Adding upstream version 4.19.249.upstream/4.19.249

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

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+I2C and SMBus
+=============
+
+I2C (pronounce: I squared C) 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.
+
+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
+===========
+
+When we talk about I2C, we use the following terms:
+  Bus    -> Algorithm
+            Adapter
+  Device -> Driver
+            Client
+
+An Algorithm driver contains general code that can be used for a whole class
+of I2C adapters. Each specific adapter driver either depends on one algorithm
+driver, or includes its own implementation.
+
+A Driver driver (yes, this sounds ridiculous, sorry) contains the general
+code to access some type of device. Each detected device gets its own
+data in the Client structure. Usually, Driver and Client are more closely
+integrated than Algorithm and Adapter.
+
+For a given configuration, you will need a driver for your I2C bus, and
+drivers for your I2C devices (usually one driver for each device).