]> git.proxmox.com Git - mirror_ubuntu-artful-kernel.git/blob - Documentation/i2c/writing-clients
Merge ath-next from ath.git
[mirror_ubuntu-artful-kernel.git] / Documentation / i2c / writing-clients
1 This is a small guide for those who want to write kernel drivers for I2C
2 or SMBus devices, using Linux as the protocol host/master (not slave).
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
13 do this is to use a unique prefix for all global symbols. This is
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
16 tutorial.
17
18
19 The driver structure
20 ====================
21
22 Usually, you will implement a single driver structure, and instantiate
23 all clients from it. Remember, a driver structure contains general access
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.
27
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
36 static struct i2c_driver foo_driver = {
37 .driver = {
38 .name = "foo",
39 .pm = &foo_pm_ops, /* optional */
40 },
41
42 .id_table = foo_idtable,
43 .probe = foo_probe,
44 .remove = foo_remove,
45 /* if device autodetection is needed: */
46 .class = I2C_CLASS_SOMETHING,
47 .detect = foo_detect,
48 .address_list = normal_i2c,
49
50 .shutdown = foo_shutdown, /* optional */
51 .command = foo_command, /* optional, deprecated */
52 }
53
54 The name field is the driver name, and must not contain spaces. It
55 should match the module name (if the driver can be compiled as a module),
56 although you can use MODULE_ALIAS (passing "foo" in this example) to add
57 another name for the module. If the driver name doesn't match the module
58 name, the module won't be automatically loaded (hotplug/coldplug).
59
60 All other fields are for call-back functions which will be explained
61 below.
62
63
64 Extra client data
65 =================
66
67 Each client structure has a special `data' field that can point to any
68 structure at all. You should use this to keep device-specific data.
69
70 /* store the value */
71 void i2c_set_clientdata(struct i2c_client *client, void *data);
72
73 /* retrieve the value */
74 void *i2c_get_clientdata(const struct i2c_client *client);
75
76 Note that starting with kernel 2.6.34, you don't have to set the `data' field
77 to NULL in remove() or if probe() failed anymore. The i2c-core does this
78 automatically on these occasions. Those are also the only times the core will
79 touch this field.
80
81
82 Accessing the client
83 ====================
84
85 Let's say we have a valid client structure. At some time, we will need
86 to gather information from the client, or write new information to the
87 client.
88
89 I have found it useful to define foo_read and foo_write functions for this.
90 For some cases, it will be easier to call the i2c functions directly,
91 but many chips have some kind of register-value idea that can easily
92 be encapsulated.
93
94 The below functions are simple examples, and should not be copied
95 literally.
96
97 int foo_read_value(struct i2c_client *client, u8 reg)
98 {
99 if (reg < 0x10) /* byte-sized register */
100 return i2c_smbus_read_byte_data(client, reg);
101 else /* word-sized register */
102 return i2c_smbus_read_word_data(client, reg);
103 }
104
105 int foo_write_value(struct i2c_client *client, u8 reg, u16 value)
106 {
107 if (reg == 0x10) /* Impossible to write - driver error! */
108 return -EINVAL;
109 else if (reg < 0x10) /* byte-sized register */
110 return i2c_smbus_write_byte_data(client, reg, value);
111 else /* word-sized register */
112 return i2c_smbus_write_word_data(client, reg, value);
113 }
114
115
116 Probing and attaching
117 =====================
118
119 The Linux I2C stack was originally written to support access to hardware
120 monitoring chips on PC motherboards, and thus used to embed some assumptions
121 that were more appropriate to SMBus (and PCs) than to I2C. One of these
122 assumptions was that most adapters and devices drivers support the SMBUS_QUICK
123 protocol to probe device presence. Another was that devices and their drivers
124 can be sufficiently configured using only such probe primitives.
125
126 As Linux and its I2C stack became more widely used in embedded systems
127 and complex components such as DVB adapters, those assumptions became more
128 problematic. Drivers for I2C devices that issue interrupts need more (and
129 different) configuration information, as do drivers handling chip variants
130 that can't be distinguished by protocol probing, or which need some board
131 specific information to operate correctly.
132
133
134 Device/Driver Binding
135 ---------------------
136
137 System infrastructure, typically board-specific initialization code or
138 boot firmware, reports what I2C devices exist. For example, there may be
139 a table, in the kernel or from the boot loader, identifying I2C devices
140 and linking them to board-specific configuration information about IRQs
141 and other wiring artifacts, chip type, and so on. That could be used to
142 create i2c_client objects for each I2C device.
143
144 I2C device drivers using this binding model work just like any other
145 kind of driver in Linux: they provide a probe() method to bind to
146 those devices, and a remove() method to unbind.
147
148 static int foo_probe(struct i2c_client *client,
149 const struct i2c_device_id *id);
150 static int foo_remove(struct i2c_client *client);
151
152 Remember that the i2c_driver does not create those client handles. The
153 handle may be used during foo_probe(). If foo_probe() reports success
154 (zero not a negative status code) it may save the handle and use it until
155 foo_remove() returns. That binding model is used by most Linux drivers.
156
157 The probe function is called when an entry in the id_table name field
158 matches the device's name. It is passed the entry that was matched so
159 the driver knows which one in the table matched.
160
161
162 Device Creation
163 ---------------
164
165 If you know for a fact that an I2C device is connected to a given I2C bus,
166 you can instantiate that device by simply filling an i2c_board_info
167 structure with the device address and driver name, and calling
168 i2c_new_device(). This will create the device, then the driver core will
169 take care of finding the right driver and will call its probe() method.
170 If a driver supports different device types, you can specify the type you
171 want using the type field. You can also specify an IRQ and platform data
172 if needed.
173
174 Sometimes you know that a device is connected to a given I2C bus, but you
175 don't know the exact address it uses. This happens on TV adapters for
176 example, where the same driver supports dozens of slightly different
177 models, and I2C device addresses change from one model to the next. In
178 that case, you can use the i2c_new_probed_device() variant, which is
179 similar to i2c_new_device(), except that it takes an additional list of
180 possible I2C addresses to probe. A device is created for the first
181 responsive address in the list. If you expect more than one device to be
182 present in the address range, simply call i2c_new_probed_device() that
183 many times.
184
185 The call to i2c_new_device() or i2c_new_probed_device() typically happens
186 in the I2C bus driver. You may want to save the returned i2c_client
187 reference for later use.
188
189
190 Device Detection
191 ----------------
192
193 Sometimes you do not know in advance which I2C devices are connected to
194 a given I2C bus. This is for example the case of hardware monitoring
195 devices on a PC's SMBus. In that case, you may want to let your driver
196 detect supported devices automatically. This is how the legacy model
197 was working, and is now available as an extension to the standard
198 driver model.
199
200 You simply have to define a detect callback which will attempt to
201 identify supported devices (returning 0 for supported ones and -ENODEV
202 for unsupported ones), a list of addresses to probe, and a device type
203 (or class) so that only I2C buses which may have that type of device
204 connected (and not otherwise enumerated) will be probed. For example,
205 a driver for a hardware monitoring chip for which auto-detection is
206 needed would set its class to I2C_CLASS_HWMON, and only I2C adapters
207 with a class including I2C_CLASS_HWMON would be probed by this driver.
208 Note that the absence of matching classes does not prevent the use of
209 a device of that type on the given I2C adapter. All it prevents is
210 auto-detection; explicit instantiation of devices is still possible.
211
212 Note that this mechanism is purely optional and not suitable for all
213 devices. You need some reliable way to identify the supported devices
214 (typically using device-specific, dedicated identification registers),
215 otherwise misdetections are likely to occur and things can get wrong
216 quickly. Keep in mind that the I2C protocol doesn't include any
217 standard way to detect the presence of a chip at a given address, let
218 alone a standard way to identify devices. Even worse is the lack of
219 semantics associated to bus transfers, which means that the same
220 transfer can be seen as a read operation by a chip and as a write
221 operation by another chip. For these reasons, explicit device
222 instantiation should always be preferred to auto-detection where
223 possible.
224
225
226 Device Deletion
227 ---------------
228
229 Each I2C device which has been created using i2c_new_device() or
230 i2c_new_probed_device() can be unregistered by calling
231 i2c_unregister_device(). If you don't call it explicitly, it will be
232 called automatically before the underlying I2C bus itself is removed, as a
233 device can't survive its parent in the device driver model.
234
235
236 Initializing the driver
237 =======================
238
239 When the kernel is booted, or when your foo driver module is inserted,
240 you have to do some initializing. Fortunately, just registering the
241 driver module is usually enough.
242
243 static int __init foo_init(void)
244 {
245 return i2c_add_driver(&foo_driver);
246 }
247 module_init(foo_init);
248
249 static void __exit foo_cleanup(void)
250 {
251 i2c_del_driver(&foo_driver);
252 }
253 module_exit(foo_cleanup);
254
255 The module_i2c_driver() macro can be used to reduce above code.
256
257 module_i2c_driver(foo_driver);
258
259 Note that some functions are marked by `__init'. These functions can
260 be removed after kernel booting (or module loading) is completed.
261 Likewise, functions marked by `__exit' are dropped by the compiler when
262 the code is built into the kernel, as they would never be called.
263
264
265 Driver Information
266 ==================
267
268 /* Substitute your own name and email address */
269 MODULE_AUTHOR("Frodo Looijaard <frodol@dds.nl>"
270 MODULE_DESCRIPTION("Driver for Barf Inc. Foo I2C devices");
271
272 /* a few non-GPL license types are also allowed */
273 MODULE_LICENSE("GPL");
274
275
276 Power Management
277 ================
278
279 If your I2C device needs special handling when entering a system low
280 power state -- like putting a transceiver into a low power mode, or
281 activating a system wakeup mechanism -- do that by implementing the
282 appropriate callbacks for the dev_pm_ops of the driver (like suspend
283 and resume).
284
285 These are standard driver model calls, and they work just like they
286 would for any other driver stack. The calls can sleep, and can use
287 I2C messaging to the device being suspended or resumed (since their
288 parent I2C adapter is active when these calls are issued, and IRQs
289 are still enabled).
290
291
292 System Shutdown
293 ===============
294
295 If your I2C device needs special handling when the system shuts down
296 or reboots (including kexec) -- like turning something off -- use a
297 shutdown() method.
298
299 Again, this is a standard driver model call, working just like it
300 would for any other driver stack: the calls can sleep, and can use
301 I2C messaging.
302
303
304 Command function
305 ================
306
307 A generic ioctl-like function call back is supported. You will seldom
308 need this, and its use is deprecated anyway, so newer design should not
309 use it.
310
311
312 Sending and receiving
313 =====================
314
315 If you want to communicate with your device, there are several functions
316 to do this. You can find all of them in <linux/i2c.h>.
317
318 If you can choose between plain I2C communication and SMBus level
319 communication, please use the latter. All adapters understand SMBus level
320 commands, but only some of them understand plain I2C!
321
322
323 Plain I2C communication
324 -----------------------
325
326 int i2c_master_send(struct i2c_client *client, const char *buf,
327 int count);
328 int i2c_master_recv(struct i2c_client *client, char *buf, int count);
329
330 These routines read and write some bytes from/to a client. The client
331 contains the i2c address, so you do not have to include it. The second
332 parameter contains the bytes to read/write, the third the number of bytes
333 to read/write (must be less than the length of the buffer, also should be
334 less than 64k since msg.len is u16.) Returned is the actual number of bytes
335 read/written.
336
337 int i2c_transfer(struct i2c_adapter *adap, struct i2c_msg *msg,
338 int num);
339
340 This sends a series of messages. Each message can be a read or write,
341 and they can be mixed in any way. The transactions are combined: no
342 stop bit is sent between transaction. The i2c_msg structure contains
343 for each message the client address, the number of bytes of the message
344 and the message data itself.
345
346 You can read the file `i2c-protocol' for more information about the
347 actual I2C protocol.
348
349
350 SMBus communication
351 -------------------
352
353 s32 i2c_smbus_xfer(struct i2c_adapter *adapter, u16 addr,
354 unsigned short flags, char read_write, u8 command,
355 int size, union i2c_smbus_data *data);
356
357 This is the generic SMBus function. All functions below are implemented
358 in terms of it. Never use this function directly!
359
360 s32 i2c_smbus_read_byte(struct i2c_client *client);
361 s32 i2c_smbus_write_byte(struct i2c_client *client, u8 value);
362 s32 i2c_smbus_read_byte_data(struct i2c_client *client, u8 command);
363 s32 i2c_smbus_write_byte_data(struct i2c_client *client,
364 u8 command, u8 value);
365 s32 i2c_smbus_read_word_data(struct i2c_client *client, u8 command);
366 s32 i2c_smbus_write_word_data(struct i2c_client *client,
367 u8 command, u16 value);
368 s32 i2c_smbus_read_block_data(struct i2c_client *client,
369 u8 command, u8 *values);
370 s32 i2c_smbus_write_block_data(struct i2c_client *client,
371 u8 command, u8 length, const u8 *values);
372 s32 i2c_smbus_read_i2c_block_data(struct i2c_client *client,
373 u8 command, u8 length, u8 *values);
374 s32 i2c_smbus_write_i2c_block_data(struct i2c_client *client,
375 u8 command, u8 length,
376 const u8 *values);
377
378 These ones were removed from i2c-core because they had no users, but could
379 be added back later if needed:
380
381 s32 i2c_smbus_write_quick(struct i2c_client *client, u8 value);
382 s32 i2c_smbus_process_call(struct i2c_client *client,
383 u8 command, u16 value);
384 s32 i2c_smbus_block_process_call(struct i2c_client *client,
385 u8 command, u8 length, u8 *values);
386
387 All these transactions return a negative errno value on failure. The 'write'
388 transactions return 0 on success; the 'read' transactions return the read
389 value, except for block transactions, which return the number of values
390 read. The block buffers need not be longer than 32 bytes.
391
392 You can read the file `smbus-protocol' for more information about the
393 actual SMBus protocol.
394
395
396 General purpose routines
397 ========================
398
399 Below all general purpose routines are listed, that were not mentioned
400 before.
401
402 /* Return the adapter number for a specific adapter */
403 int i2c_adapter_id(struct i2c_adapter *adap);