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198 lines
7.7 KiB
198 lines
7.7 KiB
===================================== |
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Linux I2C slave interface description |
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===================================== |
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by Wolfram Sang <[email protected]> in 2014-15 |
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Linux can also be an I2C slave if the I2C controller in use has slave |
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functionality. For that to work, one needs slave support in the bus driver plus |
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a hardware independent software backend providing the actual functionality. An |
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example for the latter is the slave-eeprom driver, which acts as a dual memory |
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driver. While another I2C master on the bus can access it like a regular |
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EEPROM, the Linux I2C slave can access the content via sysfs and handle data as |
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needed. The backend driver and the I2C bus driver communicate via events. Here |
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is a small graph visualizing the data flow and the means by which data is |
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transported. The dotted line marks only one example. The backend could also |
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use a character device, be in-kernel only, or something completely different:: |
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e.g. sysfs I2C slave events I/O registers |
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+-----------+ v +---------+ v +--------+ v +------------+ |
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| Userspace +........+ Backend +-----------+ Driver +-----+ Controller | |
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+-----------+ +---------+ +--------+ +------------+ |
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----------------------------------------------------------------+-- I2C |
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--------------------------------------------------------------+---- Bus |
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Note: Technically, there is also the I2C core between the backend and the |
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driver. However, at this time of writing, the layer is transparent. |
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User manual |
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=========== |
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I2C slave backends behave like standard I2C clients. So, you can instantiate |
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them as described in the document 'instantiating-devices'. The only difference |
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is that i2c slave backends have their own address space. So, you have to add |
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0x1000 to the address you would originally request. An example for |
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instantiating the slave-eeprom driver from userspace at the 7 bit address 0x64 |
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on bus 1:: |
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# echo slave-24c02 0x1064 > /sys/bus/i2c/devices/i2c-1/new_device |
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Each backend should come with separate documentation to describe its specific |
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behaviour and setup. |
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Developer manual |
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================ |
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First, the events which are used by the bus driver and the backend will be |
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described in detail. After that, some implementation hints for extending bus |
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drivers and writing backends will be given. |
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I2C slave events |
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---------------- |
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The bus driver sends an event to the backend using the following function:: |
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ret = i2c_slave_event(client, event, &val) |
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'client' describes the I2C slave device. 'event' is one of the special event |
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types described hereafter. 'val' holds an u8 value for the data byte to be |
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read/written and is thus bidirectional. The pointer to val must always be |
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provided even if val is not used for an event, i.e. don't use NULL here. 'ret' |
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is the return value from the backend. Mandatory events must be provided by the |
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bus drivers and must be checked for by backend drivers. |
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Event types: |
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* I2C_SLAVE_WRITE_REQUESTED (mandatory) |
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'val': unused |
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'ret': always 0 |
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Another I2C master wants to write data to us. This event should be sent once |
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our own address and the write bit was detected. The data did not arrive yet, so |
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there is nothing to process or return. Wakeup or initialization probably needs |
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to be done, though. |
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* I2C_SLAVE_READ_REQUESTED (mandatory) |
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'val': backend returns first byte to be sent |
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'ret': always 0 |
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Another I2C master wants to read data from us. This event should be sent once |
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our own address and the read bit was detected. After returning, the bus driver |
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should transmit the first byte. |
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* I2C_SLAVE_WRITE_RECEIVED (mandatory) |
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'val': bus driver delivers received byte |
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'ret': 0 if the byte should be acked, some errno if the byte should be nacked |
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Another I2C master has sent a byte to us which needs to be set in 'val'. If 'ret' |
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is zero, the bus driver should ack this byte. If 'ret' is an errno, then the byte |
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should be nacked. |
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* I2C_SLAVE_READ_PROCESSED (mandatory) |
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'val': backend returns next byte to be sent |
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'ret': always 0 |
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The bus driver requests the next byte to be sent to another I2C master in |
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'val'. Important: This does not mean that the previous byte has been acked, it |
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only means that the previous byte is shifted out to the bus! To ensure seamless |
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transmission, most hardware requests the next byte when the previous one is |
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still shifted out. If the master sends NACK and stops reading after the byte |
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currently shifted out, this byte requested here is never used. It very likely |
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needs to be sent again on the next I2C_SLAVE_READ_REQUEST, depending a bit on |
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your backend, though. |
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* I2C_SLAVE_STOP (mandatory) |
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'val': unused |
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'ret': always 0 |
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A stop condition was received. This can happen anytime and the backend should |
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reset its state machine for I2C transfers to be able to receive new requests. |
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Software backends |
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----------------- |
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If you want to write a software backend: |
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* use a standard i2c_driver and its matching mechanisms |
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* write the slave_callback which handles the above slave events |
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(best using a state machine) |
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* register this callback via i2c_slave_register() |
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Check the i2c-slave-eeprom driver as an example. |
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Bus driver support |
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------------------ |
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If you want to add slave support to the bus driver: |
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* implement calls to register/unregister the slave and add those to the |
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struct i2c_algorithm. When registering, you probably need to set the I2C |
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slave address and enable slave specific interrupts. If you use runtime pm, you |
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should use pm_runtime_get_sync() because your device usually needs to be |
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powered on always to be able to detect its slave address. When unregistering, |
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do the inverse of the above. |
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* Catch the slave interrupts and send appropriate i2c_slave_events to the backend. |
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Note that most hardware supports being master _and_ slave on the same bus. So, |
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if you extend a bus driver, please make sure that the driver supports that as |
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well. In almost all cases, slave support does not need to disable the master |
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functionality. |
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Check the i2c-rcar driver as an example. |
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About ACK/NACK |
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-------------- |
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It is good behaviour to always ACK the address phase, so the master knows if a |
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device is basically present or if it mysteriously disappeared. Using NACK to |
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state being busy is troublesome. SMBus demands to always ACK the address phase, |
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while the I2C specification is more loose on that. Most I2C controllers also |
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automatically ACK when detecting their slave addresses, so there is no option |
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to NACK them. For those reasons, this API does not support NACK in the address |
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phase. |
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Currently, there is no slave event to report if the master did ACK or NACK a |
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byte when it reads from us. We could make this an optional event if the need |
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arises. However, cases should be extremely rare because the master is expected |
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to send STOP after that and we have an event for that. Also, keep in mind not |
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all I2C controllers have the possibility to report that event. |
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About buffers |
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------------- |
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During development of this API, the question of using buffers instead of just |
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bytes came up. Such an extension might be possible, usefulness is unclear at |
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this time of writing. Some points to keep in mind when using buffers: |
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* Buffers should be opt-in and backend drivers will always have to support |
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byte-based transactions as the ultimate fallback anyhow because this is how |
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the majority of HW works. |
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* For backends simulating hardware registers, buffers are largely not helpful |
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because after each byte written an action should be immediately triggered. |
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For reads, the data kept in the buffer might get stale if the backend just |
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updated a register because of internal processing. |
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* A master can send STOP at any time. For partially transferred buffers, this |
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means additional code to handle this exception. Such code tends to be |
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error-prone.
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