linux/Documentation/driver-model/overview.txt

The Linux Kernel Device Model

Patrick Mochel	<mochel@osdl.org>

26 August 2002


Overview
~~~~~~~~

This driver model is a unification of all the current, disparate driver models
that are currently 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.

Current driver models implement some sort of tree-like structure (sometimes
just a list) for the devices they control. But, there is no linkage between
the different bus types.

A common data structure can provide this linkage with little overhead: when a
bus driver discovers a particular device, it can insert it into the global
tree as well as its local tree. In fact, the local tree becomes just a subset
of the global tree.

Common data fields can also be moved out of the local bus models into the
global model. Some of the manipulations of these fields can also be
consolidated. Most likely, manipulation functions will become a set
of helper functions, which the bus drivers wrap around to include any
bus-specific items.

The common device and bridge interface currently reflects the goals of the
modern PC: namely the ability to do seamless Plug and Play, power management,
and hot plug. (The model dictated by Intel and Microsoft (read: ACPI) ensures
us that any device in the system may fit any of these criteria.)

In reality, not every bus will be able to support such operations. But, most
buses will support a majority of those operations, and all future buses will.
In other words, a bus that doesn't support an operation is the exception,
instead of the other way around.



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

Common data fields have been moved out of individual bus layers into a common
data structure. But, 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 device;
};

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. PCI devices that have been converted generally do not touch the fields
of struct device. More precisely, device-specific drivers should not touch
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.