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