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diff --git a/Documentation/i2c/slave-interface b/Documentation/i2c/slave-interface new file mode 100644 index 0000000..389bb5d --- /dev/null +++ b/Documentation/i2c/slave-interface @@ -0,0 +1,179 @@ +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. + |