TEXT   355

bus.txt

Guest on 1st August 2021 06:05:14 AM

  1.  
  2. Bus Types
  3.  
  4. Definition
  5. ~~~~~~~~~~
  6. See the kerneldoc for the struct bus_type.
  7.  
  8. int bus_register(struct bus_type * bus);
  9.  
  10.  
  11. Declaration
  12. ~~~~~~~~~~~
  13.  
  14. Each bus type in the kernel (PCI, USB, etc) should declare one static
  15. object of this type. They must initialize the name field, and may
  16. optionally initialize the match callback.
  17.  
  18. struct bus_type pci_bus_type = {
  19.        .name    = "pci",
  20.        .match   = pci_bus_match,
  21. };
  22.  
  23. The structure should be exported to drivers in a header file:
  24.  
  25. extern struct bus_type pci_bus_type;
  26.  
  27.  
  28. Registration
  29. ~~~~~~~~~~~~
  30.  
  31. When a bus driver is initialized, it calls bus_register. This
  32. initializes the rest of the fields in the bus object and inserts it
  33. into a global list of bus types. Once the bus object is registered,
  34. the fields in it are usable by the bus driver.
  35.  
  36.  
  37. Callbacks
  38. ~~~~~~~~~
  39.  
  40. match(): Attaching Drivers to Devices
  41. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  42.  
  43. The format of device ID structures and the semantics for comparing
  44. them are inherently bus-specific. Drivers typically declare an array
  45. of device IDs of devices they support that reside in a bus-specific
  46. driver structure.
  47.  
  48. The purpose of the match callback is to give the bus an opportunity to
  49. determine if a particular driver supports a particular device by
  50. comparing the device IDs the driver supports with the device ID of a
  51. particular device, without sacrificing bus-specific functionality or
  52. type-safety.
  53.  
  54. When a driver is registered with the bus, the bus's list of devices is
  55. iterated over, and the match callback is called for each device that
  56. does not have a driver associated with it.
  57.  
  58.  
  59.  
  60. Device and Driver Lists
  61. ~~~~~~~~~~~~~~~~~~~~~~~
  62.  
  63. The lists of devices and drivers are intended to replace the local
  64. lists that many buses keep. They are lists of struct devices and
  65. struct device_drivers, respectively. Bus drivers are free to use the
  66. lists as they please, but conversion to the bus-specific type may be
  67. necessary.
  68.  
  69. The LDM core provides helper functions for iterating over each list.
  70.  
  71. int bus_for_each_dev(struct bus_type * bus, struct device * start, void * data,
  72.                      int (*fn)(struct device *, void *));
  73.  
  74. int bus_for_each_drv(struct bus_type * bus, struct device_driver * start,
  75.                      void * data, int (*fn)(struct device_driver *, void *));
  76.  
  77. These helpers iterate over the respective list, and call the callback
  78. for each device or driver in the list. All list accesses are
  79. synchronized by taking the bus's lock (read currently). The reference
  80. count on each object in the list is incremented before the callback is
  81. called; it is decremented after the next object has been obtained. The
  82. lock is not held when calling the callback.
  83.  
  84.  
  85. sysfs
  86. ~~~~~~~~
  87. There is a top-level directory named 'bus'.
  88.  
  89. Each bus gets a directory in the bus directory, along with two default
  90. directories:
  91.  
  92.         /sys/bus/pci/
  93.         |-- devices
  94.         `-- drivers
  95.  
  96. Drivers registered with the bus get a directory in the bus's drivers
  97. directory:
  98.  
  99.         /sys/bus/pci/
  100.         |-- devices
  101.         `-- drivers
  102.             |-- Intel ICH
  103.             |-- Intel ICH Joystick
  104.             |-- agpgart
  105.             `-- e100
  106.  
  107. Each device that is discovered on a bus of that type gets a symlink in
  108. the bus's devices directory to the device's directory in the physical
  109. hierarchy:
  110.  
  111.         /sys/bus/pci/
  112.         |-- devices
  113.         |   |-- 00:00.0 -> ../../../root/pci0/00:00.0
  114.         |   |-- 00:01.0 -> ../../../root/pci0/00:01.0
  115.         |   `-- 00:02.0 -> ../../../root/pci0/00:02.0
  116.         `-- drivers
  117.  
  118.  
  119. Exporting Attributes
  120. ~~~~~~~~~~~~~~~~~~~~
  121. struct bus_attribute {
  122.         struct attribute        attr;
  123.         ssize_t (*show)(struct bus_type *, char * buf);
  124.         ssize_t (*store)(struct bus_type *, const char * buf, size_t count);
  125. };
  126.  
  127. Bus drivers can export attributes using the BUS_ATTR_RW macro that works
  128. similarly to the DEVICE_ATTR_RW macro for devices. For example, a
  129. definition like this:
  130.  
  131. static BUS_ATTR_RW(debug);
  132.  
  133. is equivalent to declaring:
  134.  
  135. static bus_attribute bus_attr_debug;
  136.  
  137. This can then be used to add and remove the attribute from the bus's
  138. sysfs directory using:
  139.  
  140. int bus_create_file(struct bus_type *, struct bus_attribute *);
  141. void bus_remove_file(struct bus_type *, struct bus_attribute *);

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