media: i2c: add I2C Address Translator (ATR) support

An ATR is a device that looks similar to an i2c-mux: it has an I2C
slave "upstream" port and N master "downstream" ports, and forwards
transactions from upstream to the appropriate downstream port. But it
is different in that the forwarded transaction has a different slave
address. The address used on the upstream bus is called the "alias"
and is (potentially) different from the physical slave address of the
downstream chip.

Add a helper file (just like i2c-mux.c for a mux or switch) to allow
implementing ATR features in a device driver. The helper takes care of
adapter creation/destruction and translates addresses at each transaction.

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Wolfram Sang <wsa@kernel.org>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>

Documentation: i2c: add i2c-sysfs into index

Append i2c-sysfs to toctree in order to get rid of building warnings.

Fixes: 31df7195b100 ("Documentation: i2c: Add doc for I2C sysfs")
Signed-off-by: Hu Haowen <src.res@email.cn>
Signed-off-by: Wolfram Sang <wsa@kernel.org>

Documentation: i2c: add testunit docs to index

Fixes: a8335c64c5f0 ("i2c: add slave testunit driver")
Reported-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>

docs: remove the 2.6 "Upgrading I2C Drivers" guide

All the drivers have long since been upgraded, and all the important
information here is also included in the "Implementing I2C device
drivers" guide.

Signed-off-by: Stephen Kitt <steve@sk2.org>
Reviewed-by: Wolfram Sang <wsa@kernel.org>
Link: https://lore.kernel.org/r/20200806161456.8680-1-steve@sk2.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

docs: i2c: sort index logically

The index page currently lists sections in alphabetical file order without
caring about their content. Sort sections based on their content logically,
according to the following structure:

* Intro to I2C/SMBus and their usage in Linux: summary, i2c-protocol,
smbus-protocol, instantiating-devices, busses/index, i2c-topology,
muxes/i2c-mux-gpio
* Implementing drivers: writing-clients, dev-interface,
dma-considerations, fault-codes, functionality
* Debugging: gpio-fault-injection, i2c-stub
* Slave I2C: slave-interface, slave-eeprom-backend
* Advanced: ten-bit-addresses
* Obsolete info: upgrading-clients, old-module-parameters

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Reviewed-by: Jean Delvare <jdelvare@suse.de>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>

docs: i2c: Fix SPDX-License-Identifier syntax

ReST directives are introduced with two dots.

Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

docs: i2c: convert to ReST and add to driver-api bookset

Convert each file at I2C subsystem, renaming them to .rst and
adding to the driver-api book.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Acked-by: Wolfram Sang <wsa@the-dreams.de>
Acked-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Acked-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>