Commit | Line | Data |
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1da177e4 | 1 | This is a small guide for those who want to write kernel drivers for I2C |
4298cfc3 | 2 | or SMBus devices, using Linux as the protocol host/master (not slave). |
1da177e4 LT |
3 | |
4 | To set up a driver, you need to do several things. Some are optional, and | |
5 | some things can be done slightly or completely different. Use this as a | |
6 | guide, not as a rule book! | |
7 | ||
8 | ||
9 | General remarks | |
10 | =============== | |
11 | ||
12 | Try to keep the kernel namespace as clean as possible. The best way to | |
0e47858d | 13 | do this is to use a unique prefix for all global symbols. This is |
1da177e4 LT |
14 | especially important for exported symbols, but it is a good idea to do |
15 | it for non-exported symbols too. We will use the prefix `foo_' in this | |
0e47858d | 16 | tutorial. |
1da177e4 LT |
17 | |
18 | ||
19 | The driver structure | |
20 | ==================== | |
21 | ||
22 | Usually, you will implement a single driver structure, and instantiate | |
0e47858d | 23 | all clients from it. Remember, a driver structure contains general access |
f37dd80a DB |
24 | routines, and should be zero-initialized except for fields with data you |
25 | provide. A client structure holds device-specific information like the | |
26 | driver model device node, and its I2C address. | |
1da177e4 | 27 | |
2260e63a BD |
28 | static struct i2c_device_id foo_idtable[] = { |
29 | { "foo", my_id_for_foo }, | |
30 | { "bar", my_id_for_bar }, | |
31 | { } | |
32 | }; | |
33 | ||
34 | MODULE_DEVICE_TABLE(i2c, foo_idtable); | |
35 | ||
1da177e4 | 36 | static struct i2c_driver foo_driver = { |
d45d204f | 37 | .driver = { |
d45d204f JD |
38 | .name = "foo", |
39 | }, | |
4298cfc3 | 40 | |
2260e63a | 41 | .id_table = foo_ids, |
4298cfc3 DB |
42 | .probe = foo_probe, |
43 | .remove = foo_remove, | |
4735c98f JD |
44 | /* if device autodetection is needed: */ |
45 | .class = I2C_CLASS_SOMETHING, | |
46 | .detect = foo_detect, | |
47 | .address_data = &addr_data, | |
4298cfc3 | 48 | |
f37dd80a DB |
49 | .shutdown = foo_shutdown, /* optional */ |
50 | .suspend = foo_suspend, /* optional */ | |
51 | .resume = foo_resume, /* optional */ | |
0e47858d | 52 | .command = foo_command, /* optional, deprecated */ |
1da177e4 | 53 | } |
0e47858d | 54 | |
f37dd80a DB |
55 | The name field is the driver name, and must not contain spaces. It |
56 | should match the module name (if the driver can be compiled as a module), | |
57 | although you can use MODULE_ALIAS (passing "foo" in this example) to add | |
4298cfc3 DB |
58 | another name for the module. If the driver name doesn't match the module |
59 | name, the module won't be automatically loaded (hotplug/coldplug). | |
1da177e4 | 60 | |
0e47858d | 61 | All other fields are for call-back functions which will be explained |
1da177e4 LT |
62 | below. |
63 | ||
1da177e4 LT |
64 | |
65 | Extra client data | |
66 | ================= | |
67 | ||
f37dd80a | 68 | Each client structure has a special `data' field that can point to any |
0e47858d | 69 | structure at all. You should use this to keep device-specific data. |
1da177e4 | 70 | |
f37dd80a DB |
71 | /* store the value */ |
72 | void i2c_set_clientdata(struct i2c_client *client, void *data); | |
73 | ||
74 | /* retrieve the value */ | |
7d1d8999 | 75 | void *i2c_get_clientdata(const struct i2c_client *client); |
f37dd80a | 76 | |
1da177e4 LT |
77 | |
78 | Accessing the client | |
79 | ==================== | |
80 | ||
81 | Let's say we have a valid client structure. At some time, we will need | |
82 | to gather information from the client, or write new information to the | |
0e47858d | 83 | client. |
1da177e4 | 84 | |
0e47858d | 85 | I have found it useful to define foo_read and foo_write functions for this. |
1da177e4 LT |
86 | For some cases, it will be easier to call the i2c functions directly, |
87 | but many chips have some kind of register-value idea that can easily | |
eefcd75e | 88 | be encapsulated. |
1da177e4 LT |
89 | |
90 | The below functions are simple examples, and should not be copied | |
91 | literally. | |
92 | ||
0e47858d JD |
93 | int foo_read_value(struct i2c_client *client, u8 reg) |
94 | { | |
95 | if (reg < 0x10) /* byte-sized register */ | |
96 | return i2c_smbus_read_byte_data(client, reg); | |
97 | else /* word-sized register */ | |
98 | return i2c_smbus_read_word_data(client, reg); | |
99 | } | |
100 | ||
101 | int foo_write_value(struct i2c_client *client, u8 reg, u16 value) | |
102 | { | |
103 | if (reg == 0x10) /* Impossible to write - driver error! */ | |
104 | return -EINVAL; | |
105 | else if (reg < 0x10) /* byte-sized register */ | |
106 | return i2c_smbus_write_byte_data(client, reg, value); | |
107 | else /* word-sized register */ | |
108 | return i2c_smbus_write_word_data(client, reg, value); | |
109 | } | |
1da177e4 | 110 | |
1da177e4 LT |
111 | |
112 | Probing and attaching | |
113 | ===================== | |
114 | ||
4298cfc3 | 115 | The Linux I2C stack was originally written to support access to hardware |
e313353d JD |
116 | monitoring chips on PC motherboards, and thus used to embed some assumptions |
117 | that were more appropriate to SMBus (and PCs) than to I2C. One of these | |
118 | assumptions was that most adapters and devices drivers support the SMBUS_QUICK | |
119 | protocol to probe device presence. Another was that devices and their drivers | |
4298cfc3 DB |
120 | can be sufficiently configured using only such probe primitives. |
121 | ||
122 | As Linux and its I2C stack became more widely used in embedded systems | |
123 | and complex components such as DVB adapters, those assumptions became more | |
124 | problematic. Drivers for I2C devices that issue interrupts need more (and | |
125 | different) configuration information, as do drivers handling chip variants | |
126 | that can't be distinguished by protocol probing, or which need some board | |
127 | specific information to operate correctly. | |
128 | ||
129 | Accordingly, the I2C stack now has two models for associating I2C devices | |
130 | with their drivers: the original "legacy" model, and a newer one that's | |
131 | fully compatible with the Linux 2.6 driver model. These models do not mix, | |
132 | since the "legacy" model requires drivers to create "i2c_client" device | |
133 | objects after SMBus style probing, while the Linux driver model expects | |
134 | drivers to be given such device objects in their probe() routines. | |
135 | ||
e313353d JD |
136 | The legacy model is deprecated now and will soon be removed, so we no |
137 | longer document it here. | |
138 | ||
4298cfc3 DB |
139 | |
140 | Standard Driver Model Binding ("New Style") | |
141 | ------------------------------------------- | |
142 | ||
143 | System infrastructure, typically board-specific initialization code or | |
144 | boot firmware, reports what I2C devices exist. For example, there may be | |
145 | a table, in the kernel or from the boot loader, identifying I2C devices | |
146 | and linking them to board-specific configuration information about IRQs | |
147 | and other wiring artifacts, chip type, and so on. That could be used to | |
148 | create i2c_client objects for each I2C device. | |
149 | ||
150 | I2C device drivers using this binding model work just like any other | |
151 | kind of driver in Linux: they provide a probe() method to bind to | |
152 | those devices, and a remove() method to unbind. | |
153 | ||
d2653e92 JD |
154 | static int foo_probe(struct i2c_client *client, |
155 | const struct i2c_device_id *id); | |
4298cfc3 DB |
156 | static int foo_remove(struct i2c_client *client); |
157 | ||
158 | Remember that the i2c_driver does not create those client handles. The | |
159 | handle may be used during foo_probe(). If foo_probe() reports success | |
160 | (zero not a negative status code) it may save the handle and use it until | |
161 | foo_remove() returns. That binding model is used by most Linux drivers. | |
162 | ||
2260e63a BD |
163 | The probe function is called when an entry in the id_table name field |
164 | matches the device's name. It is passed the entry that was matched so | |
165 | the driver knows which one in the table matched. | |
4298cfc3 DB |
166 | |
167 | ||
e313353d JD |
168 | Device Creation |
169 | --------------- | |
ce9e0794 JD |
170 | |
171 | If you know for a fact that an I2C device is connected to a given I2C bus, | |
172 | you can instantiate that device by simply filling an i2c_board_info | |
173 | structure with the device address and driver name, and calling | |
174 | i2c_new_device(). This will create the device, then the driver core will | |
175 | take care of finding the right driver and will call its probe() method. | |
176 | If a driver supports different device types, you can specify the type you | |
177 | want using the type field. You can also specify an IRQ and platform data | |
178 | if needed. | |
179 | ||
180 | Sometimes you know that a device is connected to a given I2C bus, but you | |
181 | don't know the exact address it uses. This happens on TV adapters for | |
182 | example, where the same driver supports dozens of slightly different | |
183 | models, and I2C device addresses change from one model to the next. In | |
184 | that case, you can use the i2c_new_probed_device() variant, which is | |
185 | similar to i2c_new_device(), except that it takes an additional list of | |
186 | possible I2C addresses to probe. A device is created for the first | |
187 | responsive address in the list. If you expect more than one device to be | |
188 | present in the address range, simply call i2c_new_probed_device() that | |
189 | many times. | |
190 | ||
191 | The call to i2c_new_device() or i2c_new_probed_device() typically happens | |
192 | in the I2C bus driver. You may want to save the returned i2c_client | |
193 | reference for later use. | |
194 | ||
195 | ||
e313353d JD |
196 | Device Detection |
197 | ---------------- | |
4735c98f JD |
198 | |
199 | Sometimes you do not know in advance which I2C devices are connected to | |
200 | a given I2C bus. This is for example the case of hardware monitoring | |
201 | devices on a PC's SMBus. In that case, you may want to let your driver | |
202 | detect supported devices automatically. This is how the legacy model | |
203 | was working, and is now available as an extension to the standard | |
204 | driver model (so that we can finally get rid of the legacy model.) | |
205 | ||
206 | You simply have to define a detect callback which will attempt to | |
207 | identify supported devices (returning 0 for supported ones and -ENODEV | |
208 | for unsupported ones), a list of addresses to probe, and a device type | |
209 | (or class) so that only I2C buses which may have that type of device | |
764c1691 JD |
210 | connected (and not otherwise enumerated) will be probed. For example, |
211 | a driver for a hardware monitoring chip for which auto-detection is | |
212 | needed would set its class to I2C_CLASS_HWMON, and only I2C adapters | |
213 | with a class including I2C_CLASS_HWMON would be probed by this driver. | |
214 | Note that the absence of matching classes does not prevent the use of | |
215 | a device of that type on the given I2C adapter. All it prevents is | |
216 | auto-detection; explicit instantiation of devices is still possible. | |
4735c98f JD |
217 | |
218 | Note that this mechanism is purely optional and not suitable for all | |
219 | devices. You need some reliable way to identify the supported devices | |
220 | (typically using device-specific, dedicated identification registers), | |
221 | otherwise misdetections are likely to occur and things can get wrong | |
764c1691 JD |
222 | quickly. Keep in mind that the I2C protocol doesn't include any |
223 | standard way to detect the presence of a chip at a given address, let | |
224 | alone a standard way to identify devices. Even worse is the lack of | |
225 | semantics associated to bus transfers, which means that the same | |
226 | transfer can be seen as a read operation by a chip and as a write | |
227 | operation by another chip. For these reasons, explicit device | |
228 | instantiation should always be preferred to auto-detection where | |
229 | possible. | |
4735c98f JD |
230 | |
231 | ||
e313353d JD |
232 | Device Deletion |
233 | --------------- | |
ce9e0794 JD |
234 | |
235 | Each I2C device which has been created using i2c_new_device() or | |
236 | i2c_new_probed_device() can be unregistered by calling | |
237 | i2c_unregister_device(). If you don't call it explicitly, it will be | |
238 | called automatically before the underlying I2C bus itself is removed, as a | |
239 | device can't survive its parent in the device driver model. | |
240 | ||
241 | ||
0e47858d JD |
242 | Initializing the driver |
243 | ======================= | |
1da177e4 | 244 | |
0e47858d JD |
245 | When the kernel is booted, or when your foo driver module is inserted, |
246 | you have to do some initializing. Fortunately, just registering the | |
247 | driver module is usually enough. | |
1da177e4 | 248 | |
0e47858d JD |
249 | static int __init foo_init(void) |
250 | { | |
251 | return i2c_add_driver(&foo_driver); | |
252 | } | |
1da177e4 | 253 | |
0e47858d JD |
254 | static void __exit foo_cleanup(void) |
255 | { | |
256 | i2c_del_driver(&foo_driver); | |
257 | } | |
1da177e4 | 258 | |
0e47858d JD |
259 | /* Substitute your own name and email address */ |
260 | MODULE_AUTHOR("Frodo Looijaard <frodol@dds.nl>" | |
261 | MODULE_DESCRIPTION("Driver for Barf Inc. Foo I2C devices"); | |
1da177e4 | 262 | |
0e47858d JD |
263 | /* a few non-GPL license types are also allowed */ |
264 | MODULE_LICENSE("GPL"); | |
eefcd75e | 265 | |
0e47858d JD |
266 | module_init(foo_init); |
267 | module_exit(foo_cleanup); | |
1da177e4 | 268 | |
0e47858d JD |
269 | Note that some functions are marked by `__init'. These functions can |
270 | be removed after kernel booting (or module loading) is completed. | |
271 | Likewise, functions marked by `__exit' are dropped by the compiler when | |
272 | the code is built into the kernel, as they would never be called. | |
1da177e4 | 273 | |
fb687d73 | 274 | |
f37dd80a DB |
275 | Power Management |
276 | ================ | |
277 | ||
278 | If your I2C device needs special handling when entering a system low | |
279 | power state -- like putting a transceiver into a low power mode, or | |
280 | activating a system wakeup mechanism -- do that in the suspend() method. | |
281 | The resume() method should reverse what the suspend() method does. | |
282 | ||
283 | These are standard driver model calls, and they work just like they | |
284 | would for any other driver stack. The calls can sleep, and can use | |
285 | I2C messaging to the device being suspended or resumed (since their | |
286 | parent I2C adapter is active when these calls are issued, and IRQs | |
287 | are still enabled). | |
288 | ||
289 | ||
290 | System Shutdown | |
291 | =============== | |
292 | ||
293 | If your I2C device needs special handling when the system shuts down | |
294 | or reboots (including kexec) -- like turning something off -- use a | |
295 | shutdown() method. | |
296 | ||
297 | Again, this is a standard driver model call, working just like it | |
298 | would for any other driver stack: the calls can sleep, and can use | |
299 | I2C messaging. | |
300 | ||
301 | ||
1da177e4 LT |
302 | Command function |
303 | ================ | |
304 | ||
305 | A generic ioctl-like function call back is supported. You will seldom | |
fb687d73 | 306 | need this, and its use is deprecated anyway, so newer design should not |
0e47858d | 307 | use it. |
1da177e4 LT |
308 | |
309 | ||
310 | Sending and receiving | |
311 | ===================== | |
312 | ||
313 | If you want to communicate with your device, there are several functions | |
0e47858d | 314 | to do this. You can find all of them in <linux/i2c.h>. |
1da177e4 | 315 | |
0e47858d JD |
316 | If you can choose between plain I2C communication and SMBus level |
317 | communication, please use the latter. All adapters understand SMBus level | |
318 | commands, but only some of them understand plain I2C! | |
1da177e4 LT |
319 | |
320 | ||
0e47858d | 321 | Plain I2C communication |
1da177e4 LT |
322 | ----------------------- |
323 | ||
0e47858d JD |
324 | int i2c_master_send(struct i2c_client *client, const char *buf, |
325 | int count); | |
326 | int i2c_master_recv(struct i2c_client *client, char *buf, int count); | |
1da177e4 LT |
327 | |
328 | These routines read and write some bytes from/to a client. The client | |
329 | contains the i2c address, so you do not have to include it. The second | |
0e47858d JD |
330 | parameter contains the bytes to read/write, the third the number of bytes |
331 | to read/write (must be less than the length of the buffer.) Returned is | |
332 | the actual number of bytes read/written. | |
333 | ||
334 | int i2c_transfer(struct i2c_adapter *adap, struct i2c_msg *msg, | |
335 | int num); | |
1da177e4 LT |
336 | |
337 | This sends a series of messages. Each message can be a read or write, | |
338 | and they can be mixed in any way. The transactions are combined: no | |
339 | stop bit is sent between transaction. The i2c_msg structure contains | |
340 | for each message the client address, the number of bytes of the message | |
341 | and the message data itself. | |
342 | ||
343 | You can read the file `i2c-protocol' for more information about the | |
0e47858d | 344 | actual I2C protocol. |
1da177e4 LT |
345 | |
346 | ||
347 | SMBus communication | |
348 | ------------------- | |
349 | ||
0e47858d JD |
350 | s32 i2c_smbus_xfer(struct i2c_adapter *adapter, u16 addr, |
351 | unsigned short flags, char read_write, u8 command, | |
352 | int size, union i2c_smbus_data *data); | |
353 | ||
354 | This is the generic SMBus function. All functions below are implemented | |
355 | in terms of it. Never use this function directly! | |
356 | ||
357 | s32 i2c_smbus_read_byte(struct i2c_client *client); | |
358 | s32 i2c_smbus_write_byte(struct i2c_client *client, u8 value); | |
359 | s32 i2c_smbus_read_byte_data(struct i2c_client *client, u8 command); | |
360 | s32 i2c_smbus_write_byte_data(struct i2c_client *client, | |
361 | u8 command, u8 value); | |
362 | s32 i2c_smbus_read_word_data(struct i2c_client *client, u8 command); | |
363 | s32 i2c_smbus_write_word_data(struct i2c_client *client, | |
364 | u8 command, u16 value); | |
365 | s32 i2c_smbus_process_call(struct i2c_client *client, | |
366 | u8 command, u16 value); | |
367 | s32 i2c_smbus_read_block_data(struct i2c_client *client, | |
368 | u8 command, u8 *values); | |
369 | s32 i2c_smbus_write_block_data(struct i2c_client *client, | |
370 | u8 command, u8 length, const u8 *values); | |
371 | s32 i2c_smbus_read_i2c_block_data(struct i2c_client *client, | |
372 | u8 command, u8 length, u8 *values); | |
373 | s32 i2c_smbus_write_i2c_block_data(struct i2c_client *client, | |
374 | u8 command, u8 length, | |
375 | const u8 *values); | |
67c2e665 JD |
376 | |
377 | These ones were removed from i2c-core because they had no users, but could | |
378 | be added back later if needed: | |
379 | ||
0e47858d JD |
380 | s32 i2c_smbus_write_quick(struct i2c_client *client, u8 value); |
381 | s32 i2c_smbus_block_process_call(struct i2c_client *client, | |
382 | u8 command, u8 length, u8 *values); | |
1da177e4 | 383 | |
24a5bb7b DB |
384 | All these transactions return a negative errno value on failure. The 'write' |
385 | transactions return 0 on success; the 'read' transactions return the read | |
386 | value, except for block transactions, which return the number of values | |
387 | read. The block buffers need not be longer than 32 bytes. | |
1da177e4 LT |
388 | |
389 | You can read the file `smbus-protocol' for more information about the | |
390 | actual SMBus protocol. | |
391 | ||
392 | ||
393 | General purpose routines | |
394 | ======================== | |
395 | ||
396 | Below all general purpose routines are listed, that were not mentioned | |
397 | before. | |
398 | ||
0e47858d JD |
399 | /* Return the adapter number for a specific adapter */ |
400 | int i2c_adapter_id(struct i2c_adapter *adap); |