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diff --git a/Documentation/i2c/slave-interface b/Documentation/i2c/slave-interface deleted file mode 100644 index 7e2a228f21bc..000000000000 --- a/Documentation/i2c/slave-interface +++ /dev/null @@ -1,193 +0,0 @@ -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'. 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': 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_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. - |