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+=============================
+Introduction to I2C and SMBus
+=============================
+
+I²C (pronounce: I squared C and written I2C in the kernel documentation) is
+a protocol developed by Philips. It is a slow two-wire protocol (variable
+speed, up to 400 kHz), with a high speed extension (3.4 MHz).  It provides
+an inexpensive bus for connecting many types of devices with infrequent or
+low bandwidth communications needs.  I2C is widely used with embedded
+systems.  Some systems use variants that don't meet branding requirements,
+and so are not advertised as being I2C but come under different names,
+e.g. TWI (Two Wire Interface), IIC.
+
+The latest official I2C specification is the `"I2C-bus specification and user
+manual" (UM10204) <https://www.nxp.com/webapp/Download?colCode=UM10204>`_
+published by NXP Semiconductors. However, you need to log-in to the site to
+access the PDF. An older version of the specification (revision 6) is archived
+`here <https://web.archive.org/web/20210813122132/https://www.nxp.com/docs/en/user-guide/UM10204.pdf>`_.
+
+SMBus (System Management Bus) is based on the I2C protocol, and is mostly
+a subset of I2C protocols and signaling.  Many I2C devices will work on an
+SMBus, but some SMBus protocols add semantics beyond what is required to
+achieve I2C branding.  Modern PC mainboards rely on SMBus.  The most common
+devices connected through SMBus are RAM modules configured using I2C EEPROMs,
+and hardware monitoring chips.
+
+Because the SMBus is mostly a subset of the generalized I2C bus, we can
+use its protocols on many I2C systems.  However, there are systems that don't
+meet both SMBus and I2C electrical constraints; and others which can't
+implement all the common SMBus protocol semantics or messages.
+
+
+Terminology
+===========
+
+Using the terminology from the official documentation, the I2C bus connects
+one or more *master* chips and one or more *slave* chips.
+
+.. kernel-figure::  i2c_bus.svg
+   :alt:    Simple I2C bus with one master and 3 slaves
+
+   Simple I2C bus
+
+A **master** chip is a node that starts communications with slaves. In the
+Linux kernel implementation it is called an **adapter** or bus. Adapter
+drivers are in the ``drivers/i2c/busses/`` subdirectory.
+
+An **algorithm** contains general code that can be used to implement a
+whole class of I2C adapters. Each specific adapter driver either depends on
+an algorithm driver in the ``drivers/i2c/algos/`` subdirectory, or includes
+its own implementation.
+
+A **slave** chip is a node that responds to communications when addressed
+by the master. In Linux it is called a **client**. Client drivers are kept
+in a directory specific to the feature they provide, for example
+``drivers/media/gpio/`` for GPIO expanders and ``drivers/media/i2c/`` for
+video-related chips.
+
+For the example configuration in figure, you will need a driver for your
+I2C adapter, and drivers for your I2C devices (usually one driver for each
+device).