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 to give 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