=============================
The Linux Kernel Device Model
=============================

Patrick Mochel	<mochel@digitalimplant.org>

Drafted 26 August 2002
Updated 31 January 2006


Overview
~~~~~~~~

The Linux Kernel Driver Model is a unification of all the disparate driver
models that were previously used in the kernel. It is intended to augment the
bus-specific drivers for bridges and devices by consolidating a set of data
and operations into globally accessible data structures.

Traditional driver models implemented some sort of tree-like structure
(sometimes just a list) for the devices they control. There wasn't any
uniformity across the different bus types.

The current driver model provides a common, uniform data model for describing
a bus and the devices that can appear under the bus. The unified bus
model includes a set of common attributes which all buses carry, and a set
of common callbacks, such as device discovery during bus probing, bus
shutdown, bus power management, etc.

The common device and bridge interface reflects the goals of the modern
computer: namely the ability to do seamless device "plug and play", power
management, and hot plug. In particular, the model dictated by Intel and
Microsoft (namely ACPI) ensures that almost every device on almost any bus
on an x86-compatible system can work within this paradigm.  Of course,
not every bus is able to support all such operations, although most
buses support most of those operations.


Downstream Access
~~~~~~~~~~~~~~~~~

Common data fields have been moved out of individual bus layers into a common
data structure. These fields must still be accessed by the bus layers,
and sometimes by the device-specific drivers.

Other bus layers are encouraged to do what has been done for the PCI layer.
struct pci_dev now looks like this::

  struct pci_dev {
	...

	struct device dev;     /* Generic device interface */
	...
  };

Note first that the struct device dev within the struct pci_dev is
statically allocated. This means only one allocation on device discovery.

Note also that that struct device dev is not necessarily defined at the
front of the pci_dev structure.  This is to make people think about what
they're doing when switching between the bus driver and the global driver,
and to discourage meaningless and incorrect casts between the two.

The PCI bus layer freely accesses the fields of struct device. It knows about
the structure of struct pci_dev, and it should know the structure of struct
device. Individual PCI device drivers that have been converted to the current
driver model generally do not and should not touch the fields of struct device,
unless there is a compelling reason to do so.

The above abstraction prevents unnecessary pain during transitional phases.
If it were not done this way, then when a field was renamed or removed, every
downstream driver would break.  On the other hand, if only the bus layer
(and not the device layer) accesses the struct device, it is only the bus
layer that needs to change.


User Interface
~~~~~~~~~~~~~~

By virtue of having a complete hierarchical view of all the devices in the
system, exporting a complete hierarchical view to userspace becomes relatively
easy. This has been accomplished by implementing a special purpose virtual
file system named sysfs.

Almost all mainstream Linux distros mount this filesystem automatically; you
can see some variation of the following in the output of the "mount" command::

  $ mount
  ...
  none on /sys type sysfs (rw,noexec,nosuid,nodev)
  ...
  $

The auto-mounting of sysfs is typically accomplished by an entry similar to
the following in the /etc/fstab file::

  none     	/sys	sysfs    defaults	  	0 0

or something similar in the /lib/init/fstab file on Debian-based systems::

  none            /sys    sysfs    nodev,noexec,nosuid    0 0

If sysfs is not automatically mounted, you can always do it manually with::

	# mount -t sysfs sysfs /sys

Whenever a device is inserted into the tree, a directory is created for it.
This directory may be populated at each layer of discovery - the global layer,
the bus layer, or the device layer.

The global layer currently creates two files - 'name' and 'power'. The
former only reports the name of the device. The latter reports the
current power state of the device. It will also be used to set the current
power state.

The bus layer may also create files for the devices it finds while probing the
bus. For example, the PCI layer currently creates 'irq' and 'resource' files
for each PCI device.

A device-specific driver may also export files in its directory to expose
device-specific data or tunable interfaces.

More information about the sysfs directory layout can be found in
the other documents in this directory and in the file
Documentation/filesystems/sysfs.rst.