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180 lines
6.9 KiB
Plaintext
180 lines
6.9 KiB
Plaintext
Linux I2C slave interface description
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=====================================
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by Wolfram Sang <wsa@sang-engineering.com> in 2014-15
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Linux can also be an I2C slave in case I2C controllers have slave support.
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Besides this HW requirement, one also needs a software backend providing the
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actual functionality. An example for this is the slave-eeprom driver, which
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acts as a dual memory driver. While another I2C master on the bus can access it
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like a regular EEPROM, the Linux I2C slave can access the content via sysfs and
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retrieve/provide information as needed. The software backend driver and the I2C
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bus driver communicate via events. Here is a small graph visualizing the data
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flow and the means by which data is transported. The dotted line marks only one
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example. The backend could also use e.g. a character device, be in-kernel
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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 like described in the document 'instantiating-devices'. A quick example
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for instantiating the slave-eeprom driver from userspace:
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# echo 0-0064 > /sys/bus/i2c/drivers/i2c-slave-eeprom/bind
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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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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_forbid() because your device usually needs to be powered
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on always to be able to detect its slave address. When unregistering, do the
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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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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 slave drivers will always have to support
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byte-based transactions as the ultimate fallback because this is how the
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majority of HW works.
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* For backends simulating hardware registers, buffers are not helpful because
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on writes an action should be immediately triggered. For reads, the data in
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the buffer might get stale.
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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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