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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).
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!
9 General remarks
10 ===============
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, and `FOO_' for preprocessor variables.
19 The driver structure
20 ====================
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.
28 /* iff driver uses driver model ("new style") binding model: */
30 static struct i2c_device_id foo_idtable[] = {
31 { "foo", my_id_for_foo },
32 { "bar", my_id_for_bar },
33 { }
34 };
36 MODULE_DEVICE_TABLE(i2c, foo_idtable);
38 static struct i2c_driver foo_driver = {
39 .driver = {
40 .name = "foo",
41 },
43 /* iff driver uses driver model ("new style") binding model: */
44 .id_table = foo_ids,
45 .probe = foo_probe,
46 .remove = foo_remove,
48 /* else, driver uses "legacy" binding model: */
49 .attach_adapter = foo_attach_adapter,
50 .detach_client = foo_detach_client,
52 /* these may be used regardless of the driver binding model */
53 .shutdown = foo_shutdown, /* optional */
54 .suspend = foo_suspend, /* optional */
55 .resume = foo_resume, /* optional */
56 .command = foo_command, /* optional */
57 }
59 The name field is the driver name, and must not contain spaces. It
60 should match the module name (if the driver can be compiled as a module),
61 although you can use MODULE_ALIAS (passing "foo" in this example) to add
62 another name for the module. If the driver name doesn't match the module
63 name, the module won't be automatically loaded (hotplug/coldplug).
65 All other fields are for call-back functions which will be explained
66 below.
69 Extra client data
70 =================
72 Each client structure has a special `data' field that can point to any
73 structure at all. You should use this to keep device-specific data,
74 especially in drivers that handle multiple I2C or SMBUS devices. You
75 do not always need this, but especially for `sensors' drivers, it can
76 be very useful.
78 /* store the value */
79 void i2c_set_clientdata(struct i2c_client *client, void *data);
81 /* retrieve the value */
82 void *i2c_get_clientdata(struct i2c_client *client);
84 An example structure is below.
86 struct foo_data {
87 struct i2c_client client;
88 enum chips type; /* To keep the chips type for `sensors' drivers. */
90 /* Because the i2c bus is slow, it is often useful to cache the read
91 information of a chip for some time (for example, 1 or 2 seconds).
92 It depends of course on the device whether this is really worthwhile
93 or even sensible. */
94 struct mutex update_lock; /* When we are reading lots of information,
95 another process should not update the
96 below information */
97 char valid; /* != 0 if the following fields are valid. */
98 unsigned long last_updated; /* In jiffies */
99 /* Add the read information here too */
100 };
103 Accessing the client
104 ====================
106 Let's say we have a valid client structure. At some time, we will need
107 to gather information from the client, or write new information to the
108 client. How we will export this information to user-space is less
109 important at this moment (perhaps we do not need to do this at all for
110 some obscure clients). But we need generic reading and writing routines.
112 I have found it useful to define foo_read and foo_write function for this.
113 For some cases, it will be easier to call the i2c functions directly,
114 but many chips have some kind of register-value idea that can easily
115 be encapsulated.
117 The below functions are simple examples, and should not be copied
118 literally.
120 int foo_read_value(struct i2c_client *client, u8 reg)
121 {
122 if (reg < 0x10) /* byte-sized register */
123 return i2c_smbus_read_byte_data(client,reg);
124 else /* word-sized register */
125 return i2c_smbus_read_word_data(client,reg);
126 }
128 int foo_write_value(struct i2c_client *client, u8 reg, u16 value)
129 {
130 if (reg == 0x10) /* Impossible to write - driver error! */ {
131 return -1;
132 else if (reg < 0x10) /* byte-sized register */
133 return i2c_smbus_write_byte_data(client,reg,value);
134 else /* word-sized register */
135 return i2c_smbus_write_word_data(client,reg,value);
136 }
139 Probing and attaching
140 =====================
142 The Linux I2C stack was originally written to support access to hardware
143 monitoring chips on PC motherboards, and thus it embeds some assumptions
144 that are more appropriate to SMBus (and PCs) than to I2C. One of these
145 assumptions is that most adapters and devices drivers support the SMBUS_QUICK
146 protocol to probe device presence. Another is that devices and their drivers
147 can be sufficiently configured using only such probe primitives.
149 As Linux and its I2C stack became more widely used in embedded systems
150 and complex components such as DVB adapters, those assumptions became more
151 problematic. Drivers for I2C devices that issue interrupts need more (and
152 different) configuration information, as do drivers handling chip variants
153 that can't be distinguished by protocol probing, or which need some board
154 specific information to operate correctly.
156 Accordingly, the I2C stack now has two models for associating I2C devices
157 with their drivers: the original "legacy" model, and a newer one that's
158 fully compatible with the Linux 2.6 driver model. These models do not mix,
159 since the "legacy" model requires drivers to create "i2c_client" device
160 objects after SMBus style probing, while the Linux driver model expects
161 drivers to be given such device objects in their probe() routines.
164 Standard Driver Model Binding ("New Style")
165 -------------------------------------------
167 System infrastructure, typically board-specific initialization code or
168 boot firmware, reports what I2C devices exist. For example, there may be
169 a table, in the kernel or from the boot loader, identifying I2C devices
170 and linking them to board-specific configuration information about IRQs
171 and other wiring artifacts, chip type, and so on. That could be used to
172 create i2c_client objects for each I2C device.
174 I2C device drivers using this binding model work just like any other
175 kind of driver in Linux: they provide a probe() method to bind to
176 those devices, and a remove() method to unbind.
178 static int foo_probe(struct i2c_client *client,
179 const struct i2c_device_id *id);
180 static int foo_remove(struct i2c_client *client);
182 Remember that the i2c_driver does not create those client handles. The
183 handle may be used during foo_probe(). If foo_probe() reports success
184 (zero not a negative status code) it may save the handle and use it until
185 foo_remove() returns. That binding model is used by most Linux drivers.
187 The probe function is called when an entry in the id_table name field
188 matches the device's name. It is passed the entry that was matched so
189 the driver knows which one in the table matched.
192 Device Creation (Standard driver model)
193 ---------------------------------------
195 If you know for a fact that an I2C device is connected to a given I2C bus,
196 you can instantiate that device by simply filling an i2c_board_info
197 structure with the device address and driver name, and calling
198 i2c_new_device(). This will create the device, then the driver core will
199 take care of finding the right driver and will call its probe() method.
200 If a driver supports different device types, you can specify the type you
201 want using the type field. You can also specify an IRQ and platform data
202 if needed.
204 Sometimes you know that a device is connected to a given I2C bus, but you
205 don't know the exact address it uses. This happens on TV adapters for
206 example, where the same driver supports dozens of slightly different
207 models, and I2C device addresses change from one model to the next. In
208 that case, you can use the i2c_new_probed_device() variant, which is
209 similar to i2c_new_device(), except that it takes an additional list of
210 possible I2C addresses to probe. A device is created for the first
211 responsive address in the list. If you expect more than one device to be
212 present in the address range, simply call i2c_new_probed_device() that
213 many times.
215 The call to i2c_new_device() or i2c_new_probed_device() typically happens
216 in the I2C bus driver. You may want to save the returned i2c_client
217 reference for later use.
220 Device Deletion (Standard driver model)
221 ---------------------------------------
223 Each I2C device which has been created using i2c_new_device() or
224 i2c_new_probed_device() can be unregistered by calling
225 i2c_unregister_device(). If you don't call it explicitly, it will be
226 called automatically before the underlying I2C bus itself is removed, as a
227 device can't survive its parent in the device driver model.
230 Legacy Driver Binding Model
231 ---------------------------
233 Most i2c devices can be present on several i2c addresses; for some this
234 is determined in hardware (by soldering some chip pins to Vcc or Ground),
235 for others this can be changed in software (by writing to specific client
236 registers). Some devices are usually on a specific address, but not always;
237 and some are even more tricky. So you will probably need to scan several
238 i2c addresses for your clients, and do some sort of detection to see
239 whether it is actually a device supported by your driver.
241 To give the user a maximum of possibilities, some default module parameters
242 are defined to help determine what addresses are scanned. Several macros
243 are defined in i2c.h to help you support them, as well as a generic
244 detection algorithm.
246 You do not have to use this parameter interface; but don't try to use
247 function i2c_probe() if you don't.
250 Probing classes (Legacy model)
251 ------------------------------
253 All parameters are given as lists of unsigned 16-bit integers. Lists are
254 terminated by I2C_CLIENT_END.
255 The following lists are used internally:
257 normal_i2c: filled in by the module writer.
258 A list of I2C addresses which should normally be examined.
259 probe: insmod parameter.
260 A list of pairs. The first value is a bus number (-1 for any I2C bus),
261 the second is the address. These addresses are also probed, as if they
262 were in the 'normal' list.
263 ignore: insmod parameter.
264 A list of pairs. The first value is a bus number (-1 for any I2C bus),
265 the second is the I2C address. These addresses are never probed.
266 This parameter overrules the 'normal_i2c' list only.
267 force: insmod parameter.
268 A list of pairs. The first value is a bus number (-1 for any I2C bus),
269 the second is the I2C address. A device is blindly assumed to be on
270 the given address, no probing is done.
272 Additionally, kind-specific force lists may optionally be defined if
273 the driver supports several chip kinds. They are grouped in a
274 NULL-terminated list of pointers named forces, those first element if the
275 generic force list mentioned above. Each additional list correspond to an
276 insmod parameter of the form force_<kind>.
278 Fortunately, as a module writer, you just have to define the `normal_i2c'
279 parameter. The complete declaration could look like this:
281 /* Scan 0x4c to 0x4f */
282 static const unsigned short normal_i2c[] = { 0x4c, 0x4d, 0x4e, 0x4f,
283 I2C_CLIENT_END };
285 /* Magic definition of all other variables and things */
286 I2C_CLIENT_INSMOD;
287 /* Or, if your driver supports, say, 2 kind of devices: */
288 I2C_CLIENT_INSMOD_2(foo, bar);
290 If you use the multi-kind form, an enum will be defined for you:
291 enum chips { any_chip, foo, bar, ... }
292 You can then (and certainly should) use it in the driver code.
294 Note that you *have* to call the defined variable `normal_i2c',
295 without any prefix!
298 Attaching to an adapter (Legacy model)
299 --------------------------------------
301 Whenever a new adapter is inserted, or for all adapters if the driver is
302 being registered, the callback attach_adapter() is called. Now is the
303 time to determine what devices are present on the adapter, and to register
304 a client for each of them.
306 The attach_adapter callback is really easy: we just call the generic
307 detection function. This function will scan the bus for us, using the
308 information as defined in the lists explained above. If a device is
309 detected at a specific address, another callback is called.
311 int foo_attach_adapter(struct i2c_adapter *adapter)
312 {
313 return i2c_probe(adapter,&addr_data,&foo_detect_client);
314 }
316 Remember, structure `addr_data' is defined by the macros explained above,
317 so you do not have to define it yourself.
319 The i2c_probe function will call the foo_detect_client
320 function only for those i2c addresses that actually have a device on
321 them (unless a `force' parameter was used). In addition, addresses that
322 are already in use (by some other registered client) are skipped.
325 The detect client function (Legacy model)
326 -----------------------------------------
328 The detect client function is called by i2c_probe. The `kind' parameter
329 contains -1 for a probed detection, 0 for a forced detection, or a positive
330 number for a forced detection with a chip type forced.
332 Returning an error different from -ENODEV in a detect function will cause
333 the detection to stop: other addresses and adapters won't be scanned.
334 This should only be done on fatal or internal errors, such as a memory
335 shortage or i2c_attach_client failing.
337 For now, you can ignore the `flags' parameter. It is there for future use.
339 int foo_detect_client(struct i2c_adapter *adapter, int address,
340 int kind)
341 {
342 int err = 0;
343 int i;
344 struct i2c_client *client;
345 struct foo_data *data;
346 const char *name = "";
348 /* Let's see whether this adapter can support what we need.
349 Please substitute the things you need here! */
350 if (!i2c_check_functionality(adapter,I2C_FUNC_SMBUS_WORD_DATA |
351 I2C_FUNC_SMBUS_WRITE_BYTE))
352 goto ERROR0;
354 /* OK. For now, we presume we have a valid client. We now create the
355 client structure, even though we cannot fill it completely yet.
356 But it allows us to access several i2c functions safely */
358 if (!(data = kzalloc(sizeof(struct foo_data), GFP_KERNEL))) {
359 err = -ENOMEM;
360 goto ERROR0;
361 }
363 client = &data->client;
364 i2c_set_clientdata(client, data);
366 client->addr = address;
367 client->adapter = adapter;
368 client->driver = &foo_driver;
370 /* Now, we do the remaining detection. If no `force' parameter is used. */
372 /* First, the generic detection (if any), that is skipped if any force
373 parameter was used. */
374 if (kind < 0) {
375 /* The below is of course bogus */
376 if (foo_read(client, FOO_REG_GENERIC) != FOO_GENERIC_VALUE)
377 goto ERROR1;
378 }
380 /* Next, specific detection. This is especially important for `sensors'
381 devices. */
383 /* Determine the chip type. Not needed if a `force_CHIPTYPE' parameter
384 was used. */
385 if (kind <= 0) {
386 i = foo_read(client, FOO_REG_CHIPTYPE);
387 if (i == FOO_TYPE_1)
388 kind = chip1; /* As defined in the enum */
389 else if (i == FOO_TYPE_2)
390 kind = chip2;
391 else {
392 printk("foo: Ignoring 'force' parameter for unknown chip at "
393 "adapter %d, address 0x%02x\n",i2c_adapter_id(adapter),address);
394 goto ERROR1;
395 }
396 }
398 /* Now set the type and chip names */
399 if (kind == chip1) {
400 name = "chip1";
401 } else if (kind == chip2) {
402 name = "chip2";
403 }
405 /* Fill in the remaining client fields. */
406 strlcpy(client->name, name, I2C_NAME_SIZE);
407 data->type = kind;
408 mutex_init(&data->update_lock); /* Only if you use this field */
410 /* Any other initializations in data must be done here too. */
412 /* This function can write default values to the client registers, if
413 needed. */
414 foo_init_client(client);
416 /* Tell the i2c layer a new client has arrived */
417 if ((err = i2c_attach_client(client)))
418 goto ERROR1;
420 return 0;
422 /* OK, this is not exactly good programming practice, usually. But it is
423 very code-efficient in this case. */
425 ERROR1:
426 kfree(data);
427 ERROR0:
428 return err;
429 }
432 Removing the client (Legacy model)
433 ==================================
435 The detach_client call back function is called when a client should be
436 removed. It may actually fail, but only when panicking. This code is
437 much simpler than the attachment code, fortunately!
439 int foo_detach_client(struct i2c_client *client)
440 {
441 int err;
443 /* Try to detach the client from i2c space */
444 if ((err = i2c_detach_client(client)))
445 return err;
447 kfree(i2c_get_clientdata(client));
448 return 0;
449 }
452 Initializing the module or kernel
453 =================================
455 When the kernel is booted, or when your foo driver module is inserted,
456 you have to do some initializing. Fortunately, just attaching (registering)
457 the driver module is usually enough.
459 static int __init foo_init(void)
460 {
461 int res;
463 if ((res = i2c_add_driver(&foo_driver))) {
464 printk("foo: Driver registration failed, module not inserted.\n");
465 return res;
466 }
467 return 0;
468 }
470 static void __exit foo_cleanup(void)
471 {
472 i2c_del_driver(&foo_driver);
473 }
475 /* Substitute your own name and email address */
476 MODULE_AUTHOR("Frodo Looijaard <frodol@dds.nl>"
477 MODULE_DESCRIPTION("Driver for Barf Inc. Foo I2C devices");
479 /* a few non-GPL license types are also allowed */
480 MODULE_LICENSE("GPL");
482 module_init(foo_init);
483 module_exit(foo_cleanup);
485 Note that some functions are marked by `__init', and some data structures
486 by `__initdata'. These functions and structures can be removed after
487 kernel booting (or module loading) is completed.
490 Power Management
491 ================
493 If your I2C device needs special handling when entering a system low
494 power state -- like putting a transceiver into a low power mode, or
495 activating a system wakeup mechanism -- do that in the suspend() method.
496 The resume() method should reverse what the suspend() method does.
498 These are standard driver model calls, and they work just like they
499 would for any other driver stack. The calls can sleep, and can use
500 I2C messaging to the device being suspended or resumed (since their
501 parent I2C adapter is active when these calls are issued, and IRQs
502 are still enabled).
505 System Shutdown
506 ===============
508 If your I2C device needs special handling when the system shuts down
509 or reboots (including kexec) -- like turning something off -- use a
510 shutdown() method.
512 Again, this is a standard driver model call, working just like it
513 would for any other driver stack: the calls can sleep, and can use
514 I2C messaging.
517 Command function
518 ================
520 A generic ioctl-like function call back is supported. You will seldom
521 need this, and its use is deprecated anyway, so newer design should not
522 use it. Set it to NULL.
525 Sending and receiving
526 =====================
528 If you want to communicate with your device, there are several functions
529 to do this. You can find all of them in i2c.h.
531 If you can choose between plain i2c communication and SMBus level
532 communication, please use the last. All adapters understand SMBus level
533 commands, but only some of them understand plain i2c!
536 Plain i2c communication
537 -----------------------
539 extern int i2c_master_send(struct i2c_client *,const char* ,int);
540 extern int i2c_master_recv(struct i2c_client *,char* ,int);
542 These routines read and write some bytes from/to a client. The client
543 contains the i2c address, so you do not have to include it. The second
544 parameter contains the bytes the read/write, the third the length of the
545 buffer. Returned is the actual number of bytes read/written.
547 extern int i2c_transfer(struct i2c_adapter *adap, struct i2c_msg *msg,
548 int num);
550 This sends a series of messages. Each message can be a read or write,
551 and they can be mixed in any way. The transactions are combined: no
552 stop bit is sent between transaction. The i2c_msg structure contains
553 for each message the client address, the number of bytes of the message
554 and the message data itself.
556 You can read the file `i2c-protocol' for more information about the
557 actual i2c protocol.
560 SMBus communication
561 -------------------
563 extern s32 i2c_smbus_xfer (struct i2c_adapter * adapter, u16 addr,
564 unsigned short flags,
565 char read_write, u8 command, int size,
566 union i2c_smbus_data * data);
568 This is the generic SMBus function. All functions below are implemented
569 in terms of it. Never use this function directly!
572 extern s32 i2c_smbus_write_quick(struct i2c_client * client, u8 value);
573 extern s32 i2c_smbus_read_byte(struct i2c_client * client);
574 extern s32 i2c_smbus_write_byte(struct i2c_client * client, u8 value);
575 extern s32 i2c_smbus_read_byte_data(struct i2c_client * client, u8 command);
576 extern s32 i2c_smbus_write_byte_data(struct i2c_client * client,
577 u8 command, u8 value);
578 extern s32 i2c_smbus_read_word_data(struct i2c_client * client, u8 command);
579 extern s32 i2c_smbus_write_word_data(struct i2c_client * client,
580 u8 command, u16 value);
581 extern s32 i2c_smbus_write_block_data(struct i2c_client * client,
582 u8 command, u8 length,
583 u8 *values);
584 extern s32 i2c_smbus_read_i2c_block_data(struct i2c_client * client,
585 u8 command, u8 length, u8 *values);
587 These ones were removed in Linux 2.6.10 because they had no users, but could
588 be added back later if needed:
590 extern s32 i2c_smbus_read_block_data(struct i2c_client * client,
591 u8 command, u8 *values);
592 extern s32 i2c_smbus_write_i2c_block_data(struct i2c_client * client,
593 u8 command, u8 length,
594 u8 *values);
595 extern s32 i2c_smbus_process_call(struct i2c_client * client,
596 u8 command, u16 value);
597 extern s32 i2c_smbus_block_process_call(struct i2c_client *client,
598 u8 command, u8 length,
599 u8 *values)
601 All these transactions return -1 on failure. The 'write' transactions
602 return 0 on success; the 'read' transactions return the read value, except
603 for read_block, which returns the number of values read. The block buffers
604 need not be longer than 32 bytes.
606 You can read the file `smbus-protocol' for more information about the
607 actual SMBus protocol.
610 General purpose routines
611 ========================
613 Below all general purpose routines are listed, that were not mentioned
614 before.
616 /* This call returns a unique low identifier for each registered adapter.
617 */
618 extern int i2c_adapter_id(struct i2c_adapter *adap);