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 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;     /* Generic device interface */
  50        ...
  51};
  52
  53Note first that the struct device dev within the struct pci_dev is
  54statically allocated. This means only one allocation on device discovery.
  55
  56Note also that that struct device dev is not necessarily defined at the
  57front of the pci_dev structure.  This is to make people think about what
  58they're doing when switching between the bus driver and the global driver,
  59and to discourage meaningless and incorrect casts between the two.
  60
  61The PCI bus layer freely accesses the fields of struct device. It knows about
  62the structure of struct pci_dev, and it should know the structure of struct
  63device. Individual PCI device drivers that have been converted to the current
  64driver model generally do not and should not touch the fields of struct device,
  65unless there is a compelling reason to do so.
  66
  67The above abstraction prevents unnecessary pain during transitional phases.
  68If it were not done this way, then when a field was renamed or removed, every
  69downstream driver would break.  On the other hand, if only the bus layer
  70(and not the device layer) accesses the struct device, it is only the bus
  71layer that needs to change.
  72
  73
  74User Interface
  75~~~~~~~~~~~~~~
  76
  77By virtue of having a complete hierarchical view of all the devices in the
  78system, exporting a complete hierarchical view to userspace becomes relatively
  79easy. This has been accomplished by implementing a special purpose virtual
  80file system named sysfs.
  81
  82Almost all mainstream Linux distros mount this filesystem automatically; you
  83can see some variation of the following in the output of the "mount" command:
  84
  85$ mount
  86...
  87none on /sys type sysfs (rw,noexec,nosuid,nodev)
  88...
  89$
  90
  91The auto-mounting of sysfs is typically accomplished by an entry similar to
  92the following in the /etc/fstab file:
  93
  94none            /sys    sysfs    defaults               0 0
  95
  96or something similar in the /lib/init/fstab file on Debian-based systems:
  97
  98none            /sys    sysfs    nodev,noexec,nosuid    0 0
  99
 100If sysfs is not automatically mounted, you can always do it manually with:
 101
 102# mount -t sysfs sysfs /sys
 103
 104Whenever a device is inserted into the tree, a directory is created for it.
 105This directory may be populated at each layer of discovery - the global layer,
 106the bus layer, or the device layer.
 107
 108The global layer currently creates two files - 'name' and 'power'. The
 109former only reports the name of the device. The latter reports the
 110current power state of the device. It will also be used to set the current
 111power state. 
 112
 113The bus layer may also create files for the devices it finds while probing the
 114bus. For example, the PCI layer currently creates 'irq' and 'resource' files
 115for each PCI device.
 116
 117A device-specific driver may also export files in its directory to expose
 118device-specific data or tunable interfaces.
 119
 120More information about the sysfs directory layout can be found in
 121the other documents in this directory and in the file 
 122Documentation/filesystems/sysfs.txt.
 123
 124
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