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() 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 Remember, structure `addr_data' is defined by the macros explained above,
263 so you do not have to define it yourself.
265 The i2c_probe function will call the foo_detect_client
266 function only for those i2c addresses that actually have a device on
267 them (unless a `force' parameter was used). In addition, addresses that
268 are already in use (by some other registered client) are skipped.
271 The detect client function
272 --------------------------
274 The detect client function is called by i2c_probe. The `kind' parameter
275 contains -1 for a probed detection, 0 for a forced detection, or a positive
276 number for a forced detection with a chip type forced.
278 Below, some things are only needed if this is a `sensors' driver. Those
279 parts are between /* SENSORS ONLY START */ and /* SENSORS ONLY END */
282 This function should only return an error (any value != 0) if there is
283 some reason why no more detection should be done anymore. If the
284 detection just fails for this address, return 0.
286 For now, you can ignore the `flags' parameter. It is there for future use.
288 int foo_detect_client(struct i2c_adapter *adapter, int address,
289 unsigned short flags, int kind)
293 struct i2c_client *new_client;
294 struct foo_data *data;
295 const char *client_name = ""; /* For non-`sensors' drivers, put the real
298 /* Let's see whether this adapter can support what we need.
299 Please substitute the things you need here!
300 For `sensors' drivers, add `! is_isa &&' to the if statement */
301 if (!i2c_check_functionality(adapter,I2C_FUNC_SMBUS_WORD_DATA |
302 I2C_FUNC_SMBUS_WRITE_BYTE))
305 /* SENSORS ONLY START */
306 const char *type_name = "";
307 int is_isa = i2c_is_isa_adapter(adapter);
309 /* Do this only if the chip can additionally be found on the ISA bus
314 /* Discard immediately if this ISA range is already used */
315 if (check_region(address,FOO_EXTENT))
318 /* Probe whether there is anything on this address.
319 Some example code is below, but you will have to adapt this
320 for your own driver */
322 if (kind < 0) /* Only if no force parameter was used */ {
323 /* We may need long timeouts at least for some chips. */
324 #define REALLY_SLOW_IO
325 i = inb_p(address + 1);
326 if (inb_p(address + 2) != i)
328 if (inb_p(address + 3) != i)
330 if (inb_p(address + 7) != i)
332 #undef REALLY_SLOW_IO
334 /* Let's just hope nothing breaks here */
335 i = inb_p(address + 5) & 0x7f;
336 outb_p(~i & 0x7f,address+5);
337 if ((inb_p(address + 5) & 0x7f) != (~i & 0x7f)) {
344 /* SENSORS ONLY END */
346 /* OK. For now, we presume we have a valid client. We now create the
347 client structure, even though we cannot fill it completely yet.
348 But it allows us to access several i2c functions safely */
350 /* Note that we reserve some space for foo_data too. If you don't
351 need it, remove it. We do it here to help to lessen memory
353 if (! (new_client = kmalloc(sizeof(struct i2c_client) +
354 sizeof(struct foo_data),
360 /* This is tricky, but it will set the data to the right value. */
361 client->data = new_client + 1;
362 data = (struct foo_data *) (client->data);
364 new_client->addr = address;
365 new_client->data = data;
366 new_client->adapter = adapter;
367 new_client->driver = &foo_driver;
368 new_client->flags = 0;
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. */
375 /* The below is of course bogus */
376 if (foo_read(new_client,FOO_REG_GENERIC) != FOO_GENERIC_VALUE)
380 /* SENSORS ONLY START */
382 /* Next, specific detection. This is especially important for `sensors'
385 /* Determine the chip type. Not needed if a `force_CHIPTYPE' parameter
388 i = foo_read(new_client,FOO_REG_CHIPTYPE);
390 kind = chip1; /* As defined in the enum */
391 else if (i == FOO_TYPE_2)
394 printk("foo: Ignoring 'force' parameter for unknown chip at "
395 "adapter %d, address 0x%02x\n",i2c_adapter_id(adapter),address);
400 /* Now set the type and chip names */
402 type_name = "chip1"; /* For /proc entry */
403 client_name = "CHIP 1";
404 } else if (kind == chip2) {
405 type_name = "chip2"; /* For /proc entry */
406 client_name = "CHIP 2";
409 /* Reserve the ISA region */
411 request_region(address,FOO_EXTENT,type_name);
413 /* SENSORS ONLY END */
415 /* Fill in the remaining client fields. */
416 strcpy(new_client->name,client_name);
418 /* SENSORS ONLY BEGIN */
420 /* SENSORS ONLY END */
422 data->valid = 0; /* Only if you use this field */
423 init_MUTEX(&data->update_lock); /* Only if you use this field */
425 /* Any other initializations in data must be done here too. */
427 /* Tell the i2c layer a new client has arrived */
428 if ((err = i2c_attach_client(new_client)))
431 /* SENSORS ONLY BEGIN */
432 /* Register a new directory entry with module sensors. See below for
433 the `template' structure. */
434 if ((i = i2c_register_entry(new_client, type_name,
435 foo_dir_table_template,THIS_MODULE)) < 0) {
441 /* SENSORS ONLY END */
443 /* This function can write default values to the client registers, if
445 foo_init_client(new_client);
448 /* OK, this is not exactly good programming practice, usually. But it is
449 very code-efficient in this case. */
452 i2c_detach_client(new_client);
455 /* SENSORS ONLY START */
457 release_region(address,FOO_EXTENT);
458 /* SENSORS ONLY END */
469 The detach_client call back function is called when a client should be
470 removed. It may actually fail, but only when panicking. This code is
471 much simpler than the attachment code, fortunately!
473 int foo_detach_client(struct i2c_client *client)
477 /* SENSORS ONLY START */
478 /* Deregister with the `i2c-proc' module. */
479 i2c_deregister_entry(((struct lm78_data *)(client->data))->sysctl_id);
480 /* SENSORS ONLY END */
482 /* Try to detach the client from i2c space */
483 if ((err = i2c_detach_client(client)))
486 /* HYBRID SENSORS CHIP ONLY START */
487 if i2c_is_isa_client(client)
488 release_region(client->addr,LM78_EXTENT);
489 /* HYBRID SENSORS CHIP ONLY END */
491 kfree(client); /* Frees client data too, if allocated at the same time */
496 Initializing the module or kernel
497 =================================
499 When the kernel is booted, or when your foo driver module is inserted,
500 you have to do some initializing. Fortunately, just attaching (registering)
501 the driver module is usually enough.
503 /* Keep track of how far we got in the initialization process. If several
504 things have to initialized, and we fail halfway, only those things
505 have to be cleaned up! */
506 static int __initdata foo_initialized = 0;
508 static int __init foo_init(void)
511 printk("foo version %s (%s)\n",FOO_VERSION,FOO_DATE);
513 if ((res = i2c_add_driver(&foo_driver))) {
514 printk("foo: Driver registration failed, module not inserted.\n");
522 void foo_cleanup(void)
524 if (foo_initialized == 1) {
525 if ((res = i2c_del_driver(&foo_driver))) {
526 printk("foo: Driver registration failed, module not removed.\n");
533 /* Substitute your own name and email address */
534 MODULE_AUTHOR("Frodo Looijaard <frodol@dds.nl>"
535 MODULE_DESCRIPTION("Driver for Barf Inc. Foo I2C devices");
537 module_init(foo_init);
538 module_exit(foo_cleanup);
540 Note that some functions are marked by `__init', and some data structures
541 by `__init_data'. Hose functions and structures can be removed after
542 kernel booting (or module loading) is completed.
547 A generic ioctl-like function call back is supported. You will seldom
548 need this. You may even set it to NULL.
550 /* No commands defined */
551 int foo_command(struct i2c_client *client, unsigned int cmd, void *arg)
557 Sending and receiving
558 =====================
560 If you want to communicate with your device, there are several functions
561 to do this. You can find all of them in i2c.h.
563 If you can choose between plain i2c communication and SMBus level
564 communication, please use the last. All adapters understand SMBus level
565 commands, but only some of them understand plain i2c!
568 Plain i2c communication
569 -----------------------
571 extern int i2c_master_send(struct i2c_client *,const char* ,int);
572 extern int i2c_master_recv(struct i2c_client *,char* ,int);
574 These routines read and write some bytes from/to a client. The client
575 contains the i2c address, so you do not have to include it. The second
576 parameter contains the bytes the read/write, the third the length of the
577 buffer. Returned is the actual number of bytes read/written.
579 extern int i2c_transfer(struct i2c_adapter *adap, struct i2c_msg *msg,
582 This sends a series of messages. Each message can be a read or write,
583 and they can be mixed in any way. The transactions are combined: no
584 stop bit is sent between transaction. The i2c_msg structure contains
585 for each message the client address, the number of bytes of the message
586 and the message data itself.
588 You can read the file `i2c-protocol' for more information about the
595 extern s32 i2c_smbus_xfer (struct i2c_adapter * adapter, u16 addr,
596 unsigned short flags,
597 char read_write, u8 command, int size,
598 union i2c_smbus_data * data);
600 This is the generic SMBus function. All functions below are implemented
601 in terms of it. Never use this function directly!
604 extern s32 i2c_smbus_write_quick(struct i2c_client * client, u8 value);
605 extern s32 i2c_smbus_read_byte(struct i2c_client * client);
606 extern s32 i2c_smbus_write_byte(struct i2c_client * client, u8 value);
607 extern s32 i2c_smbus_read_byte_data(struct i2c_client * client, u8 command);
608 extern s32 i2c_smbus_write_byte_data(struct i2c_client * client,
609 u8 command, u8 value);
610 extern s32 i2c_smbus_read_word_data(struct i2c_client * client, u8 command);
611 extern s32 i2c_smbus_write_word_data(struct i2c_client * client,
612 u8 command, u16 value);
613 extern s32 i2c_smbus_write_block_data(struct i2c_client * client,
614 u8 command, u8 length,
617 These ones were removed in Linux 2.6.10 because they had no users, but could
618 be added back later if needed:
620 extern s32 i2c_smbus_read_i2c_block_data(struct i2c_client * client,
621 u8 command, u8 *values);
622 extern s32 i2c_smbus_read_block_data(struct i2c_client * client,
623 u8 command, u8 *values);
624 extern s32 i2c_smbus_write_i2c_block_data(struct i2c_client * client,
625 u8 command, u8 length,
627 extern s32 i2c_smbus_process_call(struct i2c_client * client,
628 u8 command, u16 value);
629 extern s32 i2c_smbus_block_process_call(struct i2c_client *client,
630 u8 command, u8 length,
633 All these transactions return -1 on failure. The 'write' transactions
634 return 0 on success; the 'read' transactions return the read value, except
635 for read_block, which returns the number of values read. The block buffers
636 need not be longer than 32 bytes.
638 You can read the file `smbus-protocol' for more information about the
639 actual SMBus protocol.
642 General purpose routines
643 ========================
645 Below all general purpose routines are listed, that were not mentioned
648 /* This call returns a unique low identifier for each registered adapter,
649 * or -1 if the adapter was not registered.
651 extern int i2c_adapter_id(struct i2c_adapter *adap);
654 The sensors sysctl/proc interface
655 =================================
657 This section only applies if you write `sensors' drivers.
659 Each sensors driver creates a directory in /proc/sys/dev/sensors for each
660 registered client. The directory is called something like foo-i2c-4-65.
661 The sensors module helps you to do this as easily as possible.
666 You will need to define a ctl_table template. This template will automatically
667 be copied to a newly allocated structure and filled in where necessary when
668 you call sensors_register_entry.
670 First, I will give an example definition.
671 static ctl_table foo_dir_table_template[] = {
672 { FOO_SYSCTL_FUNC1, "func1", NULL, 0, 0644, NULL, &i2c_proc_real,
673 &i2c_sysctl_real,NULL,&foo_func },
674 { FOO_SYSCTL_FUNC2, "func2", NULL, 0, 0644, NULL, &i2c_proc_real,
675 &i2c_sysctl_real,NULL,&foo_func },
676 { FOO_SYSCTL_DATA, "data", NULL, 0, 0644, NULL, &i2c_proc_real,
677 &i2c_sysctl_real,NULL,&foo_data },
681 In the above example, three entries are defined. They can either be
682 accessed through the /proc interface, in the /proc/sys/dev/sensors/*
683 directories, as files named func1, func2 and data, or alternatively
684 through the sysctl interface, in the appropriate table, with identifiers
685 FOO_SYSCTL_FUNC1, FOO_SYSCTL_FUNC2 and FOO_SYSCTL_DATA.
687 The third, sixth and ninth parameters should always be NULL, and the
688 fourth should always be 0. The fifth is the mode of the /proc file;
689 0644 is safe, as the file will be owned by root:root.
691 The seventh and eighth parameters should be &i2c_proc_real and
692 &i2c_sysctl_real if you want to export lists of reals (scaled
693 integers). You can also use your own function for them, as usual.
694 Finally, the last parameter is the call-back to gather the data
695 (see below) if you use the *_proc_real functions.
701 The call back functions (foo_func and foo_data in the above example)
702 can be called in several ways; the operation parameter determines
705 * If operation == SENSORS_PROC_REAL_INFO, you must return the
706 magnitude (scaling) in nrels_mag;
707 * If operation == SENSORS_PROC_REAL_READ, you must read information
708 from the chip and return it in results. The number of integers
709 to display should be put in nrels_mag;
710 * If operation == SENSORS_PROC_REAL_WRITE, you must write the
711 supplied information to the chip. nrels_mag will contain the number
712 of integers, results the integers themselves.
714 The *_proc_real functions will display the elements as reals for the
715 /proc interface. If you set the magnitude to 2, and supply 345 for
716 SENSORS_PROC_REAL_READ, it would display 3.45; and if the user would
717 write 45.6 to the /proc file, it would be returned as 4560 for
718 SENSORS_PROC_REAL_WRITE. A magnitude may even be negative!
722 /* FOO_FROM_REG and FOO_TO_REG translate between scaled values and
723 register values. Note the use of the read cache. */
724 void foo_in(struct i2c_client *client, int operation, int ctl_name,
725 int *nrels_mag, long *results)
727 struct foo_data *data = client->data;
728 int nr = ctl_name - FOO_SYSCTL_FUNC1; /* reduce to 0 upwards */
730 if (operation == SENSORS_PROC_REAL_INFO)
732 else if (operation == SENSORS_PROC_REAL_READ) {
733 /* Update the readings cache (if necessary) */
734 foo_update_client(client);
735 /* Get the readings from the cache */
736 results[0] = FOO_FROM_REG(data->foo_func_base[nr]);
737 results[1] = FOO_FROM_REG(data->foo_func_more[nr]);
738 results[2] = FOO_FROM_REG(data->foo_func_readonly[nr]);
740 } else if (operation == SENSORS_PROC_REAL_WRITE) {
741 if (*nrels_mag >= 1) {
742 /* Update the cache */
743 data->foo_base[nr] = FOO_TO_REG(results[0]);
744 /* Update the chip */
745 foo_write_value(client,FOO_REG_FUNC_BASE(nr),data->foo_base[nr]);
747 if (*nrels_mag >= 2) {
748 /* Update the cache */
749 data->foo_more[nr] = FOO_TO_REG(results[1]);
750 /* Update the chip */
751 foo_write_value(client,FOO_REG_FUNC_MORE(nr),data->foo_more[nr]);