linux/Documentation/driver-model/bus.txt
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   1
   2Bus Types 
   3
   4Definition
   5~~~~~~~~~~
   6
   7struct bus_type {
   8        char                    * name;
   9
  10        struct subsystem        subsys;
  11        struct kset             drivers;
  12        struct kset             devices;
  13
  14        struct bus_attribute    * bus_attrs;
  15        struct device_attribute * dev_attrs;
  16        struct driver_attribute * drv_attrs;
  17
  18        int             (*match)(struct device * dev, struct device_driver * drv);
  19        int             (*hotplug) (struct device *dev, char **envp, 
  20                                    int num_envp, char *buffer, int buffer_size);
  21        int             (*suspend)(struct device * dev, pm_message_t state);
  22        int             (*resume)(struct device * dev);
  23};
  24
  25int bus_register(struct bus_type * bus);
  26
  27
  28Declaration
  29~~~~~~~~~~~
  30
  31Each bus type in the kernel (PCI, USB, etc) should declare one static
  32object of this type. They must initialize the name field, and may
  33optionally initialize the match callback.
  34
  35struct bus_type pci_bus_type = {
  36       .name    = "pci",
  37       .match   = pci_bus_match,
  38};
  39
  40The structure should be exported to drivers in a header file:
  41
  42extern struct bus_type pci_bus_type;
  43
  44
  45Registration
  46~~~~~~~~~~~~
  47
  48When a bus driver is initialized, it calls bus_register. This
  49initializes the rest of the fields in the bus object and inserts it
  50into a global list of bus types. Once the bus object is registered, 
  51the fields in it are usable by the bus driver. 
  52
  53
  54Callbacks
  55~~~~~~~~~
  56
  57match(): Attaching Drivers to Devices
  58~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  59
  60The format of device ID structures and the semantics for comparing
  61them are inherently bus-specific. Drivers typically declare an array
  62of device IDs of devices they support that reside in a bus-specific
  63driver structure. 
  64
  65The purpose of the match callback is provide the bus an opportunity to
  66determine if a particular driver supports a particular device by
  67comparing the device IDs the driver supports with the device ID of a
  68particular device, without sacrificing bus-specific functionality or
  69type-safety. 
  70
  71When a driver is registered with the bus, the bus's list of devices is
  72iterated over, and the match callback is called for each device that
  73does not have a driver associated with it. 
  74
  75
  76
  77Device and Driver Lists
  78~~~~~~~~~~~~~~~~~~~~~~~
  79
  80The lists of devices and drivers are intended to replace the local
  81lists that many buses keep. They are lists of struct devices and
  82struct device_drivers, respectively. Bus drivers are free to use the
  83lists as they please, but conversion to the bus-specific type may be
  84necessary. 
  85
  86The LDM core provides helper functions for iterating over each list.
  87
  88int bus_for_each_dev(struct bus_type * bus, struct device * start, void * data,
  89                     int (*fn)(struct device *, void *));
  90
  91int bus_for_each_drv(struct bus_type * bus, struct device_driver * start, 
  92                     void * data, int (*fn)(struct device_driver *, void *));
  93
  94These helpers iterate over the respective list, and call the callback
  95for each device or driver in the list. All list accesses are
  96synchronized by taking the bus's lock (read currently). The reference
  97count on each object in the list is incremented before the callback is
  98called; it is decremented after the next object has been obtained. The
  99lock is not held when calling the callback. 
 100
 101
 102sysfs
 103~~~~~~~~
 104There is a top-level directory named 'bus'.
 105
 106Each bus gets a directory in the bus directory, along with two default
 107directories:
 108
 109        /sys/bus/pci/
 110        |-- devices
 111        `-- drivers
 112
 113Drivers registered with the bus get a directory in the bus's drivers
 114directory:
 115
 116        /sys/bus/pci/
 117        |-- devices
 118        `-- drivers
 119            |-- Intel ICH
 120            |-- Intel ICH Joystick
 121            |-- agpgart
 122            `-- e100
 123
 124Each device that is discovered on a bus of that type gets a symlink in
 125the bus's devices directory to the device's directory in the physical
 126hierarchy:
 127
 128        /sys/bus/pci/
 129        |-- devices
 130        |   |-- 00:00.0 -> ../../../root/pci0/00:00.0
 131        |   |-- 00:01.0 -> ../../../root/pci0/00:01.0
 132        |   `-- 00:02.0 -> ../../../root/pci0/00:02.0
 133        `-- drivers
 134
 135
 136Exporting Attributes
 137~~~~~~~~~~~~~~~~~~~~
 138struct bus_attribute {
 139        struct attribute        attr;
 140        ssize_t (*show)(struct bus_type *, char * buf);
 141        ssize_t (*store)(struct bus_type *, const char * buf, size_t count);
 142};
 143
 144Bus drivers can export attributes using the BUS_ATTR macro that works
 145similarly to the DEVICE_ATTR macro for devices. For example, a definition 
 146like this:
 147
 148static BUS_ATTR(debug,0644,show_debug,store_debug);
 149
 150is equivalent to declaring:
 151
 152static bus_attribute bus_attr_debug;
 153
 154This can then be used to add and remove the attribute from the bus's
 155sysfs directory using:
 156
 157int bus_create_file(struct bus_type *, struct bus_attribute *);
 158void bus_remove_file(struct bus_type *, struct bus_attribute *);
 159
 160
 161