1Linux I2C slave interface description
2=====================================
3
4by Wolfram Sang <wsa@sang-engineering.com> in 2014-15
5
6Linux can also be an I2C slave if the I2C controller in use has slave
7functionality. For that to work, one needs slave support in the bus driver plus
8a hardware independent software backend providing the actual functionality. An
9example for the latter is the slave-eeprom driver, which acts as a dual memory
10driver. While another I2C master on the bus can access it like a regular
11EEPROM, the Linux I2C slave can access the content via sysfs and handle data as
12needed. The backend driver and the I2C bus driver communicate via events. Here
13is a small graph visualizing the data flow and the means by which data is
14transported. The dotted line marks only one example. The backend could also
15use a character device, be in-kernel only, or something completely different:
16
17
18              e.g. sysfs        I2C slave events        I/O registers
19  +-----------+   v    +---------+     v     +--------+  v  +------------+
20  | Userspace +........+ Backend +-----------+ Driver +-----+ Controller |
21  +-----------+        +---------+           +--------+     +------------+
22                                                                | |
23  ----------------------------------------------------------------+--  I2C
24  --------------------------------------------------------------+----  Bus
25
26Note: Technically, there is also the I2C core between the backend and the
27driver. However, at this time of writing, the layer is transparent.
28
29
30User manual
31===========
32
33I2C slave backends behave like standard I2C clients. So, you can instantiate
34them as described in the document 'instantiating-devices'. The only difference
35is that i2c slave backends have their own address space. So, you have to add
360x1000 to the address you would originally request. An example for
37instantiating the slave-eeprom driver from userspace at the 7 bit address 0x64
38on bus 1:
39
40  # echo slave-24c02 0x1064 > /sys/bus/i2c/devices/i2c-1/new_device
41
42Each backend should come with separate documentation to describe its specific
43behaviour and setup.
44
45
46Developer manual
47================
48
49First, the events which are used by the bus driver and the backend will be
50described in detail. After that, some implementation hints for extending bus
51drivers and writing backends will be given.
52
53
54I2C slave events
55----------------
56
57The bus driver sends an event to the backend using the following function:
58
59	ret = i2c_slave_event(client, event, &val)
60
61'client' describes the i2c slave device. 'event' is one of the special event
62types described hereafter. 'val' holds an u8 value for the data byte to be
63read/written and is thus bidirectional. The pointer to val must always be
64provided even if val is not used for an event, i.e. don't use NULL here. 'ret'
65is the return value from the backend. Mandatory events must be provided by the
66bus drivers and must be checked for by backend drivers.
67
68Event types:
69
70* I2C_SLAVE_WRITE_REQUESTED (mandatory)
71
72'val': unused
73'ret': always 0
74
75Another I2C master wants to write data to us. This event should be sent once
76our own address and the write bit was detected. The data did not arrive yet, so
77there is nothing to process or return. Wakeup or initialization probably needs
78to be done, though.
79
80* I2C_SLAVE_READ_REQUESTED (mandatory)
81
82'val': backend returns first byte to be sent
83'ret': always 0
84
85Another I2C master wants to read data from us. This event should be sent once
86our own address and the read bit was detected. After returning, the bus driver
87should transmit the first byte.
88
89* I2C_SLAVE_WRITE_RECEIVED (mandatory)
90
91'val': bus driver delivers received byte
92'ret': 0 if the byte should be acked, some errno if the byte should be nacked
93
94Another I2C master has sent a byte to us which needs to be set in 'val'. If 'ret'
95is zero, the bus driver should ack this byte. If 'ret' is an errno, then the byte
96should be nacked.
97
98* I2C_SLAVE_READ_PROCESSED (mandatory)
99
100'val': backend returns next byte to be sent
101'ret': always 0
102
103The bus driver requests the next byte to be sent to another I2C master in
104'val'. Important: This does not mean that the previous byte has been acked, it
105only means that the previous byte is shifted out to the bus! To ensure seamless
106transmission, most hardware requests the next byte when the previous one is
107still shifted out. If the master sends NACK and stops reading after the byte
108currently shifted out, this byte requested here is never used. It very likely
109needs to be sent again on the next I2C_SLAVE_READ_REQUEST, depending a bit on
110your backend, though.
111
112* I2C_SLAVE_STOP (mandatory)
113
114'val': unused
115'ret': always 0
116
117A stop condition was received. This can happen anytime and the backend should
118reset its state machine for I2C transfers to be able to receive new requests.
119
120
121Software backends
122-----------------
123
124If you want to write a software backend:
125
126* use a standard i2c_driver and its matching mechanisms
127* write the slave_callback which handles the above slave events
128  (best using a state machine)
129* register this callback via i2c_slave_register()
130
131Check the i2c-slave-eeprom driver as an example.
132
133
134Bus driver support
135------------------
136
137If you want to add slave support to the bus driver:
138
139* implement calls to register/unregister the slave and add those to the
140  struct i2c_algorithm. When registering, you probably need to set the i2c
141  slave address and enable slave specific interrupts. If you use runtime pm, you
142  should use pm_runtime_forbid() because your device usually needs to be powered
143  on always to be able to detect its slave address. When unregistering, do the
144  inverse of the above.
145
146* Catch the slave interrupts and send appropriate i2c_slave_events to the backend.
147
148Check the i2c-rcar driver as an example.
149
150
151About ACK/NACK
152--------------
153
154It is good behaviour to always ACK the address phase, so the master knows if a
155device is basically present or if it mysteriously disappeared. Using NACK to
156state being busy is troublesome. SMBus demands to always ACK the address phase,
157while the I2C specification is more loose on that. Most I2C controllers also
158automatically ACK when detecting their slave addresses, so there is no option
159to NACK them. For those reasons, this API does not support NACK in the address
160phase.
161
162Currently, there is no slave event to report if the master did ACK or NACK a
163byte when it reads from us. We could make this an optional event if the need
164arises. However, cases should be extremely rare because the master is expected
165to send STOP after that and we have an event for that. Also, keep in mind not
166all I2C controllers have the possibility to report that event.
167
168
169About buffers
170-------------
171
172During development of this API, the question of using buffers instead of just
173bytes came up. Such an extension might be possible, usefulness is unclear at
174this time of writing. Some points to keep in mind when using buffers:
175
176* Buffers should be opt-in and slave drivers will always have to support
177  byte-based transactions as the ultimate fallback because this is how the
178  majority of HW works.
179
180* For backends simulating hardware registers, buffers are not helpful because
181  on writes an action should be immediately triggered. For reads, the data in
182  the buffer might get stale.
183
184* A master can send STOP at any time. For partially transferred buffers, this
185  means additional code to handle this exception. Such code tends to be
186  error-prone.
187
188