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1 | Linux I2C slave interface description |
2 | ===================================== | |
3 | ||
4 | by Wolfram Sang <wsa@sang-engineering.com> in 2014-15 | |
5 | ||
6 | Linux can also be an I2C slave in case I2C controllers have slave support. | |
7 | Besides this HW requirement, one also needs a software backend providing the | |
8 | actual functionality. An example for this is the slave-eeprom driver, which | |
9 | acts as a dual memory driver. While another I2C master on the bus can access it | |
10 | like a regular EEPROM, the Linux I2C slave can access the content via sysfs and | |
11 | retrieve/provide information as needed. The software backend driver and the I2C | |
12 | bus driver communicate via events. Here is a small graph visualizing the data | |
13 | flow and the means by which data is transported. The dotted line marks only one | |
14 | example. The backend could also use e.g. a character device, be in-kernel | |
15 | 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 | ||
26 | Note: Technically, there is also the I2C core between the backend and the | |
27 | driver. However, at this time of writing, the layer is transparent. | |
28 | ||
29 | ||
30 | User manual | |
31 | =========== | |
32 | ||
33 | I2C slave backends behave like standard I2C clients. So, you can instantiate | |
34 | them like described in the document 'instantiating-devices'. A quick example | |
35 | for instantiating the slave-eeprom driver from userspace: | |
36 | ||
37 | # echo 0-0064 > /sys/bus/i2c/drivers/i2c-slave-eeprom/bind | |
38 | ||
39 | Each backend should come with separate documentation to describe its specific | |
40 | behaviour and setup. | |
41 | ||
42 | ||
43 | Developer manual | |
44 | ================ | |
45 | ||
46 | I2C slave events | |
47 | ---------------- | |
48 | ||
49 | The bus driver sends an event to the backend using the following function: | |
50 | ||
51 | ret = i2c_slave_event(client, event, &val) | |
52 | ||
53 | 'client' describes the i2c slave device. 'event' is one of the special event | |
54 | types described hereafter. 'val' holds an u8 value for the data byte to be | |
55 | read/written and is thus bidirectional. The pointer to val must always be | |
56 | provided even if val is not used for an event, i.e. don't use NULL here. 'ret' | |
57 | is the return value from the backend. Mandatory events must be provided by the | |
58 | bus drivers and must be checked for by backend drivers. | |
59 | ||
60 | Event types: | |
61 | ||
62 | * I2C_SLAVE_WRITE_REQUESTED (mandatory) | |
63 | ||
64 | 'val': unused | |
65 | 'ret': always 0 | |
66 | ||
67 | Another I2C master wants to write data to us. This event should be sent once | |
68 | our own address and the write bit was detected. The data did not arrive yet, so | |
69 | there is nothing to process or return. Wakeup or initialization probably needs | |
70 | to be done, though. | |
71 | ||
72 | * I2C_SLAVE_READ_REQUESTED (mandatory) | |
73 | ||
74 | 'val': backend returns first byte to be sent | |
75 | 'ret': always 0 | |
76 | ||
77 | Another I2C master wants to read data from us. This event should be sent once | |
78 | our own address and the read bit was detected. After returning, the bus driver | |
79 | should transmit the first byte. | |
80 | ||
81 | * I2C_SLAVE_WRITE_RECEIVED (mandatory) | |
82 | ||
83 | 'val': bus driver delivers received byte | |
84 | 'ret': 0 if the byte should be acked, some errno if the byte should be nacked | |
85 | ||
86 | Another I2C master has sent a byte to us which needs to be set in 'val'. If 'ret' | |
87 | is zero, the bus driver should ack this byte. If 'ret' is an errno, then the byte | |
88 | should be nacked. | |
89 | ||
90 | * I2C_SLAVE_READ_PROCESSED (mandatory) | |
91 | ||
92 | 'val': backend returns next byte to be sent | |
93 | 'ret': always 0 | |
94 | ||
95 | The bus driver requests the next byte to be sent to another I2C master in | |
96 | 'val'. Important: This does not mean that the previous byte has been acked, it | |
97 | only means that the previous byte is shifted out to the bus! To ensure seamless | |
98 | transmission, most hardware requests the next byte when the previous one is | |
99 | still shifted out. If the master sends NACK and stops reading after the byte | |
100 | currently shifted out, this byte requested here is never used. It very likely | |
101 | needs to be sent again on the next I2C_SLAVE_READ_REQUEST, depending a bit on | |
102 | your backend, though. | |
103 | ||
104 | * I2C_SLAVE_STOP (mandatory) | |
105 | ||
106 | 'val': unused | |
107 | 'ret': always 0 | |
108 | ||
109 | A stop condition was received. This can happen anytime and the backend should | |
110 | reset its state machine for I2C transfers to be able to receive new requests. | |
111 | ||
112 | ||
113 | Software backends | |
114 | ----------------- | |
115 | ||
116 | If you want to write a software backend: | |
117 | ||
118 | * use a standard i2c_driver and its matching mechanisms | |
119 | * write the slave_callback which handles the above slave events | |
120 | (best using a state machine) | |
121 | * register this callback via i2c_slave_register() | |
122 | ||
123 | Check the i2c-slave-eeprom driver as an example. | |
124 | ||
125 | ||
126 | Bus driver support | |
127 | ------------------ | |
128 | ||
129 | If you want to add slave support to the bus driver: | |
130 | ||
131 | * implement calls to register/unregister the slave and add those to the | |
132 | struct i2c_algorithm. When registering, you probably need to set the i2c | |
133 | slave address and enable slave specific interrupts. If you use runtime pm, you | |
134 | should use pm_runtime_forbid() because your device usually needs to be powered | |
135 | on always to be able to detect its slave address. When unregistering, do the | |
136 | inverse of the above. | |
137 | ||
138 | * Catch the slave interrupts and send appropriate i2c_slave_events to the backend. | |
139 | ||
140 | Check the i2c-rcar driver as an example. | |
141 | ||
142 | ||
143 | About ACK/NACK | |
144 | -------------- | |
145 | ||
146 | It is good behaviour to always ACK the address phase, so the master knows if a | |
147 | device is basically present or if it mysteriously disappeared. Using NACK to | |
148 | state being busy is troublesome. SMBus demands to always ACK the address phase, | |
149 | while the I2C specification is more loose on that. Most I2C controllers also | |
150 | automatically ACK when detecting their slave addresses, so there is no option | |
151 | to NACK them. For those reasons, this API does not support NACK in the address | |
152 | phase. | |
153 | ||
154 | Currently, there is no slave event to report if the master did ACK or NACK a | |
155 | byte when it reads from us. We could make this an optional event if the need | |
156 | arises. However, cases should be extremely rare because the master is expected | |
157 | to send STOP after that and we have an event for that. Also, keep in mind not | |
158 | all I2C controllers have the possibility to report that event. | |
159 | ||
160 | ||
161 | About buffers | |
162 | ------------- | |
163 | ||
164 | During development of this API, the question of using buffers instead of just | |
165 | bytes came up. Such an extension might be possible, usefulness is unclear at | |
166 | this time of writing. Some points to keep in mind when using buffers: | |
167 | ||
168 | * Buffers should be opt-in and slave drivers will always have to support | |
169 | byte-based transactions as the ultimate fallback because this is how the | |
170 | majority of HW works. | |
171 | ||
172 | * For backends simulating hardware registers, buffers are not helpful because | |
173 | on writes an action should be immediately triggered. For reads, the data in | |
174 | the buffer might get stale. | |
175 | ||
176 | * A master can send STOP at any time. For partially transferred buffers, this | |
177 | means additional code to handle this exception. Such code tends to be | |
178 | error-prone. | |
179 |