linux/Documentation/driver-model/binding.txt
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   2Driver Binding
   3
   4Driver binding is the process of associating a device with a device
   5driver that can control it. Bus drivers have typically handled this
   6because there have been bus-specific structures to represent the
   7devices and the drivers. With generic device and device driver
   8structures, most of the binding can take place using common code.
   9
  10
  11Bus
  12~~~
  13
  14The bus type structure contains a list of all devices that are on that bus
  15type in the system. When device_register is called for a device, it is
  16inserted into the end of this list. The bus object also contains a
  17list of all drivers of that bus type. When driver_register is called
  18for a driver, it is inserted at the end of this list. These are the
  19two events which trigger driver binding.
  20
  21
  22device_register
  23~~~~~~~~~~~~~~~
  24
  25When a new device is added, the bus's list of drivers is iterated over
  26to find one that supports it. In order to determine that, the device
  27ID of the device must match one of the device IDs that the driver
  28supports. The format and semantics for comparing IDs is bus-specific. 
  29Instead of trying to derive a complex state machine and matching
  30algorithm, it is up to the bus driver to provide a callback to compare
  31a device against the IDs of a driver. The bus returns 1 if a match was
  32found; 0 otherwise.
  33
  34int match(struct device * dev, struct device_driver * drv);
  35
  36If a match is found, the device's driver field is set to the driver
  37and the driver's probe callback is called. This gives the driver a
  38chance to verify that it really does support the hardware, and that
  39it's in a working state. 
  40
  41Device Class
  42~~~~~~~~~~~~
  43
  44Upon the successful completion of probe, the device is registered with
  45the class to which it belongs. Device drivers belong to one and only one
  46class, and that is set in the driver's devclass field. 
  47devclass_add_device is called to enumerate the device within the class
  48and actually register it with the class, which happens with the
  49class's register_dev callback.
  50
  51
  52Driver
  53~~~~~~
  54
  55When a driver is attached to a device, the device is inserted into the
  56driver's list of devices. 
  57
  58
  59sysfs
  60~~~~~
  61
  62A symlink is created in the bus's 'devices' directory that points to
  63the device's directory in the physical hierarchy.
  64
  65A symlink is created in the driver's 'devices' directory that points
  66to the device's directory in the physical hierarchy.
  67
  68A directory for the device is created in the class's directory. A
  69symlink is created in that directory that points to the device's
  70physical location in the sysfs tree. 
  71
  72A symlink can be created (though this isn't done yet) in the device's
  73physical directory to either its class directory, or the class's
  74top-level directory. One can also be created to point to its driver's
  75directory also. 
  76
  77
  78driver_register
  79~~~~~~~~~~~~~~~
  80
  81The process is almost identical for when a new driver is added. 
  82The bus's list of devices is iterated over to find a match. Devices
  83that already have a driver are skipped. All the devices are iterated
  84over, to bind as many devices as possible to the driver.
  85
  86
  87Removal
  88~~~~~~~
  89
  90When a device is removed, the reference count for it will eventually
  91go to 0. When it does, the remove callback of the driver is called. It
  92is removed from the driver's list of devices and the reference count
  93of the driver is decremented. All symlinks between the two are removed.
  94
  95When a driver is removed, the list of devices that it supports is
  96iterated over, and the driver's remove callback is called for each
  97one. The device is removed from that list and the symlinks removed. 
  98
  99
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