Documentation/i2c/i2c-address-translators.rst

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.. SPDX-License-Identifier: GPL-2.0

=======================
I2C Address Translators
=======================

Author: Luca Ceresoli <luca@lucaceresoli.net>
Author: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>

Description
-----------

An I2C Address Translator (ATR) is a device with an I2C slave parent
("upstream") port and N I2C master child ("downstream") ports, and
forwards transactions from upstream to the appropriate downstream port
with a modified slave address. The address used on the parent bus is
called the "alias" and is (potentially) different from the physical
slave address of the child bus. Address translation is done by the
hardware.

An ATR looks similar to an i2c-mux except:
 - the address on the parent and child busses can be different
 - there is normally no need to select the child port; the alias used on the
   parent bus implies it

The ATR functionality can be provided by a chip with many other features.
The kernel i2c-atr provides a helper to implement an ATR within a driver.

The ATR creates a new I2C "child" adapter on each child bus. Adding
devices on the child bus ends up in invoking the driver code to select
an available alias. Maintaining an appropriate pool of available aliases
and picking one for each new device is up to the driver implementer. The
ATR maintains a table of currently assigned alias and uses it to modify
all I2C transactions directed to devices on the child buses.

A typical example follows.

Topology::

                      Slave X @ 0x10
              .-----.   |
  .-----.     |     |---+---- B
  | CPU |--A--| ATR |
  `-----'     |     |---+---- C
              `-----'   |
                      Slave Y @ 0x10

Alias table:

A, B and C are three physical I2C busses, electrically independent from
each other. The ATR receives the transactions initiated on bus A and
propagates them on bus B or bus C or none depending on the device address
in the transaction and based on the alias table.

Alias table:

.. table::

   ===============   =====
   Client            Alias
   ===============   =====
   X (bus B, 0x10)   0x20
   Y (bus C, 0x10)   0x30
   ===============   =====

Transaction:

 - Slave X driver requests a transaction (on adapter B), slave address 0x10
 - ATR driver finds slave X is on bus B and has alias 0x20, rewrites
   messages with address 0x20, forwards to adapter A

Annotation

Implementation Notes