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diff --git a/Documentation/driver-model/overview.txt b/Documentation/driver-model/overview.txt new file mode 100644 index 00000000..07236ed9 --- /dev/null +++ b/Documentation/driver-model/overview.txt @@ -0,0 +1,107 @@ +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. + |