1 files changed, 20 insertions, 9 deletions

diff --git a/Documentation/i2c/summary b/Documentation/i2c/summary
index 41dde8776791..aea60bf7e8f0 100644
--- a/Documentation/i2c/summary
+++ b/Documentation/i2c/summary
@@ -4,17 +4,23 @@ I2C and SMBus
 =============
 
 I2C (pronounce: I squared C) is a protocol developed by Philips. It is a 
-slow two-wire protocol (10-400 kHz), but it suffices for many types of 
-devices.
+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 a subset of the I2C protocol. Many
-modern mainboards have a System Management Bus. There are a lot of 
-devices which can be connected to a SMBus; the most notable are modern 
-memory chips with EEPROM memories and chips for hardware monitoring.
+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 just a special case of the generalized I2C bus, we
-can simulate the SMBus protocol on plain I2C busses. The reverse is
-regretfully impossible.
+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
@@ -29,6 +35,7 @@ When we talk about I2C, we use the following terms:
 An Algorithm driver contains general code that can be used for a whole class
 of I2C adapters. Each specific adapter driver depends on one algorithm
 driver.
+
 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
@@ -40,6 +47,10 @@ a separate Adapter and Algorithm driver), and drivers for your I2C devices
 in this package. See the lm_sensors project http://www.lm-sensors.nu
 for device drivers.
 
+At this time, Linux only operates I2C (or SMBus) in master mode; you can't
+use these APIs to make a Linux system behave as a slave/device, either to
+speak a custom protocol or to emulate some other device.
+
 
 Included Bus Drivers
 ====================