TEXT   43

class.txt

Guest on 1st August 2021 06:05:46 AM

  1.  
  2. Device Classes
  3.  
  4.  
  5. Introduction
  6. ~~~~~~~~~~~~
  7. A device class describes a type of device, like an audio or network
  8. device. The following device classes have been identified:
  9.  
  10. <Insert List of Device Classes Here>
  11.  
  12.  
  13. Each device class defines a set of semantics and a programming interface
  14. that devices of that class adhere to. Device drivers are the
  15. implementation of that programming interface for a particular device on
  16. a particular bus.
  17.  
  18. Device classes are agnostic with respect to what bus a device resides
  19. on.
  20.  
  21.  
  22. Programming Interface
  23. ~~~~~~~~~~~~~~~~~~~~~
  24. The device class structure looks like:
  25.  
  26.  
  27. typedef int (*devclass_add)(struct device *);
  28. typedef void (*devclass_remove)(struct device *);
  29.  
  30. See the kerneldoc for the struct class.
  31.  
  32. A typical device class definition would look like:
  33.  
  34. struct device_class input_devclass = {
  35.         .name           = "input",
  36.         .add_device     = input_add_device,
  37.         .remove_device  = input_remove_device,
  38. };
  39.  
  40. Each device class structure should be exported in a header file so it
  41. can be used by drivers, extensions and interfaces.
  42.  
  43. Device classes are registered and unregistered with the core using:
  44.  
  45. int devclass_register(struct device_class * cls);
  46. void devclass_unregister(struct device_class * cls);
  47.  
  48.  
  49. Devices
  50. ~~~~~~~
  51. As devices are bound to drivers, they are added to the device class
  52. that the driver belongs to. Before the driver model core, this would
  53. typically happen during the driver's probe() callback, once the device
  54. has been initialized. It now happens after the probe() callback
  55. finishes from the core.
  56.  
  57. The device is enumerated in the class. Each time a device is added to
  58. the class, the class's devnum field is incremented and assigned to the
  59. device. The field is never decremented, so if the device is removed
  60. from the class and re-added, it will receive a different enumerated
  61. value.
  62.  
  63. The class is allowed to create a class-specific structure for the
  64. device and store it in the device's class_data pointer.
  65.  
  66. There is no list of devices in the device class. Each driver has a
  67. list of devices that it supports. The device class has a list of
  68. drivers of that particular class. To access all of the devices in the
  69. class, iterate over the device lists of each driver in the class.
  70.  
  71.  
  72. Device Drivers
  73. ~~~~~~~~~~~~~~
  74. Device drivers are added to device classes when they are registered
  75. with the core. A driver specifies the class it belongs to by setting
  76. the struct device_driver::devclass field.
  77.  
  78.  
  79. sysfs directory structure
  80. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  81. There is a top-level sysfs directory named 'class'.
  82.  
  83. Each class gets a directory in the class directory, along with two
  84. default subdirectories:
  85.  
  86.         class/
  87.         `-- input
  88.             |-- devices
  89.             `-- drivers
  90.  
  91.  
  92. Drivers registered with the class get a symlink in the drivers/ directory
  93. that points to the driver's directory (under its bus directory):
  94.  
  95.    class/
  96.    `-- input
  97.        |-- devices
  98.        `-- drivers
  99.            `-- usb:usb_mouse -> ../../../bus/drivers/usb_mouse/
  100.  
  101.  
  102. Each device gets a symlink in the devices/ directory that points to the
  103. device's directory in the physical hierarchy:
  104.  
  105.    class/
  106.    `-- input
  107.        |-- devices
  108.        |   `-- 1 -> ../../../root/pci0/00:1f.0/usb_bus/00:1f.2-1:0/
  109.        `-- drivers
  110.  
  111.  
  112. Exporting Attributes
  113. ~~~~~~~~~~~~~~~~~~~~
  114. struct devclass_attribute {
  115.         struct attribute        attr;
  116.         ssize_t (*show)(struct device_class *, char * buf, size_t count, loff_t off);
  117.         ssize_t (*store)(struct device_class *, const char * buf, size_t count, loff_t off);
  118. };
  119.  
  120. Class drivers can export attributes using the DEVCLASS_ATTR macro that works
  121. similarly to the DEVICE_ATTR macro for devices. For example, a definition
  122. like this:
  123.  
  124. static DEVCLASS_ATTR(debug,0644,show_debug,store_debug);
  125.  
  126. is equivalent to declaring:
  127.  
  128. static devclass_attribute devclass_attr_debug;
  129.  
  130. The bus driver can add and remove the attribute from the class's
  131. sysfs directory using:
  132.  
  133. int devclass_create_file(struct device_class *, struct devclass_attribute *);
  134. void devclass_remove_file(struct device_class *, struct devclass_attribute *);
  135.  
  136. In the example above, the file will be named 'debug' in placed in the
  137. class's directory in sysfs.
  138.  
  139.  
  140. Interfaces
  141. ~~~~~~~~~~
  142. There may exist multiple mechanisms for accessing the same device of a
  143. particular class type. Device interfaces describe these mechanisms.
  144.  
  145. When a device is added to a device class, the core attempts to add it
  146. to every interface that is registered with the device class.

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