115 lines
4.5 KiB
Plaintext
115 lines
4.5 KiB
Plaintext
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.
|
|
|