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>

2 files changed, 97 insertions, 0 deletions

diff --git a/Documentation/i2c/i2c-address-translators.rst b/Documentation/i2c/i2c-address-translators.rst
new file mode 100644
index 000000000000..b22ce9f41ecf
--- /dev/null
+++ b/Documentation/i2c/i2c-address-translators.rst
@@ -0,0 +1,96 @@
+.. 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
+ - Physical I2C transaction on bus A, slave address 0x20
+ - ATR chip detects transaction on address 0x20, finds it in table,
+   propagates transaction on bus B with address translated to 0x10,
+   keeps clock streched on bus A waiting for reply
+ - Slave X chip (on bus B) detects transaction at its own physical
+   address 0x10 and replies normally
+ - ATR chip stops clock stretching and forwards reply on bus A,
+   with address translated back to 0x20
+ - ATR driver receives the reply, rewrites messages with address 0x10
+   as they were initially
+ - Slave X driver gets back the msgs[], with reply and address 0x10
+
+Usage:
+
+ 1. In the driver (typically in the probe function) add an ATR by
+    calling i2c_atr_new() passing attach/detach callbacks
+ 2. When the attach callback is called pick an appropriate alias,
+    configure it in the chip and return the chosen alias in the
+    alias_id parameter
+ 3. When the detach callback is called, deconfigure the alias from
+    the chip and put the alias back in the pool for later usage
+
+I2C ATR functions and data structures
+-------------------------------------
+
+.. kernel-doc:: include/linux/i2c-atr.h
diff --git a/Documentation/i2c/index.rst b/Documentation/i2c/index.rst
index 6270f1fd7d4e..2b213d4ce89c 100644
--- a/Documentation/i2c/index.rst
+++ b/Documentation/i2c/index.rst
@@ -18,6 +18,7 @@ Introduction
    i2c-topology
    muxes/i2c-mux-gpio
    i2c-sysfs
+   i2c-address-translators
 
 Writing device drivers
 ======================