Linux I2C slave interface description
=====================================

by Wolfram Sang <wsa@sang-engineering.com> in 2014-15

Linux can also be an I2C slave in case I2C controllers have slave support.
Besides this HW requirement, one also needs a software backend providing the
actual functionality. An example for this is the slave-eeprom driver, which
acts as a dual memory driver. While another I2C master on the bus can access it
like a regular EEPROM, the Linux I2C slave can access the content via sysfs and
retrieve/provide information as needed. The software backend driver and the I2C
bus driver communicate via events. Here is a small graph visualizing the data
flow and the means by which data is transported. The dotted line marks only one
example. The backend could also use e.g. a character device, be in-kernel
only, or something completely different:


              e.g. sysfs        I2C slave events        I/O registers
  +-----------+   v    +---------+     v     +--------+  v  +------------+
  | Userspace +........+ Backend +-----------+ Driver +-----+ Controller |
  +-----------+        +---------+           +--------+     +------------+
                                                                | |
  ----------------------------------------------------------------+--  I2C
  --------------------------------------------------------------+----  Bus

Note: Technically, there is also the I2C core between the backend and the
driver. However, at this time of writing, the layer is transparent.


User manual
===========

I2C slave backends behave like standard I2C clients. So, you can instantiate
them like described in the document 'instantiating-devices'. A quick example
for instantiating the slave-eeprom driver from userspace:

  # echo 0-0064 > /sys/bus/i2c/drivers/i2c-slave-eeprom/bind

Each backend should come with separate documentation to describe its specific
behaviour and setup.


Developer manual
================

I2C slave events
----------------

The bus driver sends an event to the backend using the following function:

	ret = i2c_slave_event(client, event, &val)

'client' describes the i2c slave device. 'event' is one of the special event
types described hereafter. 'val' holds an u8 value for the data byte to be
read/written and is thus bidirectional. The pointer to val must always be
provided even if val is not used for an event, i.e. don't use NULL here. 'ret'
is the return value from the backend. Mandatory events must be provided by the
bus drivers and must be checked for by backend drivers.

Event types:

* I2C_SLAVE_WRITE_REQUESTED (mandatory)

'val': unused
'ret': always 0

Another I2C master wants to write data to us. This event should be sent once
our own address and the write bit was detected. The data did not arrive yet, so
there is nothing to process or return. Wakeup or initialization probably needs
to be done, though.

* I2C_SLAVE_READ_REQUESTED (mandatory)

'val': backend returns first byte to be sent
'ret': always 0

Another I2C master wants to read data from us. This event should be sent once
our own address and the read bit was detected. After returning, the bus driver
should transmit the first byte.

* I2C_SLAVE_WRITE_RECEIVED (mandatory)

'val': bus driver delivers received byte
'ret': 0 if the byte should be acked, some errno if the byte should be nacked

Another I2C master has sent a byte to us which needs to be set in 'val'. If 'ret'
is zero, the bus driver should ack this byte. If 'ret' is an errno, then the byte
should be nacked.

* I2C_SLAVE_READ_PROCESSED (mandatory)

'val': backend returns next byte to be sent
'ret': always 0

The bus driver requests the next byte to be sent to another I2C master in
'val'. Important: This does not mean that the previous byte has been acked, it
only means that the previous byte is shifted out to the bus! To ensure seamless
transmission, most hardware requests the next byte when the previous one is
still shifted out. If the master sends NACK and stops reading after the byte
currently shifted out, this byte requested here is never used. It very likely
needs to be sent again on the next I2C_SLAVE_READ_REQUEST, depending a bit on
your backend, though.

* I2C_SLAVE_STOP (mandatory)

'val': unused
'ret': always 0

A stop condition was received. This can happen anytime and the backend should
reset its state machine for I2C transfers to be able to receive new requests.


Software backends
-----------------

If you want to write a software backend:

* use a standard i2c_driver and its matching mechanisms
* write the slave_callback which handles the above slave events
  (best using a state machine)
* register this callback via i2c_slave_register()

Check the i2c-slave-eeprom driver as an example.


Bus driver support
------------------

If you want to add slave support to the bus driver:

* implement calls to register/unregister the slave and add those to the
  struct i2c_algorithm. When registering, you probably need to set the i2c
  slave address and enable slave specific interrupts. If you use runtime pm, you
  should use pm_runtime_forbid() because your device usually needs to be powered
  on always to be able to detect its slave address. When unregistering, do the
  inverse of the above.

* Catch the slave interrupts and send appropriate i2c_slave_events to the backend.

Check the i2c-rcar driver as an example.


About ACK/NACK
--------------

It is good behaviour to always ACK the address phase, so the master knows if a
device is basically present or if it mysteriously disappeared. Using NACK to
state being busy is troublesome. SMBus demands to always ACK the address phase,
while the I2C specification is more loose on that. Most I2C controllers also
automatically ACK when detecting their slave addresses, so there is no option
to NACK them. For those reasons, this API does not support NACK in the address
phase.

Currently, there is no slave event to report if the master did ACK or NACK a
byte when it reads from us. We could make this an optional event if the need
arises. However, cases should be extremely rare because the master is expected
to send STOP after that and we have an event for that. Also, keep in mind not
all I2C controllers have the possibility to report that event.


About buffers
-------------

During development of this API, the question of using buffers instead of just
bytes came up. Such an extension might be possible, usefulness is unclear at
this time of writing. Some points to keep in mind when using buffers:

* Buffers should be opt-in and slave drivers will always have to support
  byte-based transactions as the ultimate fallback because this is how the
  majority of HW works.

* For backends simulating hardware registers, buffers are not helpful because
  on writes an action should be immediately triggered. For reads, the data in
  the buffer might get stale.

* A master can send STOP at any time. For partially transferred buffers, this
  means additional code to handle this exception. Such code tends to be
  error-prone.