i2c: Improve smbus-protocol documentation

SMBus Protocol Summary

======================

The following is a summary of the SMBus protocol. It applies to

all revisions of the protocol (1.0, 1.1, and 2.0).

Certain protocol features which are not supported by

Some adapters understand only the SMBus (System Management Bus) protocol,

which is a subset from the I2C protocol. Fortunately, many devices use

only the same subset, which makes it possible to put them on an SMBus.

If you write a driver for some I2C device, please try to use the SMBus

commands if at all possible (if the device uses only that subset of the

I2C protocol). This makes it possible to use the device driver on both

translated to I2C on I2C adapters, but plain I2C commands can not be

handled at all on most pure SMBus adapters).

Below is a list of SMBus commands.

Below is a list of SMBus protocol operations, and the functions executing

them.  Note that the names used in the SMBus protocol specifications usually

don't match these function names.  For some of the operations which pass a

single data byte, the functions using SMBus protocol operation names execute

a different protocol operation entirely.

Key to symbols

==============

[..]: Data sent by I2C device, as opposed to data sent by the host adapter.

SMBus Write Quick

=================

SMBus Quick Command:  i2c_smbus_write_quick()

=============================================

This sends a single bit to the device, at the place of the Rd/Wr bit.

There is no equivalent Read Quick command.

A Addr Rd/Wr [A] P

SMBus Read Byte

===============

SMBus Receive Byte:  i2c_smbus_read_byte()

==========================================

This reads a single byte from a device, without specifying a device

register. Some devices are so simple that this interface is enough; for

S Addr Rd [A] [Data] NA P

SMBus Write Byte

================

SMBus Send Byte:  i2c_smbus_write_byte()

========================================

This is the reverse of Read Byte: it sends a single byte to a device.

See Read Byte for more information.

This operation is the reverse of Receive Byte: it sends a single byte

to a device.  See Receive Byte for more information.

S Addr Wr [A] Data [A] P

SMBus Read Byte Data

====================

SMBus Read Byte:  i2c_smbus_read_byte_data()

============================================

This reads a single byte from a device, from a designated register.

The register is specified through the Comm byte.

S Addr Wr [A] Comm [A] S Addr Rd [A] [Data] NA P

SMBus Read Word Data

====================

SMBus Read Word:  i2c_smbus_read_word_data()

============================================

This command is very like Read Byte Data; again, data is read from a

This operation is very like Read Byte; again, data is read from a

device, from a designated register that is specified through the Comm

byte. But this time, the data is a complete word (16 bits).

S Addr Wr [A] Comm [A] S Addr Rd [A] [DataLow] A [DataHigh] NA P

SMBus Write Byte Data

=====================

SMBus Write Byte:  i2c_smbus_write_byte_data()

==============================================

This writes a single byte to a device, to a designated register. The

register is specified through the Comm byte. This is the opposite of

the Read Byte Data command.

the Read Byte operation.

S Addr Wr [A] Comm [A] Data [A] P

SMBus Write Word Data

=====================

SMBus Write Word:  i2c_smbus_write_word_data()

==============================================

This is the opposite operation of the Read Word Data command. 16 bits

This is the opposite of the Read Word operation. 16 bits

of data is written to a device, to the designated register that is

specified through the Comm byte. 

                             S Addr Rd [A] [DataLow] A [DataHigh] NA P

SMBus Block Read

================

SMBus Block Read:  i2c_smbus_read_block_data()

==============================================

This command reads a block of up to 32 bytes from a device, from a 

designated register that is specified through the Comm byte. The amount

           S Addr Rd [A] [Count] A [Data] A [Data] A ... A [Data] NA P

SMBus Block Write

=================

SMBus Block Write:  i2c_smbus_write_block_data()

================================================

The opposite of the Block Read command, this writes up to 32 bytes to 

a device, to a designated register that is specified through the

S Addr Wr [A] Comm [A] Count [A] Data [A] Data [A] ... [A] Data [A] P

SMBus Block Process Call

========================

SMBus Block Write - Block Read Process Call

===========================================

SMBus Block Process Call was introduced in Revision 2.0 of the specification.

SMBus Block Write - Block Read Process Call was introduced in

Revision 2.0 of the specification.

This command selects a device register (through the Comm byte), sends

1 to 31 bytes of data to it, and reads 1 to 31 bytes of data in return.

Packet Error Checking (PEC)

===========================

Packet Error Checking was introduced in Revision 1.1 of the specification.

PEC adds a CRC-8 error-checking byte to all transfers.

PEC adds a CRC-8 error-checking byte to transfers using it, immediately

before the terminating STOP.

Address Resolution Protocol (ARP)

=================================

The Address Resolution Protocol was introduced in Revision 2.0 of

the specification. It is a higher-layer protocol which uses the

messages above.

I2C Block Transactions

======================

The following I2C block transactions are supported by the

SMBus layer and are described here for completeness.

They are *NOT* defined by the SMBus specification.

I2C block transactions do not limit the number of bytes transferred

but the SMBus layer places a limit of 32 bytes.

I2C Block Read

==============

I2C Block Read:  i2c_smbus_read_i2c_block_data()

================================================

This command reads a block of bytes from a device, from a 

designated register that is specified through the Comm byte.

           S Addr Rd [A] [Data] A [Data] A ... A [Data] NA P

I2C Block Write

===============

I2C Block Write:  i2c_smbus_write_i2c_block_data()

==================================================

The opposite of the Block Read command, this writes bytes to 

a device, to a designated register that is specified through the

supported as they are indistinguishable from data.

S Addr Wr [A] Comm [A] Data [A] Data [A] ... [A] Data [A] P