1I\ :sup:`2`\ C and SMBus Subsystem
2==================================
3
4I\ :sup:`2`\ C (or without fancy typography, "I2C") is an acronym for
5the "Inter-IC" bus, a simple bus protocol which is widely used where low
6data rate communications suffice. Since it's also a licensed trademark,
7some vendors use another name (such as "Two-Wire Interface", TWI) for
8the same bus. I2C only needs two signals (SCL for clock, SDA for data),
9conserving board real estate and minimizing signal quality issues. Most
10I2C devices use seven bit addresses, and bus speeds of up to 400 kHz;
11there's a high speed extension (3.4 MHz) that's not yet found wide use.
12I2C is a multi-master bus; open drain signaling is used to arbitrate
13between masters, as well as to handshake and to synchronize clocks from
14slower clients.
15
16The Linux I2C programming interfaces support only the master side of bus
17interactions, not the slave side. The programming interface is
18structured around two kinds of driver, and two kinds of device. An I2C
19"Adapter Driver" abstracts the controller hardware; it binds to a
20physical device (perhaps a PCI device or platform_device) and exposes a
21:c:type:`struct i2c_adapter <i2c_adapter>` representing each
22I2C bus segment it manages. On each I2C bus segment will be I2C devices
23represented by a :c:type:`struct i2c_client <i2c_client>`.
24Those devices will be bound to a :c:type:`struct i2c_driver
25<i2c_driver>`, which should follow the standard Linux driver
26model. (At this writing, a legacy model is more widely used.) There are
27functions to perform various I2C protocol operations; at this writing
28all such functions are usable only from task context.
29
30The System Management Bus (SMBus) is a sibling protocol. Most SMBus
31systems are also I2C conformant. The electrical constraints are tighter
32for SMBus, and it standardizes particular protocol messages and idioms.
33Controllers that support I2C can also support most SMBus operations, but
34SMBus controllers don't support all the protocol options that an I2C
35controller will. There are functions to perform various SMBus protocol
36operations, either using I2C primitives or by issuing SMBus commands to
37i2c_adapter devices which don't support those I2C operations.
38
39.. kernel-doc:: include/linux/i2c.h
40   :internal:
41
42.. kernel-doc:: drivers/i2c/i2c-boardinfo.c
43   :functions: i2c_register_board_info
44
45.. kernel-doc:: drivers/i2c/i2c-core.c
46   :export:
47