Documentation/driver-model/overview.txt - kernel/omap - Git at Google

 The Linux Kernel Device Model

 Patrick Mochel	<[email protected]>

 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 busses 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 a 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;
 };

 Note first that it is statically allocated. This means only one allocation on
 device discovery. Note also that it is at the _end_ of struct pci_dev. This is
 to make people think about what they're doing when switching between the bus
 driver and the global driver; and to prevent against mindless 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 strong compelling reason to do so.

 This abstraction is prevention of unnecessary pain during transitional phases.
 If the name of the field changes or is removed, then every downstream driver
 will break. On the other hand, if only the bus layer (and not the device
 layer) accesses struct device, it is only that 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. It is hence possible for the user to mount the
 whole sysfs filesystem anywhere in userspace.

 This can be done permanently by providing the following entry into the
 /etc/fstab (under the provision that the mount point does exist, of course):

 none     	/sys	sysfs    defaults		0	0

 Or by hand on the command line:

 # 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.txt.
	The Linux Kernel Device Model

	Patrick Mochel <[email protected]>

	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 busses 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 a 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;
	};

	Note first that it is statically allocated. This means only one allocation on
	device discovery. Note also that it is at the _end_ of struct pci_dev. This is
	to make people think about what they're doing when switching between the bus
	driver and the global driver; and to prevent against mindless 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 strong compelling reason to do so.

	This abstraction is prevention of unnecessary pain during transitional phases.
	If the name of the field changes or is removed, then every downstream driver
	will break. On the other hand, if only the bus layer (and not the device
	layer) accesses struct device, it is only that 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. It is hence possible for the user to mount the
	whole sysfs filesystem anywhere in userspace.

	This can be done permanently by providing the following entry into the
	/etc/fstab (under the provision that the mount point does exist, of course):

	none /sys sysfs defaults 0 0

	Or by hand on the command line:

	# 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.txt.