1 This is a small guide for those who want to write kernel drivers for I2C
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!
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.
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, a client structure specific information like the actual I2C
27 static struct i2c_driver foo_driver = {
29 .name = "Foo version 2.3 driver",
30 .flags = I2C_DF_NOTIFY,
31 .attach_adapter = &foo_attach_adapter,
32 .detach_client = &foo_detach_client,
33 .command = &foo_command /* may be NULL */
36 The name can be chosen freely, and may be upto 40 characters long. Please
37 use something descriptive here.
39 Don't worry about the flags field; just put I2C_DF_NOTIFY into it. This
40 means that your driver will be notified when new adapters are found.
41 This is almost always what you want.
43 All other fields are for call-back functions which will be explained
46 There use to be two additional fields in this structure, inc_use et dec_use,
47 for module usage count, but these fields were obsoleted and removed.
53 The client structure has a special `data' field that can point to any
54 structure at all. You can use this to keep client-specific data. You
55 do not always need this, but especially for `sensors' drivers, it can
58 An example structure is below.
61 struct semaphore lock; /* For ISA access in `sensors' drivers. */
62 int sysctl_id; /* To keep the /proc directory entry for
64 enum chips type; /* To keep the chips type for `sensors' drivers. */
66 /* Because the i2c bus is slow, it is often useful to cache the read
67 information of a chip for some time (for example, 1 or 2 seconds).
68 It depends of course on the device whether this is really worthwhile
70 struct semaphore update_lock; /* When we are reading lots of information,
71 another process should not update the
73 char valid; /* != 0 if the following fields are valid. */
74 unsigned long last_updated; /* In jiffies */
75 /* Add the read information here too */
82 Let's say we have a valid client structure. At some time, we will need
83 to gather information from the client, or write new information to the
84 client. How we will export this information to user-space is less
85 important at this moment (perhaps we do not need to do this at all for
86 some obscure clients). But we need generic reading and writing routines.
88 I have found it useful to define foo_read and foo_write function for this.
89 For some cases, it will be easier to call the i2c functions directly,
90 but many chips have some kind of register-value idea that can easily
91 be encapsulated. Also, some chips have both ISA and I2C interfaces, and
92 it useful to abstract from this (only for `sensors' drivers).
94 The below functions are simple examples, and should not be copied
97 int foo_read_value(struct i2c_client *client, u8 reg)
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);
105 int foo_write_value(struct i2c_client *client, u8 reg, u16 value)
107 if (reg == 0x10) /* Impossible to write - driver error! */ {
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);
115 For sensors code, you may have to cope with ISA registers too. Something
116 like the below often works. Note the locking!
118 int foo_read_value(struct i2c_client *client, u8 reg)
121 if (i2c_is_isa_client(client)) {
122 down(&(((struct foo_data *) (client->data)) -> lock));
123 outb_p(reg,client->addr + FOO_ADDR_REG_OFFSET);
124 res = inb_p(client->addr + FOO_DATA_REG_OFFSET);
125 up(&(((struct foo_data *) (client->data)) -> lock));
128 return i2c_smbus_read_byte_data(client,reg);
131 Writing is done the same way.
134 Probing and attaching
135 =====================
137 Most i2c devices can be present on several i2c addresses; for some this
138 is determined in hardware (by soldering some chip pins to Vcc or Ground),
139 for others this can be changed in software (by writing to specific client
140 registers). Some devices are usually on a specific address, but not always;
141 and some are even more tricky. So you will probably need to scan several
142 i2c addresses for your clients, and do some sort of detection to see
143 whether it is actually a device supported by your driver.
145 To give the user a maximum of possibilities, some default module parameters
146 are defined to help determine what addresses are scanned. Several macros
147 are defined in i2c.h to help you support them, as well as a generic
150 You do not have to use this parameter interface; but don't try to use
151 function i2c_probe() (or i2c_detect()) if you don't.
153 NOTE: If you want to write a `sensors' driver, the interface is slightly
154 different! See below.
158 Probing classes (i2c)
159 ---------------------
161 All parameters are given as lists of unsigned 16-bit integers. Lists are
162 terminated by I2C_CLIENT_END.
163 The following lists are used internally:
165 normal_i2c: filled in by the module writer.
166 A list of I2C addresses which should normally be examined.
167 probe: insmod parameter.
168 A list of pairs. The first value is a bus number (-1 for any I2C bus),
169 the second is the address. These addresses are also probed, as if they
170 were in the 'normal' list.
171 ignore: insmod parameter.
172 A list of pairs. The first value is a bus number (-1 for any I2C bus),
173 the second is the I2C address. These addresses are never probed.
174 This parameter overrules 'normal' and 'probe', but not the 'force' lists.
175 force: insmod parameter.
176 A list of pairs. The first value is a bus number (-1 for any I2C bus),
177 the second is the I2C address. A device is blindly assumed to be on
178 the given address, no probing is done.
180 Fortunately, as a module writer, you just have to define the `normal_i2c'
181 parameter. The complete declaration could look like this:
183 /* Scan 0x37, and 0x48 to 0x4f */
184 static unsigned short normal_i2c[] = { 0x37, 0x48, 0x49, 0x4a, 0x4b, 0x4c,
185 0x4d, 0x4e, 0x4f, I2C_CLIENT_END };
187 /* Magic definition of all other variables and things */
190 Note that you *have* to call the defined variable `normal_i2c',
194 Probing classes (sensors)
195 -------------------------
197 If you write a `sensors' driver, you use a slightly different interface.
198 Also, we use a enum of chip types. Don't forget to include `sensors.h'.
200 The following lists are used internally. They are all lists of integers.
202 normal_i2c: filled in by the module writer. Terminated by I2C_CLIENT_END.
203 A list of I2C addresses which should normally be examined.
204 probe: insmod parameter. Initialize this list with I2C_CLIENT_END values.
205 A list of pairs. The first value is a bus number (ANY_I2C_BUS for any
206 I2C bus), the second is the address. These addresses are also probed,
207 as if they were in the 'normal' list.
208 ignore: insmod parameter. Initialize this list with I2C_CLIENT_END values.
209 A list of pairs. The first value is a bus number (ANY_I2C_BUS for any
210 I2C bus), the second is the I2C address. These addresses are never
211 probed. This parameter overrules 'normal' and 'probe', but not the
214 Also used is a list of pointers to sensors_force_data structures:
215 force_data: insmod parameters. A list, ending with an element of which
216 the force field is NULL.
217 Each element contains the type of chip and a list of pairs.
218 The first value is a bus number (ANY_I2C_BUS for any I2C bus), the
219 second is the address.
220 These are automatically translated to insmod variables of the form
223 So we have a generic insmod variabled `force', and chip-specific variables
226 Fortunately, as a module writer, you just have to define the `normal_i2c'
227 parameter, and define what chip names are used. The complete declaration
228 could look like this:
229 /* Scan i2c addresses 0x37, and 0x48 to 0x4f */
230 static unsigned short normal_i2c[] = { 0x37, 0x48, 0x49, 0x4a, 0x4b, 0x4c,
231 0x4d, 0x4e, 0x4f, I2C_CLIENT_END };
233 /* Define chips foo and bar, as well as all module parameters and things */
234 SENSORS_INSMOD_2(foo,bar);
236 If you have one chip, you use macro SENSORS_INSMOD_1(chip), if you have 2
237 you use macro SENSORS_INSMOD_2(chip1,chip2), etc. If you do not want to
238 bother with chip types, you can use SENSORS_INSMOD_0.
240 A enum is automatically defined as follows:
241 enum chips { any_chip, chip1, chip2, ... }
244 Attaching to an adapter
245 -----------------------
247 Whenever a new adapter is inserted, or for all adapters if the driver is
248 being registered, the callback attach_adapter() is called. Now is the
249 time to determine what devices are present on the adapter, and to register
250 a client for each of them.
252 The attach_adapter callback is really easy: we just call the generic
253 detection function. This function will scan the bus for us, using the
254 information as defined in the lists explained above. If a device is
255 detected at a specific address, another callback is called.
257 int foo_attach_adapter(struct i2c_adapter *adapter)
259 return i2c_probe(adapter,&addr_data,&foo_detect_client);
262 For `sensors' drivers, use the i2c_detect function instead:
264 int foo_attach_adapter(struct i2c_adapter *adapter)
266 return i2c_detect(adapter,&addr_data,&foo_detect_client);
269 Remember, structure `addr_data' is defined by the macros explained above,
270 so you do not have to define it yourself.
272 The i2c_probe or i2c_detect function will call the foo_detect_client
273 function only for those i2c addresses that actually have a device on
274 them (unless a `force' parameter was used). In addition, addresses that
275 are already in use (by some other registered client) are skipped.
278 The detect client function
279 --------------------------
281 The detect client function is called by i2c_probe or i2c_detect.
282 The `kind' parameter contains 0 if this call is due to a `force'
283 parameter, and -1 otherwise (for i2c_detect, it contains 0 if
284 this call is due to the generic `force' parameter, and the chip type
285 number if it is due to a specific `force' parameter).
287 Below, some things are only needed if this is a `sensors' driver. Those
288 parts are between /* SENSORS ONLY START */ and /* SENSORS ONLY END */
291 This function should only return an error (any value != 0) if there is
292 some reason why no more detection should be done anymore. If the
293 detection just fails for this address, return 0.
295 For now, you can ignore the `flags' parameter. It is there for future use.
297 int foo_detect_client(struct i2c_adapter *adapter, int address,
298 unsigned short flags, int kind)
302 struct i2c_client *new_client;
303 struct foo_data *data;
304 const char *client_name = ""; /* For non-`sensors' drivers, put the real
307 /* Let's see whether this adapter can support what we need.
308 Please substitute the things you need here!
309 For `sensors' drivers, add `! is_isa &&' to the if statement */
310 if (!i2c_check_functionality(adapter,I2C_FUNC_SMBUS_WORD_DATA |
311 I2C_FUNC_SMBUS_WRITE_BYTE))
314 /* SENSORS ONLY START */
315 const char *type_name = "";
316 int is_isa = i2c_is_isa_adapter(adapter);
318 /* Do this only if the chip can additionally be found on the ISA bus
323 /* Discard immediately if this ISA range is already used */
324 if (check_region(address,FOO_EXTENT))
327 /* Probe whether there is anything on this address.
328 Some example code is below, but you will have to adapt this
329 for your own driver */
331 if (kind < 0) /* Only if no force parameter was used */ {
332 /* We may need long timeouts at least for some chips. */
333 #define REALLY_SLOW_IO
334 i = inb_p(address + 1);
335 if (inb_p(address + 2) != i)
337 if (inb_p(address + 3) != i)
339 if (inb_p(address + 7) != i)
341 #undef REALLY_SLOW_IO
343 /* Let's just hope nothing breaks here */
344 i = inb_p(address + 5) & 0x7f;
345 outb_p(~i & 0x7f,address+5);
346 if ((inb_p(address + 5) & 0x7f) != (~i & 0x7f)) {
353 /* SENSORS ONLY END */
355 /* OK. For now, we presume we have a valid client. We now create the
356 client structure, even though we cannot fill it completely yet.
357 But it allows us to access several i2c functions safely */
359 /* Note that we reserve some space for foo_data too. If you don't
360 need it, remove it. We do it here to help to lessen memory
362 if (! (new_client = kmalloc(sizeof(struct i2c_client) +
363 sizeof(struct foo_data),
369 /* This is tricky, but it will set the data to the right value. */
370 client->data = new_client + 1;
371 data = (struct foo_data *) (client->data);
373 new_client->addr = address;
374 new_client->data = data;
375 new_client->adapter = adapter;
376 new_client->driver = &foo_driver;
377 new_client->flags = 0;
379 /* Now, we do the remaining detection. If no `force' parameter is used. */
381 /* First, the generic detection (if any), that is skipped if any force
382 parameter was used. */
384 /* The below is of course bogus */
385 if (foo_read(new_client,FOO_REG_GENERIC) != FOO_GENERIC_VALUE)
389 /* SENSORS ONLY START */
391 /* Next, specific detection. This is especially important for `sensors'
394 /* Determine the chip type. Not needed if a `force_CHIPTYPE' parameter
397 i = foo_read(new_client,FOO_REG_CHIPTYPE);
399 kind = chip1; /* As defined in the enum */
400 else if (i == FOO_TYPE_2)
403 printk("foo: Ignoring 'force' parameter for unknown chip at "
404 "adapter %d, address 0x%02x\n",i2c_adapter_id(adapter),address);
409 /* Now set the type and chip names */
411 type_name = "chip1"; /* For /proc entry */
412 client_name = "CHIP 1";
413 } else if (kind == chip2) {
414 type_name = "chip2"; /* For /proc entry */
415 client_name = "CHIP 2";
418 /* Reserve the ISA region */
420 request_region(address,FOO_EXTENT,type_name);
422 /* SENSORS ONLY END */
424 /* Fill in the remaining client fields. */
425 strcpy(new_client->name,client_name);
427 /* SENSORS ONLY BEGIN */
429 /* SENSORS ONLY END */
431 data->valid = 0; /* Only if you use this field */
432 init_MUTEX(&data->update_lock); /* Only if you use this field */
434 /* Any other initializations in data must be done here too. */
436 /* Tell the i2c layer a new client has arrived */
437 if ((err = i2c_attach_client(new_client)))
440 /* SENSORS ONLY BEGIN */
441 /* Register a new directory entry with module sensors. See below for
442 the `template' structure. */
443 if ((i = i2c_register_entry(new_client, type_name,
444 foo_dir_table_template,THIS_MODULE)) < 0) {
450 /* SENSORS ONLY END */
452 /* This function can write default values to the client registers, if
454 foo_init_client(new_client);
457 /* OK, this is not exactly good programming practice, usually. But it is
458 very code-efficient in this case. */
461 i2c_detach_client(new_client);
464 /* SENSORS ONLY START */
466 release_region(address,FOO_EXTENT);
467 /* SENSORS ONLY END */
478 The detach_client call back function is called when a client should be
479 removed. It may actually fail, but only when panicking. This code is
480 much simpler than the attachment code, fortunately!
482 int foo_detach_client(struct i2c_client *client)
486 /* SENSORS ONLY START */
487 /* Deregister with the `i2c-proc' module. */
488 i2c_deregister_entry(((struct lm78_data *)(client->data))->sysctl_id);
489 /* SENSORS ONLY END */
491 /* Try to detach the client from i2c space */
492 if ((err = i2c_detach_client(client)))
495 /* HYBRID SENSORS CHIP ONLY START */
496 if i2c_is_isa_client(client)
497 release_region(client->addr,LM78_EXTENT);
498 /* HYBRID SENSORS CHIP ONLY END */
500 kfree(client); /* Frees client data too, if allocated at the same time */
505 Initializing the module or kernel
506 =================================
508 When the kernel is booted, or when your foo driver module is inserted,
509 you have to do some initializing. Fortunately, just attaching (registering)
510 the driver module is usually enough.
512 /* Keep track of how far we got in the initialization process. If several
513 things have to initialized, and we fail halfway, only those things
514 have to be cleaned up! */
515 static int __initdata foo_initialized = 0;
517 static int __init foo_init(void)
520 printk("foo version %s (%s)\n",FOO_VERSION,FOO_DATE);
522 if ((res = i2c_add_driver(&foo_driver))) {
523 printk("foo: Driver registration failed, module not inserted.\n");
531 void foo_cleanup(void)
533 if (foo_initialized == 1) {
534 if ((res = i2c_del_driver(&foo_driver))) {
535 printk("foo: Driver registration failed, module not removed.\n");
542 /* Substitute your own name and email address */
543 MODULE_AUTHOR("Frodo Looijaard <frodol@dds.nl>"
544 MODULE_DESCRIPTION("Driver for Barf Inc. Foo I2C devices");
546 module_init(foo_init);
547 module_exit(foo_cleanup);
549 Note that some functions are marked by `__init', and some data structures
550 by `__init_data'. Hose functions and structures can be removed after
551 kernel booting (or module loading) is completed.
556 A generic ioctl-like function call back is supported. You will seldom
557 need this. You may even set it to NULL.
559 /* No commands defined */
560 int foo_command(struct i2c_client *client, unsigned int cmd, void *arg)
566 Sending and receiving
567 =====================
569 If you want to communicate with your device, there are several functions
570 to do this. You can find all of them in i2c.h.
572 If you can choose between plain i2c communication and SMBus level
573 communication, please use the last. All adapters understand SMBus level
574 commands, but only some of them understand plain i2c!
577 Plain i2c communication
578 -----------------------
580 extern int i2c_master_send(struct i2c_client *,const char* ,int);
581 extern int i2c_master_recv(struct i2c_client *,char* ,int);
583 These routines read and write some bytes from/to a client. The client
584 contains the i2c address, so you do not have to include it. The second
585 parameter contains the bytes the read/write, the third the length of the
586 buffer. Returned is the actual number of bytes read/written.
588 extern int i2c_transfer(struct i2c_adapter *adap, struct i2c_msg *msg,
591 This sends a series of messages. Each message can be a read or write,
592 and they can be mixed in any way. The transactions are combined: no
593 stop bit is sent between transaction. The i2c_msg structure contains
594 for each message the client address, the number of bytes of the message
595 and the message data itself.
597 You can read the file `i2c-protocol' for more information about the
604 extern s32 i2c_smbus_xfer (struct i2c_adapter * adapter, u16 addr,
605 unsigned short flags,
606 char read_write, u8 command, int size,
607 union i2c_smbus_data * data);
609 This is the generic SMBus function. All functions below are implemented
610 in terms of it. Never use this function directly!
613 extern s32 i2c_smbus_write_quick(struct i2c_client * client, u8 value);
614 extern s32 i2c_smbus_read_byte(struct i2c_client * client);
615 extern s32 i2c_smbus_write_byte(struct i2c_client * client, u8 value);
616 extern s32 i2c_smbus_read_byte_data(struct i2c_client * client, u8 command);
617 extern s32 i2c_smbus_write_byte_data(struct i2c_client * client,
618 u8 command, u8 value);
619 extern s32 i2c_smbus_read_word_data(struct i2c_client * client, u8 command);
620 extern s32 i2c_smbus_write_word_data(struct i2c_client * client,
621 u8 command, u16 value);
622 extern s32 i2c_smbus_write_block_data(struct i2c_client * client,
623 u8 command, u8 length,
626 These ones were removed in Linux 2.6.10 because they had no users, but could
627 be added back later if needed:
629 extern s32 i2c_smbus_read_i2c_block_data(struct i2c_client * client,
630 u8 command, u8 *values);
631 extern s32 i2c_smbus_read_block_data(struct i2c_client * client,
632 u8 command, u8 *values);
633 extern s32 i2c_smbus_write_i2c_block_data(struct i2c_client * client,
634 u8 command, u8 length,
636 extern s32 i2c_smbus_process_call(struct i2c_client * client,
637 u8 command, u16 value);
638 extern s32 i2c_smbus_block_process_call(struct i2c_client *client,
639 u8 command, u8 length,
642 All these transactions return -1 on failure. The 'write' transactions
643 return 0 on success; the 'read' transactions return the read value, except
644 for read_block, which returns the number of values read. The block buffers
645 need not be longer than 32 bytes.
647 You can read the file `smbus-protocol' for more information about the
648 actual SMBus protocol.
651 General purpose routines
652 ========================
654 Below all general purpose routines are listed, that were not mentioned
657 /* This call returns a unique low identifier for each registered adapter,
658 * or -1 if the adapter was not registered.
660 extern int i2c_adapter_id(struct i2c_adapter *adap);
663 The sensors sysctl/proc interface
664 =================================
666 This section only applies if you write `sensors' drivers.
668 Each sensors driver creates a directory in /proc/sys/dev/sensors for each
669 registered client. The directory is called something like foo-i2c-4-65.
670 The sensors module helps you to do this as easily as possible.
675 You will need to define a ctl_table template. This template will automatically
676 be copied to a newly allocated structure and filled in where necessary when
677 you call sensors_register_entry.
679 First, I will give an example definition.
680 static ctl_table foo_dir_table_template[] = {
681 { FOO_SYSCTL_FUNC1, "func1", NULL, 0, 0644, NULL, &i2c_proc_real,
682 &i2c_sysctl_real,NULL,&foo_func },
683 { FOO_SYSCTL_FUNC2, "func2", NULL, 0, 0644, NULL, &i2c_proc_real,
684 &i2c_sysctl_real,NULL,&foo_func },
685 { FOO_SYSCTL_DATA, "data", NULL, 0, 0644, NULL, &i2c_proc_real,
686 &i2c_sysctl_real,NULL,&foo_data },
690 In the above example, three entries are defined. They can either be
691 accessed through the /proc interface, in the /proc/sys/dev/sensors/*
692 directories, as files named func1, func2 and data, or alternatively
693 through the sysctl interface, in the appropriate table, with identifiers
694 FOO_SYSCTL_FUNC1, FOO_SYSCTL_FUNC2 and FOO_SYSCTL_DATA.
696 The third, sixth and ninth parameters should always be NULL, and the
697 fourth should always be 0. The fifth is the mode of the /proc file;
698 0644 is safe, as the file will be owned by root:root.
700 The seventh and eighth parameters should be &i2c_proc_real and
701 &i2c_sysctl_real if you want to export lists of reals (scaled
702 integers). You can also use your own function for them, as usual.
703 Finally, the last parameter is the call-back to gather the data
704 (see below) if you use the *_proc_real functions.
710 The call back functions (foo_func and foo_data in the above example)
711 can be called in several ways; the operation parameter determines
714 * If operation == SENSORS_PROC_REAL_INFO, you must return the
715 magnitude (scaling) in nrels_mag;
716 * If operation == SENSORS_PROC_REAL_READ, you must read information
717 from the chip and return it in results. The number of integers
718 to display should be put in nrels_mag;
719 * If operation == SENSORS_PROC_REAL_WRITE, you must write the
720 supplied information to the chip. nrels_mag will contain the number
721 of integers, results the integers themselves.
723 The *_proc_real functions will display the elements as reals for the
724 /proc interface. If you set the magnitude to 2, and supply 345 for
725 SENSORS_PROC_REAL_READ, it would display 3.45; and if the user would
726 write 45.6 to the /proc file, it would be returned as 4560 for
727 SENSORS_PROC_REAL_WRITE. A magnitude may even be negative!
731 /* FOO_FROM_REG and FOO_TO_REG translate between scaled values and
732 register values. Note the use of the read cache. */
733 void foo_in(struct i2c_client *client, int operation, int ctl_name,
734 int *nrels_mag, long *results)
736 struct foo_data *data = client->data;
737 int nr = ctl_name - FOO_SYSCTL_FUNC1; /* reduce to 0 upwards */
739 if (operation == SENSORS_PROC_REAL_INFO)
741 else if (operation == SENSORS_PROC_REAL_READ) {
742 /* Update the readings cache (if necessary) */
743 foo_update_client(client);
744 /* Get the readings from the cache */
745 results[0] = FOO_FROM_REG(data->foo_func_base[nr]);
746 results[1] = FOO_FROM_REG(data->foo_func_more[nr]);
747 results[2] = FOO_FROM_REG(data->foo_func_readonly[nr]);
749 } else if (operation == SENSORS_PROC_REAL_WRITE) {
750 if (*nrels_mag >= 1) {
751 /* Update the cache */
752 data->foo_base[nr] = FOO_TO_REG(results[0]);
753 /* Update the chip */
754 foo_write_value(client,FOO_REG_FUNC_BASE(nr),data->foo_base[nr]);
756 if (*nrels_mag >= 2) {
757 /* Update the cache */
758 data->foo_more[nr] = FOO_TO_REG(results[1]);
759 /* Update the chip */
760 foo_write_value(client,FOO_REG_FUNC_MORE(nr),data->foo_more[nr]);