mirror of
https://mirrors.bfsu.edu.cn/git/linux.git
synced 2024-11-16 16:54:20 +08:00
670e9f34ee
Remove many duplicated words under Documentation/ and do other small
cleanups.
Examples:
"and and" --> "and"
"in in" --> "in"
"the the" --> "the"
"the the" --> "to the"
...
Signed-off-by: Paolo Ornati <ornati@fastwebnet.it>
Signed-off-by: Adrian Bunk <bunk@stusta.de>
108 lines
4.2 KiB
Plaintext
108 lines
4.2 KiB
Plaintext
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 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.
|
|
|