[PATCH 2/3] Documentation: i2c: describe the new slave mode

From: Wolfram Sang
Date: Thu Mar 12 2015 - 08:43:12 EST

From: Wolfram Sang <wsa+renesas@xxxxxxxxxxxxxxxxxxxx>

Signed-off-by: Wolfram Sang <wsa+renesas@xxxxxxxxxxxxxxxxxxxx>
Documentation/i2c/slave-interface | 178 ++++++++++++++++++++++++++++++++++++++
Documentation/i2c/summary | 4 -
2 files changed, 178 insertions(+), 4 deletions(-)
create mode 100644 Documentation/i2c/slave-interface

diff --git a/Documentation/i2c/slave-interface b/Documentation/i2c/slave-interface
new file mode 100644
index 00000000000000..3df6c37191c86c
--- /dev/null
+++ b/Documentation/i2c/slave-interface
@@ -0,0 +1,178 @@
+Linux I2C slave interface description
+by Wolfram Sang <wsa@xxxxxxxxxxxxxxxxxxxx> in 2014-15
+Finally, 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, or use in-kernel mechanisms 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. '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:
+'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.
+'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.
+'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.
+'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 or
+even has been put on the wires! Most hardware requests the next byte when the
+previous one is still to be shifted out to ensure seamless transmission. If the
+master stops reading after the previous byte, the next byte is never used. It
+probably needs to be sent again on the next I2C_SLAVE_READ_REQUEST, depending a
+bit on your backend.
+* 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 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.
+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 I2C specification is more loose on that. Most I2C controllers also
+automatically ACK when detecting its slave address, so there is no option to
+NACK it. For those reasons, this API does not support NACK in the address
+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.
diff --git a/Documentation/i2c/summary b/Documentation/i2c/summary
index 13ab076dcd9248..809541ab352f03 100644
--- a/Documentation/i2c/summary
+++ b/Documentation/i2c/summary
@@ -41,7 +41,3 @@ integrated than Algorithm and Adapter.

For a given configuration, you will need a driver for your I2C bus, and
drivers for your I2C devices (usually one driver for each device).
-At this time, Linux only operates I2C (or SMBus) in master mode; you can't
-use these APIs to make a Linux system behave as a slave/device, either to
-speak a custom protocol or to emulate some other device.

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