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diff --git a/Documentation/i2c/slave-interface.rst b/Documentation/i2c/slave-interface.rst new file mode 100644 index 000000000..3f0d320bc --- /dev/null +++ b/Documentation/i2c/slave-interface.rst @@ -0,0 +1,201 @@ +===================================== +Linux I2C slave interface description +===================================== + +by Wolfram Sang <wsa@sang-engineering.com> in 2014-15 + +Linux can also be an I2C slave if the I2C controller in use has slave +functionality. For that to work, one needs slave support in the bus driver plus +a hardware independent software backend providing the actual functionality. An +example for the latter 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 handle data as +needed. The 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 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 as described in the document instantiating-devices.rst. The only +difference is that i2c slave backends have their own address space. So, you +have to add 0x1000 to the address you would originally request. An example for +instantiating the slave-eeprom driver from userspace at the 7 bit address 0x64 +on bus 1:: + + # echo slave-24c02 0x1064 > /sys/bus/i2c/devices/i2c-1/new_device + +Each backend should come with separate documentation to describe its specific +behaviour and setup. + + +Developer manual +================ + +First, the events which are used by the bus driver and the backend will be +described in detail. After that, some implementation hints for extending bus +drivers and writing backends will be given. + + +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': 0 if the backend is ready, otherwise some errno + +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. After returning, the bus driver must +always ack the address phase. If 'ret' is zero, backend initialization or +wakeup is done and further data may be received. If 'ret' is an errno, the bus +driver should nack all incoming bytes until the next stop condition to enforce +a retry of the transmission. + +* 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_get_sync() 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. + +Note that most hardware supports being master _and_ slave on the same bus. So, +if you extend a bus driver, please make sure that the driver supports that as +well. In almost all cases, slave support does not need to disable the master +functionality. + +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 backend drivers will always have to support + byte-based transactions as the ultimate fallback anyhow because this is how + the majority of HW works. + +* For backends simulating hardware registers, buffers are largely not helpful + because after each byte written an action should be immediately triggered. + For reads, the data kept in the buffer might get stale if the backend just + updated a register because of internal processing. + +* 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. |