplished by an entry similar to
94the following in the /etc/fstab file::
95
96  none     	/sys	sysfs    defaults	  	0 0
97
98or something similar in the /lib/init/fstab file on Debian-based systems::
99
100  none            /sys    sysfs    nodev,noexec,nosuid    0 0
101
102If sysfs is not automatically mounted, you can always do it manually with::
103
104	# mount -t sysfs sysfs /sys
105
106Whenever a device is inserted into the tree, a directory is created for it.
107This directory may be populated at each layer of discovery - the global layer,
108the bus layer, or the device layer.
109
110The global layer currently creates two files - 'name' and 'power'. The
111former only reports the name of the device. The latter reports the
112current power state of the device. It will also be used to set the current
113power state.
114
115The bus layer may also create files for the devices it finds while probing the
116bus. For example, the PCI layer currently creates 'irq' and 'resource' files
117for each PCI device.
118
119A device-specific driver may also export files in its directory to expose
120device-specific data or tunable interfaces.
121
122More information about the sysfs directory layout can be found in
123the other documents in this directory and in the file
124Documentation/filesystems/sysfs.rst.
125