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