TEXT   28

bus.txt

Guest on 25th July 2021 05:15:48 PM

  1.  
  2. Bus Types
  3.  
  4. Definition
  5. ~~~~~~~~~~
  6.  
  7. struct bus_type {
  8.         char                    * name;
  9.         rwlock_t                lock;
  10.         atomic_t                refcount;
  11.  
  12.         struct list_head        node;
  13.         struct list_head        devices;
  14.         struct list_head        drivers;
  15.  
  16.         struct driver_dir_entry dir;
  17.         struct driver_dir_entry device_dir;
  18.         struct driver_dir_entry driver_dir;
  19.  
  20.         int     (*match)        (struct device * dev, struct device_driver * drv);
  21.         struct device (*add)    (struct device * parent, char * bus_id);
  22. };
  23.  
  24. int bus_register(struct bus_type * bus);
  25.  
  26.  
  27. Declaration
  28. ~~~~~~~~~~~
  29.  
  30. Each bus type in the kernel (PCI, USB, etc) should declare one static
  31. object of this type. They must initialize the name field, and may
  32. optionally initialize the match callback.
  33.  
  34. struct bus_type pci_bus_type = {
  35.        name:    "pci",
  36.        match:   pci_bus_match,
  37. };
  38.  
  39. The structure should be exported to drivers in a header file:
  40.  
  41. extern struct bus_type pci_bus_type;
  42.  
  43.  
  44. Registration
  45. ~~~~~~~~~~~~
  46.  
  47. When a bus driver is initialized, it calls bus_register. This
  48. initializes the rest of the fields in the bus object and inserts it
  49. into a global list of bus types. Once the bus object is registered,
  50. the fields in it (e.g. the rwlock_t) are usable by the bus driver.
  51.  
  52.  
  53. Callbacks
  54. ~~~~~~~~~
  55.  
  56. match(): Attaching Drivers to Devices
  57. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  58.  
  59. The format of device ID structures and the semantics for comparing
  60. them are inherently bus-specific. Drivers typically declare an array
  61. of device IDs of device they support that reside in a bus-specific
  62. driver structure.
  63.  
  64. The purpose of the match callback is provide the bus an opportunity to
  65. determine if a particular driver supports a particular device by
  66. comparing the device IDs the driver supports with the device ID of a
  67. particular device, without sacrificing bus-specific functionality or
  68. type-safety.
  69.  
  70. When a driver is registered with the bus, the bus's list of devices is
  71. iterated over, and the match callback is called for each device that
  72. does not have a driver associated with it.
  73.  
  74. add(): Adding a child device
  75. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  76.  
  77. The add callback is available to notify the bus about a child device
  78. at a particular location.
  79.  
  80. The parent parameter is the parent device of the child to be added. If
  81. parent == NULL, the bus should add the device as a child of a default
  82. parent device or as a child of the root. This policy decision is up to
  83. the bus driver.
  84.  
  85. The format of the bus_id field should be consistent with the format of
  86. the bus_id field of the rest of the devices on the bus. This requires
  87. the caller to know the format.
  88.  
  89. On return, the bus driver should return a pointer to the device that
  90. was created. If the device was not created, the bus driver should
  91. return an appropriate error code. Refer to include/linux/err.h for
  92. helper functions to encode errors. Some sample code:
  93.  
  94. struct device * pci_bus_add(struct device * parent, char * bus_id)
  95. {
  96.         ...
  97.         /* the device already exists */
  98.         return ERR_PTR(-EEXIST);
  99.         ...
  100. }
  101.  
  102. The caller can check the return value using IS_ERR():
  103.  
  104.     struct device * newdev = pci_bus_type.add(parent,bus_id);
  105.     if (IS_ERR(newdev)) {
  106.         ...
  107.     }
  108.  
  109.  
  110. Device and Driver Lists
  111. ~~~~~~~~~~~~~~~~~~~~~~~
  112.  
  113. The lists of devices and drivers are intended to replace the local
  114. lists that many buses keep. They are lists of struct devices and
  115. struct device_drivers, respectively. Bus drivers are free to use the
  116. lists as they please, but conversion to the bus-specific type may be
  117. necessary.
  118.  
  119. The LDM core provides helper functions for iterating over each list.
  120.  
  121. int bus_for_each_dev(struct bus_type * bus, void * data,
  122.                      int (*callback)(struct device * dev, void * data));
  123. int bus_for_each_drv(struct bus_type * bus, void * data,
  124.                      int (*callback)(struct device_driver * drv, void * data));
  125.  
  126. These helpers iterate over the respective list, and call the callback
  127. for each device or driver in the list. All list accesses are
  128. synchronized by taking the bus's lock (read currently). The reference
  129. count on each object in the list is incremented before the callback is
  130. called; it is decremented after the next object has been obtained. The
  131. lock is not held when calling the callback.
  132.  
  133.  
  134. driverfs
  135. ~~~~~~~~
  136. There is a top-level directory named 'bus'.
  137.  
  138. Each bus gets a directory in the bus directory, along with two default
  139. directories:
  140.  
  141.         /sys/bus/pci/
  142.         |-- devices
  143.         `-- drivers
  144.  
  145. Drivers registered with the bus get a directory in the bus's drivers
  146. directory:
  147.  
  148.         /sys/bus/pci/
  149.         |-- devices
  150.         `-- drivers
  151.             |-- Intel ICH
  152.             |-- Intel ICH Joystick
  153.             |-- agpgart
  154.             `-- e100
  155.  
  156. Each device that is discovered a bus of that type gets a symlink in
  157. the bus's devices directory to the device's directory in the physical
  158. hierarchy:
  159.  
  160.         /sys/bus/pci/
  161.         |-- devices
  162.         |   |-- 00:00.0 -> ../../../root/pci0/00:00.0
  163.         |   |-- 00:01.0 -> ../../../root/pci0/00:01.0
  164.         |   `-- 00:02.0 -> ../../../root/pci0/00:02.0
  165.         `-- drivers
  166.  
  167.  
  168. Exporting Attributes
  169. ~~~~~~~~~~~~~~~~~~~~
  170. struct bus_attribute {
  171.         struct attribute        attr;
  172.         ssize_t (*show)(struct bus_type *, char * buf, size_t count, loff_t off);
  173.         ssize_t (*store)(struct bus_type *, const char * buf, size_t count, loff_t off);
  174. };
  175.  
  176. Bus drivers can export attributes using the BUS_ATTR macro that works
  177. similarly to the DEVICE_ATTR macro for devices. For example, a definition
  178. like this:
  179.  
  180. static BUS_ATTR(debug,0644,show_debug,store_debug);
  181.  
  182. is equivalent to declaring:
  183.  
  184. static bus_attribute bus_attr_debug;
  185.  
  186. This can then be used to add and remove the attribute from the bus's
  187. driverfs directory using:
  188.  
  189. int bus_create_file(struct bus_type *, struct bus_attribute *);
  190. void bus_remove_file(struct bus_type *, struct bus_attribute *);

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