linux/Documentation/driver-model/bus.txt
<<
>>
Prefs
   1
   2Bus Types 
   3
   4Definition
   5~~~~~~~~~~
   6See the kerneldoc for the struct bus_type.
   7
   8int bus_register(struct bus_type * bus);
   9
  10
  11Declaration
  12~~~~~~~~~~~
  13
  14Each bus type in the kernel (PCI, USB, etc) should declare one static
  15object of this type. They must initialize the name field, and may
  16optionally initialize the match callback.
  17
  18struct bus_type pci_bus_type = {
  19       .name    = "pci",
  20       .match   = pci_bus_match,
  21};
  22
  23The structure should be exported to drivers in a header file:
  24
  25extern struct bus_type pci_bus_type;
  26
  27
  28Registration
  29~~~~~~~~~~~~
  30
  31When a bus driver is initialized, it calls bus_register. This
  32initializes the rest of the fields in the bus object and inserts it
  33into a global list of bus types. Once the bus object is registered, 
  34the fields in it are usable by the bus driver. 
  35
  36
  37Callbacks
  38~~~~~~~~~
  39
  40match(): Attaching Drivers to Devices
  41~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  42
  43The format of device ID structures and the semantics for comparing
  44them are inherently bus-specific. Drivers typically declare an array
  45of device IDs of devices they support that reside in a bus-specific
  46driver structure. 
  47
  48The purpose of the match callback is provide the bus an opportunity to
  49determine if a particular driver supports a particular device by
  50comparing the device IDs the driver supports with the device ID of a
  51particular device, without sacrificing bus-specific functionality or
  52type-safety. 
  53
  54When a driver is registered with the bus, the bus's list of devices is
  55iterated over, and the match callback is called for each device that
  56does not have a driver associated with it. 
  57
  58
  59
  60Device and Driver Lists
  61~~~~~~~~~~~~~~~~~~~~~~~
  62
  63The lists of devices and drivers are intended to replace the local
  64lists that many buses keep. They are lists of struct devices and
  65struct device_drivers, respectively. Bus drivers are free to use the
  66lists as they please, but conversion to the bus-specific type may be
  67necessary. 
  68
  69The LDM core provides helper functions for iterating over each list.
  70
  71int bus_for_each_dev(struct bus_type * bus, struct device * start, void * data,
  72                     int (*fn)(struct device *, void *));
  73
  74int bus_for_each_drv(struct bus_type * bus, struct device_driver * start, 
  75                     void * data, int (*fn)(struct device_driver *, void *));
  76
  77These helpers iterate over the respective list, and call the callback
  78for each device or driver in the list. All list accesses are
  79synchronized by taking the bus's lock (read currently). The reference
  80count on each object in the list is incremented before the callback is
  81called; it is decremented after the next object has been obtained. The
  82lock is not held when calling the callback. 
  83
  84
  85sysfs
  86~~~~~~~~
  87There is a top-level directory named 'bus'.
  88
  89Each bus gets a directory in the bus directory, along with two default
  90directories:
  91
  92        /sys/bus/pci/
  93        |-- devices
  94        `-- drivers
  95
  96Drivers registered with the bus get a directory in the bus's drivers
  97directory:
  98
  99        /sys/bus/pci/
 100        |-- devices
 101        `-- drivers
 102            |-- Intel ICH
 103            |-- Intel ICH Joystick
 104            |-- agpgart
 105            `-- e100
 106
 107Each device that is discovered on a bus of that type gets a symlink in
 108the bus's devices directory to the device's directory in the physical
 109hierarchy:
 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
 119Exporting Attributes
 120~~~~~~~~~~~~~~~~~~~~
 121struct 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
 127Bus drivers can export attributes using the BUS_ATTR macro that works
 128similarly to the DEVICE_ATTR macro for devices. For example, a definition 
 129like this:
 130
 131static BUS_ATTR(debug,0644,show_debug,store_debug);
 132
 133is equivalent to declaring:
 134
 135static bus_attribute bus_attr_debug;
 136
 137This can then be used to add and remove the attribute from the bus's
 138sysfs directory using:
 139
 140int bus_create_file(struct bus_type *, struct bus_attribute *);
 141void bus_remove_file(struct bus_type *, struct bus_attribute *);
 142
 143
 144
lxr.linux.no kindly hosted by Redpill Linpro AS, provider of Linux consulting and operations services since 1995.