1 ===============================
2 Implementing I2C device drivers
3 ===============================
4
5 This is a small guide for those who want to write kernel drivers for I2C
6 or SMBus devices, using Linux as the protocol host/master (not slave).
7
8 To set up a driver, you need to do several things. Some are optional, and
9 some things can be done slightly or completely different. Use this as a
10 guide, not as a rule book!
11
12
13 General remarks
14 ===============
15
16 Try to keep the kernel namespace as clean as possible. The best way to
17 do this is to use a unique prefix for all global symbols. This is
18 especially important for exported symbols, but it is a good idea to do
19 it for non-exported symbols too. We will use the prefix ``foo_`` in this
20 tutorial.
21
22
23 The driver structure
24 ====================
25
26 Usually, you will implement a single driver structure, and instantiate
27 all clients from it. Remember, a driver structure contains general access
28 routines, and should be zero-initialized except for fields with data you
29 provide. A client structure holds device-specific information like the
30 driver model device node, and its I2C address.
31
32 ::
33
34 static struct i2c_device_id foo_idtable[] = {
35 { "foo", my_id_for_foo },
36 { "bar", my_id_for_bar },
37 { }
38 };
39
40 MODULE_DEVICE_TABLE(i2c, foo_idtable);
41
42 static struct i2c_driver foo_driver = {
43 .driver = {
44 .name = "foo",
45 .pm = &foo_pm_ops, /* optional */
46 },
47
48 .id_table = foo_idtable,
49 .probe = foo_probe,
50 .remove = foo_remove,
51
52 .shutdown = foo_shutdown, /* optional */
53 .command = foo_command, /* optional, deprecated */
54 }
55
56 The name field is the driver name, and must not contain spaces. It
57 should match the module name (if the driver can be compiled as a module),
58 although you can use MODULE_ALIAS (passing "foo" in this example) to add
59 another name for the module. If the driver name doesn't match the module
60 name, the module won't be automatically loaded (hotplug/coldplug).
61
62 All other fields are for call-back functions which will be explained
63 below.
64
65
66 Extra client data
67 =================
68
69 Each client structure has a special ``data`` field that can point to any
70 structure at all. You should use this to keep device-specific data.
71
72 ::
73
74 /* store the value */
75 void i2c_set_clientdata(struct i2c_client *client, void *data);
76
77 /* retrieve the value */
78 void *i2c_get_clientdata(const struct i2c_client *client);
79
80 Note that starting with kernel 2.6.34, you don't have to set the ``data`` field
81 to NULL in remove() or if probe() failed anymore. The i2c-core does this
82 automatically on these occasions. Those are also the only times the core will
83 touch this field.
84
85
86 Accessing the client
87 ====================
88
89 Let's say we have a valid client structure. At some time, we will need
90 to gather information from the client, or write new information to the
91 client.
92
93 I have found it useful to define foo_read and foo_write functions for this.
94 For some cases, it will be easier to call the I2C functions directly,
95 but many chips have some kind of register-value idea that can easily
96 be encapsulated.
97
98 The below functions are simple examples, and should not be copied
99 literally::
100
101 int foo_read_value(struct i2c_client *client, u8 reg)
102 {
103 if (reg < 0x10) /* byte-sized register */
104 return i2c_smbus_read_byte_data(client, reg);
105 else /* word-sized register */
106 return i2c_smbus_read_word_data(client, reg);
107 }
108
109 int foo_write_value(struct i2c_client *client, u8 reg, u16 value)
110 {
111 if (reg == 0x10) /* Impossible to write - driver error! */
112 return -EINVAL;
113 else if (reg < 0x10) /* byte-sized register */
114 return i2c_smbus_write_byte_data(client, reg, value);
115 else /* word-sized register */
116 return i2c_smbus_write_word_data(client, reg, value);
117 }
118
119
120 Probing and attaching
121 =====================
122
123 The Linux I2C stack was originally written to support access to hardware
124 monitoring chips on PC motherboards, and thus used to embed some assumptions
125 that were more appropriate to SMBus (and PCs) than to I2C. One of these
126 assumptions was that most adapters and devices drivers support the SMBUS_QUICK
127 protocol to probe device presence. Another was that devices and their drivers
128 can be sufficiently configured using only such probe primitives.
129
130 As Linux and its I2C stack became more widely used in embedded systems
131 and complex components such as DVB adapters, those assumptions became more
132 problematic. Drivers for I2C devices that issue interrupts need more (and
133 different) configuration information, as do drivers handling chip variants
134 that can't be distinguished by protocol probing, or which need some board
135 specific information to operate correctly.
136
137
138 Device/Driver Binding
139 ---------------------
140
141 System infrastructure, typically board-specific initialization code or
142 boot firmware, reports what I2C devices exist. For example, there may be
143 a table, in the kernel or from the boot loader, identifying I2C devices
144 and linking them to board-specific configuration information about IRQs
145 and other wiring artifacts, chip type, and so on. That could be used to
146 create i2c_client objects for each I2C device.
147
148 I2C device drivers using this binding model work just like any other
149 kind of driver in Linux: they provide a probe() method to bind to
150 those devices, and a remove() method to unbind.
151
152 ::
153
154 static int foo_probe(struct i2c_client *client);
155 static void foo_remove(struct i2c_client *client);
156
157 Remember that the i2c_driver does not create those client handles. The
158 handle may be used during foo_probe(). If foo_probe() reports success
159 (zero not a negative status code) it may save the handle and use it until
160 foo_remove() returns. That binding model is used by most Linux drivers.
161
162 The probe function is called when an entry in the id_table name field
163 matches the device's name. If the probe function needs that entry, it
164 can retrieve it using
165
166 ::
167
168 const struct i2c_device_id *id = i2c_match_id(foo_idtable, client);
169
170
171 Device Creation
172 ---------------
173
174 If you know for a fact that an I2C device is connected to a given I2C bus,
175 you can instantiate that device by simply filling an i2c_board_info
176 structure with the device address and driver name, and calling
177 i2c_new_client_device(). This will create the device, then the driver core
178 will take care of finding the right driver and will call its probe() method.
179 If a driver supports different device types, you can specify the type you
180 want using the type field. You can also specify an IRQ and platform data
181 if needed.
182
183 Sometimes you know that a device is connected to a given I2C bus, but you
184 don't know the exact address it uses. This happens on TV adapters for
185 example, where the same driver supports dozens of slightly different
186 models, and I2C device addresses change from one model to the next. In
187 that case, you can use the i2c_new_scanned_device() variant, which is
188 similar to i2c_new_client_device(), except that it takes an additional list
189 of possible I2C addresses to probe. A device is created for the first
190 responsive address in the list. If you expect more than one device to be
191 present in the address range, simply call i2c_new_scanned_device() that
192 many times.
193
194 The call to i2c_new_client_device() or i2c_new_scanned_device() typically
195 happens in the I2C bus driver. You may want to save the returned i2c_client
196 reference for later use.
197
198
199 Device Detection
200 ----------------
201
202 The device detection mechanism comes with a number of disadvantages.
203 You need some reliable way to identify the supported devices
204 (typically using device-specific, dedicated identification registers),
205 otherwise misdetections are likely to occur and things can get wrong
206 quickly. Keep in mind that the I2C protocol doesn't include any
207 standard way to detect the presence of a chip at a given address, let
208 alone a standard way to identify devices. Even worse is the lack of
209 semantics associated to bus transfers, which means that the same
210 transfer can be seen as a read operation by a chip and as a write
211 operation by another chip. For these reasons, device detection is
212 considered a legacy mechanism and shouldn't be used in new code.
213
214
215 Device Deletion
216 ---------------
217
218 Each I2C device which has been created using i2c_new_client_device()
219 or i2c_new_scanned_device() can be unregistered by calling
220 i2c_unregister_device(). If you don't call it explicitly, it will be
221 called automatically before the underlying I2C bus itself is removed,
222 as a device can't survive its parent in the device driver model.
223
224
225 Initializing the driver
226 =======================
227
228 When the kernel is booted, or when your foo driver module is inserted,
229 you have to do some initializing. Fortunately, just registering the
230 driver module is usually enough.
231
232 ::
233
234 static int __init foo_init(void)
235 {
236 return i2c_add_driver(&foo_driver);
237 }
238 module_init(foo_init);
239
240 static void __exit foo_cleanup(void)
241 {
242 i2c_del_driver(&foo_driver);
243 }
244 module_exit(foo_cleanup);
245
246 The module_i2c_driver() macro can be used to reduce above code.
247
248 module_i2c_driver(foo_driver);
249
250 Note that some functions are marked by ``__init``. These functions can
251 be removed after kernel booting (or module loading) is completed.
252 Likewise, functions marked by ``__exit`` are dropped by the compiler when
253 the code is built into the kernel, as they would never be called.
254
255
256 Driver Information
257 ==================
258
259 ::
260
261 /* Substitute your own name and email address */
262 MODULE_AUTHOR("Frodo Looijaard <frodol@dds.nl>"
263 MODULE_DESCRIPTION("Driver for Barf Inc. Foo I2C devices");
264
265 /* a few non-GPL license types are also allowed */
266 MODULE_LICENSE("GPL");
267
268
269 Power Management
270 ================
271
272 If your I2C device needs special handling when entering a system low
273 power state -- like putting a transceiver into a low power mode, or
274 activating a system wakeup mechanism -- do that by implementing the
275 appropriate callbacks for the dev_pm_ops of the driver (like suspend
276 and resume).
277
278 These are standard driver model calls, and they work just like they
279 would for any other driver stack. The calls can sleep, and can use
280 I2C messaging to the device being suspended or resumed (since their
281 parent I2C adapter is active when these calls are issued, and IRQs
282 are still enabled).
283
284
285 System Shutdown
286 ===============
287
288 If your I2C device needs special handling when the system shuts down
289 or reboots (including kexec) -- like turning something off -- use a
290 shutdown() method.
291
292 Again, this is a standard driver model call, working just like it
293 would for any other driver stack: the calls can sleep, and can use
294 I2C messaging.
295
296
297 Command function
298 ================
299
300 A generic ioctl-like function call back is supported. You will seldom
301 need this, and its use is deprecated anyway, so newer design should not
302 use it.
303
304
305 Sending and receiving
306 =====================
307
308 If you want to communicate with your device, there are several functions
309 to do this. You can find all of them in <linux/i2c.h>.
310
311 If you can choose between plain I2C communication and SMBus level
312 communication, please use the latter. All adapters understand SMBus level
313 commands, but only some of them understand plain I2C!
314
315
316 Plain I2C communication
317 -----------------------
318
319 ::
320
321 int i2c_master_send(struct i2c_client *client, const char *buf,
322 int count);
323 int i2c_master_recv(struct i2c_client *client, char *buf, int count);
324
325 These routines read and write some bytes from/to a client. The client
326 contains the I2C address, so you do not have to include it. The second
327 parameter contains the bytes to read/write, the third the number of bytes
328 to read/write (must be less than the length of the buffer, also should be
329 less than 64k since msg.len is u16.) Returned is the actual number of bytes
330 read/written.
331
332 ::
333
334 int i2c_transfer(struct i2c_adapter *adap, struct i2c_msg *msg,
335 int num);
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 condition is issued between transaction. The i2c_msg structure
340 contains for each message the client address, the number of bytes of the
341 message and the message data itself.
342
343 You can read the file i2c-protocol.rst for more information about the
344 actual I2C protocol.
345
346
347 SMBus communication
348 -------------------
349
350 ::
351
352 s32 i2c_smbus_xfer(struct i2c_adapter *adapter, u16 addr,
353 unsigned short flags, char read_write, u8 command,
354 int size, union i2c_smbus_data *data);
355
356 This is the generic SMBus function. All functions below are implemented
357 in terms of it. Never use this function directly!
358
359 ::
360
361 s32 i2c_smbus_read_byte(struct i2c_client *client);
362 s32 i2c_smbus_write_byte(struct i2c_client *client, u8 value);
363 s32 i2c_smbus_read_byte_data(struct i2c_client *client, u8 command);
364 s32 i2c_smbus_write_byte_data(struct i2c_client *client,
365 u8 command, u8 value);
366 s32 i2c_smbus_read_word_data(struct i2c_client *client, u8 command);
367 s32 i2c_smbus_write_word_data(struct i2c_client *client,
368 u8 command, u16 value);
369 s32 i2c_smbus_read_block_data(struct i2c_client *client,
370 u8 command, u8 *values);
371 s32 i2c_smbus_write_block_data(struct i2c_client *client,
372 u8 command, u8 length, const u8 *values);
373 s32 i2c_smbus_read_i2c_block_data(struct i2c_client *client,
374 u8 command, u8 length, u8 *values);
375 s32 i2c_smbus_write_i2c_block_data(struct i2c_client *client,
376 u8 command, u8 length,
377 const u8 *values);
378
379 These ones were removed from i2c-core because they had no users, but could
380 be added back later if needed::
381
382 s32 i2c_smbus_write_quick(struct i2c_client *client, u8 value);
383 s32 i2c_smbus_process_call(struct i2c_client *client,
384 u8 command, u16 value);
385 s32 i2c_smbus_block_process_call(struct i2c_client *client,
386 u8 command, u8 length, u8 *values);
387
388 All these transactions return a negative errno value on failure. The 'write'
389 transactions return 0 on success; the 'read' transactions return the read
390 value, except for block transactions, which return the number of values
391 read. The block buffers need not be longer than 32 bytes.
392
393 You can read the file smbus-protocol.rst for more information about the
394 actual SMBus protocol.
395
396
397 General purpose routines
398 ========================
399
400 Below all general purpose routines are listed, that were not mentioned
401 before::
402
403 /* Return the adapter number for a specific adapter */
404 int i2c_adapter_id(struct i2c_adapter *adap);
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