linux/Documentation/driver-model/overview.txt
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   1The Linux Kernel Device Model
   2
   3Patrick Mochel  <mochel@digitalimplant.org>
   4
   5Drafted 26 August 2002
   6Updated 31 January 2006
   7
   8
   9Overview
  10~~~~~~~~
  11
  12The Linux Kernel Driver Model is a unification of all the disparate driver
  13models that were previously used in the kernel. It is intended to augment the
  14bus-specific drivers for bridges and devices by consolidating a set of data
  15and operations into globally accessible data structures.
  16
  17Traditional driver models implemented some sort of tree-like structure
  18(sometimes just a list) for the devices they control. There wasn't any
  19uniformity across the different bus types.
  20
  21The current driver model provides a common, uniform data model for describing
  22a bus and the devices that can appear under the bus. The unified bus
  23model includes a set of common attributes which all busses carry, and a set
  24of common callbacks, such as device discovery during bus probing, bus
  25shutdown, bus power management, etc.
  26
  27The common device and bridge interface reflects the goals of the modern
  28computer: namely the ability to do seamless device "plug and play", power
  29management, and hot plug. In particular, the model dictated by Intel and
  30Microsoft (namely ACPI) ensures that almost every device on almost any bus
  31on an x86-compatible system can work within this paradigm.  Of course,
  32not every bus is able to support all such operations, although most
  33buses support a most of those operations.
  34
  35
  36Downstream Access
  37~~~~~~~~~~~~~~~~~
  38
  39Common data fields have been moved out of individual bus layers into a common
  40data structure. These fields must still be accessed by the bus layers,
  41and sometimes by the device-specific drivers.
  42
  43Other bus layers are encouraged to do what has been done for the PCI layer.
  44struct pci_dev now looks like this:
  45
  46struct pci_dev {
  47        ...
  48
  49        struct device dev;
  50};
  51
  52Note first that it is statically allocated. This means only one allocation on
  53device discovery. Note also that it is at the _end_ of struct pci_dev. This is
  54to make people think about what they're doing when switching between the bus
  55driver and the global driver; and to prevent against mindless casts between
  56the two.
  57
  58The PCI bus layer freely accesses the fields of struct device. It knows about
  59the structure of struct pci_dev, and it should know the structure of struct
  60device. Individual PCI device drivers that have been converted to the current
  61driver model generally do not and should not touch the fields of struct device,
  62unless there is a strong compelling reason to do so.
  63
  64This abstraction is prevention of unnecessary pain during transitional phases.
  65If the name of the field changes or is removed, then every downstream driver
  66will break. On the other hand, if only the bus layer (and not the device
  67layer) accesses struct device, it is only that layer that needs to change.
  68
  69
  70User Interface
  71~~~~~~~~~~~~~~
  72
  73By virtue of having a complete hierarchical view of all the devices in the
  74system, exporting a complete hierarchical view to userspace becomes relatively
  75easy. This has been accomplished by implementing a special purpose virtual
  76file system named sysfs. It is hence possible for the user to mount the
  77whole sysfs filesystem anywhere in userspace.
  78
  79This can be done permanently by providing the following entry into the
  80/etc/fstab (under the provision that the mount point does exist, of course):
  81
  82none            /sys    sysfs    defaults               0       0
  83
  84Or by hand on the command line:
  85
  86# mount -t sysfs sysfs /sys
  87
  88Whenever a device is inserted into the tree, a directory is created for it.
  89This directory may be populated at each layer of discovery - the global layer,
  90the bus layer, or the device layer.
  91
  92The global layer currently creates two files - 'name' and 'power'. The
  93former only reports the name of the device. The latter reports the
  94current power state of the device. It will also be used to set the current
  95power state. 
  96
  97The bus layer may also create files for the devices it finds while probing the
  98bus. For example, the PCI layer currently creates 'irq' and 'resource' files
  99for each PCI device.
 100
 101A device-specific driver may also export files in its directory to expose
 102device-specific data or tunable interfaces.
 103
 104More information about the sysfs directory layout can be found in
 105the other documents in this directory and in the file 
 106Documentation/filesystems/sysfs.txt.
 107
 108