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114 lines
4.5 KiB
Text
114 lines
4.5 KiB
Text
The Linux Kernel Device Model
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Patrick Mochel <mochel@osdl.org>
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26 August 2002
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Overview
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~~~~~~~~
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This driver model is a unification of all the current, disparate driver models
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that are currently in the kernel. It is intended to augment the
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bus-specific drivers for bridges and devices by consolidating a set of data
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and operations into globally accessible data structures.
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Current driver models implement some sort of tree-like structure (sometimes
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just a list) for the devices they control. But, there is no linkage between
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the different bus types.
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A common data structure can provide this linkage with little overhead: when a
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bus driver discovers a particular device, it can insert it into the global
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tree as well as its local tree. In fact, the local tree becomes just a subset
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of the global tree.
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Common data fields can also be moved out of the local bus models into the
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global model. Some of the manipulations of these fields can also be
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consolidated. Most likely, manipulation functions will become a set
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of helper functions, which the bus drivers wrap around to include any
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bus-specific items.
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The common device and bridge interface currently reflects the goals of the
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modern PC: namely the ability to do seamless Plug and Play, power management,
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and hot plug. (The model dictated by Intel and Microsoft (read: ACPI) ensures
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us that any device in the system may fit any of these criteria.)
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In reality, not every bus will be able to support such operations. But, most
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buses will support a majority of those operations, and all future buses will.
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In other words, a bus that doesn't support an operation is the exception,
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instead of the other way around.
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Downstream Access
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~~~~~~~~~~~~~~~~~
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Common data fields have been moved out of individual bus layers into a common
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data structure. But, these fields must still be accessed by the bus layers,
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and sometimes by the device-specific drivers.
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Other bus layers are encouraged to do what has been done for the PCI layer.
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struct pci_dev now looks like this:
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struct pci_dev {
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...
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struct device device;
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};
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Note first that it is statically allocated. This means only one allocation on
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device discovery. Note also that it is at the _end_ of struct pci_dev. This is
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to make people think about what they're doing when switching between the bus
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driver and the global driver; and to prevent against mindless casts between
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the two.
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The PCI bus layer freely accesses the fields of struct device. It knows about
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the structure of struct pci_dev, and it should know the structure of struct
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device. PCI devices that have been converted generally do not touch the fields
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of struct device. More precisely, device-specific drivers should not touch
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fields of struct device unless there is a strong compelling reason to do so.
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This abstraction is prevention of unnecessary pain during transitional phases.
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If the name of the field changes or is removed, then every downstream driver
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will break. On the other hand, if only the bus layer (and not the device
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layer) accesses struct device, it is only that layer that needs to change.
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User Interface
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~~~~~~~~~~~~~~
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By virtue of having a complete hierarchical view of all the devices in the
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system, exporting a complete hierarchical view to userspace becomes relatively
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easy. This has been accomplished by implementing a special purpose virtual
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file system named sysfs. It is hence possible for the user to mount the
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whole sysfs filesystem anywhere in userspace.
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This can be done permanently by providing the following entry into the
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/etc/fstab (under the provision that the mount point does exist, of course):
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none /sys sysfs defaults 0 0
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Or by hand on the command line:
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# mount -t sysfs sysfs /sys
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Whenever a device is inserted into the tree, a directory is created for it.
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This directory may be populated at each layer of discovery - the global layer,
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the bus layer, or the device layer.
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The global layer currently creates two files - 'name' and 'power'. The
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former only reports the name of the device. The latter reports the
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current power state of the device. It will also be used to set the current
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power state.
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The bus layer may also create files for the devices it finds while probing the
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bus. For example, the PCI layer currently creates 'irq' and 'resource' files
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for each PCI device.
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A device-specific driver may also export files in its directory to expose
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device-specific data or tunable interfaces.
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More information about the sysfs directory layout can be found in
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the other documents in this directory and in the file
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Documentation/filesystems/sysfs.txt.
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