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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 two-wire protocol with variable
+speed (typically up to 400 kHz, high speed modes up to 5 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/docs/en/user-guide/UM10204.pdf>`_
+published by NXP Semiconductors, version 7 as of this writing.
+
+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
+===========
+
+The I2C bus connects one or more controller chips and one or more target chips.
+
+.. kernel-figure::  i2c_bus.svg
+   :alt:    Simple I2C bus with one controller and 3 targets
+
+   Simple I2C bus
+
+A **controller** chip is a node that starts communications with targets. In the
+Linux kernel implementation it is also called an "adapter" or "bus". Controller
+drivers are usually in the ``drivers/i2c/busses/`` subdirectory.
+
+An **algorithm** contains general code that can be used to implement a whole
+class of I2C controllers. Each specific controller driver either depends on an
+algorithm driver in the ``drivers/i2c/algos/`` subdirectory, or includes its
+own implementation.
+
+A **target** chip is a node that responds to communications when addressed by a
+controller. In the Linux kernel implementation it is also called a "client".
+While targets are usually separate external chips, Linux can also act as a
+target (needs hardware support) and respond to another controller on the bus.
+This is then called a **local target**. In contrast, an external chip is called
+a **remote target**.
+
+Target drivers are kept in a directory specific to the feature they provide,
+for example ``drivers/gpio/`` for GPIO expanders and ``drivers/media/i2c/`` for
+video-related chips.
+
+For the example configuration in the figure above, you will need one driver for
+the I2C controller, and drivers for your I2C targets. Usually one driver for
+each target.
+
+Synonyms
+--------
+
+As mentioned above, the Linux I2C implementation historically uses the terms
+"adapter" for controller and "client" for target. A number of data structures
+have these synonyms in their name. So, when discussing implementation details,
+you should be aware of these terms as well. The official wording is preferred,
+though.
+
+Outdated terminology
+--------------------
+
+In earlier I2C specifications, controller was named "master" and target was
+named "slave". These terms have been obsoleted with v7 of the specification and
+their use is also discouraged by the Linux Kernel Code of Conduct. You may
+still find them in references to documentation which has not been updated. The
+general attitude, however, is to use the inclusive terms: controller and
+target. Work to replace the old terminology in the Linux Kernel is on-going.