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binding.txt

Guest on 1st August 2021 06:04:25 AM

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

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