I\ :sup:`2`\ C and SMBus Subsystem================================== I\ :sup:`2`\ C (or without fancy typography, "I2C") is an acronym forthe "Inter-IC" bus, a simple bus protocol which is widely used where lowdata rate communications suffice. Since it's also a licensed trademark,some vendors use another name (such as "Two-Wire Interface", TWI) forthe same bus. I2C only needs two signals (SCL for clock, SDA for data),conserving board real estate and minimizing signal quality issues. MostI2C devices use seven bit addresses, and bus speeds of up to 400 kHz;there's a high speed extension (3.4 MHz) that's not yet found wide use.I2C is a multi-master bus; open drain signaling is used to arbitratebetween masters, as well as to handshake and to synchronize clocks fromslower clients. The Linux I2C programming interfaces support the master side of businteractions and the slave side. The programming interface isstructured around two kinds of driver, and two kinds of device. An I2C"Adapter Driver" abstracts the controller hardware; it binds to aphysical device (perhaps a PCI device or platform_device) and exposes a:c:type:`struct i2c_adapter <i2c_adapter>` representing eachI2C bus segment it manages. On each I2C bus segment will be I2C devicesrepresented by a :c:type:`struct i2c_client <i2c_client>`.Those devices will be bound to a :c:type:`struct i2c_driver<i2c_driver>`, which should follow the standard Linux driver model. Thereare functions to perform various I2C protocol operations; at this writingall such functions are usable only from task context. The System Management Bus (SMBus) is a sibling protocol. Most SMBussystems are also I2C conformant. The electrical constraints are tighterfor SMBus, and it standardizes particular protocol messages and idioms.Controllers that support I2C can also support most SMBus operations, butSMBus controllers don't support all the protocol options that an I2Ccontroller will. There are functions to perform various SMBus protocoloperations, either using I2C primitives or by issuing SMBus commands toi2c_adapter devices which don't support those I2C operations. .. kernel-doc:: include/linux/i2c.h   :internal: .. kernel-doc:: drivers/i2c/i2c-boardinfo.c   :functions: i2c_register_board_info .. kernel-doc:: drivers/i2c/i2c-core-base.c   :export: .. kernel-doc:: drivers/i2c/i2c-core-smbus.c   :export: