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 field must match the driver name, including the case. It must not
37 contain spaces, and may be up to 31 characters long.
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
50 The client structure has a special `data' field that can point to any
51 structure at all. You can use this to keep client-specific data. You
52 do not always need this, but especially for `sensors' drivers, it can
55 An example structure is below.
58 struct i2c_client client;
59 struct semaphore lock; /* For ISA access in `sensors' drivers. */
60 int sysctl_id; /* To keep the /proc directory entry for
62 enum chips type; /* To keep the chips type for `sensors' drivers. */
64 /* Because the i2c bus is slow, it is often useful to cache the read
65 information of a chip for some time (for example, 1 or 2 seconds).
66 It depends of course on the device whether this is really worthwhile
68 struct semaphore update_lock; /* When we are reading lots of information,
69 another process should not update the
71 char valid; /* != 0 if the following fields are valid. */
72 unsigned long last_updated; /* In jiffies */
73 /* Add the read information here too */
80 Let's say we have a valid client structure. At some time, we will need
81 to gather information from the client, or write new information to the
82 client. How we will export this information to user-space is less
83 important at this moment (perhaps we do not need to do this at all for
84 some obscure clients). But we need generic reading and writing routines.
86 I have found it useful to define foo_read and foo_write function for this.
87 For some cases, it will be easier to call the i2c functions directly,
88 but many chips have some kind of register-value idea that can easily
89 be encapsulated. Also, some chips have both ISA and I2C interfaces, and
90 it useful to abstract from this (only for `sensors' drivers).
92 The below functions are simple examples, and should not be copied
95 int foo_read_value(struct i2c_client *client, u8 reg)
97 if (reg < 0x10) /* byte-sized register */
98 return i2c_smbus_read_byte_data(client,reg);
99 else /* word-sized register */
100 return i2c_smbus_read_word_data(client,reg);
103 int foo_write_value(struct i2c_client *client, u8 reg, u16 value)
105 if (reg == 0x10) /* Impossible to write - driver error! */ {
107 else if (reg < 0x10) /* byte-sized register */
108 return i2c_smbus_write_byte_data(client,reg,value);
109 else /* word-sized register */
110 return i2c_smbus_write_word_data(client,reg,value);
113 For sensors code, you may have to cope with ISA registers too. Something
114 like the below often works. Note the locking!
116 int foo_read_value(struct i2c_client *client, u8 reg)
119 if (i2c_is_isa_client(client)) {
120 down(&(((struct foo_data *) (client->data)) -> lock));
121 outb_p(reg,client->addr + FOO_ADDR_REG_OFFSET);
122 res = inb_p(client->addr + FOO_DATA_REG_OFFSET);
123 up(&(((struct foo_data *) (client->data)) -> lock));
126 return i2c_smbus_read_byte_data(client,reg);
129 Writing is done the same way.
132 Probing and attaching
133 =====================
135 Most i2c devices can be present on several i2c addresses; for some this
136 is determined in hardware (by soldering some chip pins to Vcc or Ground),
137 for others this can be changed in software (by writing to specific client
138 registers). Some devices are usually on a specific address, but not always;
139 and some are even more tricky. So you will probably need to scan several
140 i2c addresses for your clients, and do some sort of detection to see
141 whether it is actually a device supported by your driver.
143 To give the user a maximum of possibilities, some default module parameters
144 are defined to help determine what addresses are scanned. Several macros
145 are defined in i2c.h to help you support them, as well as a generic
148 You do not have to use this parameter interface; but don't try to use
149 function i2c_probe() if you don't.
151 NOTE: If you want to write a `sensors' driver, the interface is slightly
152 different! See below.
159 All parameters are given as lists of unsigned 16-bit integers. Lists are
160 terminated by I2C_CLIENT_END.
161 The following lists are used internally:
163 normal_i2c: filled in by the module writer.
164 A list of I2C addresses which should normally be examined.
165 probe: insmod parameter.
166 A list of pairs. The first value is a bus number (-1 for any I2C bus),
167 the second is the address. These addresses are also probed, as if they
168 were in the 'normal' list.
169 ignore: insmod parameter.
170 A list of pairs. The first value is a bus number (-1 for any I2C bus),
171 the second is the I2C address. These addresses are never probed.
172 This parameter overrules the 'normal_i2c' list only.
173 force: insmod parameter.
174 A list of pairs. The first value is a bus number (-1 for any I2C bus),
175 the second is the I2C address. A device is blindly assumed to be on
176 the given address, no probing is done.
178 Additionally, kind-specific force lists may optionally be defined if
179 the driver supports several chip kinds. They are grouped in a
180 NULL-terminated list of pointers named forces, those first element if the
181 generic force list mentioned above. Each additional list correspond to an
182 insmod parameter of the form force_<kind>.
184 Fortunately, as a module writer, you just have to define the `normal_i2c'
185 parameter. The complete declaration could look like this:
187 /* Scan 0x37, and 0x48 to 0x4f */
188 static unsigned short normal_i2c[] = { 0x37, 0x48, 0x49, 0x4a, 0x4b, 0x4c,
189 0x4d, 0x4e, 0x4f, I2C_CLIENT_END };
191 /* Magic definition of all other variables and things */
193 /* Or, if your driver supports, say, 2 kind of devices: */
194 I2C_CLIENT_INSMOD_2(foo, bar);
196 If you use the multi-kind form, an enum will be defined for you:
197 enum chips { any_chip, foo, bar, ... }
198 You can then (and certainly should) use it in the driver code.
200 Note that you *have* to call the defined variable `normal_i2c',
204 Attaching to an adapter
205 -----------------------
207 Whenever a new adapter is inserted, or for all adapters if the driver is
208 being registered, the callback attach_adapter() is called. Now is the
209 time to determine what devices are present on the adapter, and to register
210 a client for each of them.
212 The attach_adapter callback is really easy: we just call the generic
213 detection function. This function will scan the bus for us, using the
214 information as defined in the lists explained above. If a device is
215 detected at a specific address, another callback is called.
217 int foo_attach_adapter(struct i2c_adapter *adapter)
219 return i2c_probe(adapter,&addr_data,&foo_detect_client);
222 Remember, structure `addr_data' is defined by the macros explained above,
223 so you do not have to define it yourself.
225 The i2c_probe function will call the foo_detect_client
226 function only for those i2c addresses that actually have a device on
227 them (unless a `force' parameter was used). In addition, addresses that
228 are already in use (by some other registered client) are skipped.
231 The detect client function
232 --------------------------
234 The detect client function is called by i2c_probe. The `kind' parameter
235 contains -1 for a probed detection, 0 for a forced detection, or a positive
236 number for a forced detection with a chip type forced.
238 Below, some things are only needed if this is a `sensors' driver. Those
239 parts are between /* SENSORS ONLY START */ and /* SENSORS ONLY END */
242 Returning an error different from -ENODEV in a detect function will cause
243 the detection to stop: other addresses and adapters won't be scanned.
244 This should only be done on fatal or internal errors, such as a memory
245 shortage or i2c_attach_client failing.
247 For now, you can ignore the `flags' parameter. It is there for future use.
249 int foo_detect_client(struct i2c_adapter *adapter, int address,
250 unsigned short flags, int kind)
254 struct i2c_client *new_client;
255 struct foo_data *data;
256 const char *client_name = ""; /* For non-`sensors' drivers, put the real
259 /* Let's see whether this adapter can support what we need.
260 Please substitute the things you need here!
261 For `sensors' drivers, add `! is_isa &&' to the if statement */
262 if (!i2c_check_functionality(adapter,I2C_FUNC_SMBUS_WORD_DATA |
263 I2C_FUNC_SMBUS_WRITE_BYTE))
266 /* SENSORS ONLY START */
267 const char *type_name = "";
268 int is_isa = i2c_is_isa_adapter(adapter);
270 /* Do this only if the chip can additionally be found on the ISA bus
275 /* Discard immediately if this ISA range is already used */
276 if (check_region(address,FOO_EXTENT))
279 /* Probe whether there is anything on this address.
280 Some example code is below, but you will have to adapt this
281 for your own driver */
283 if (kind < 0) /* Only if no force parameter was used */ {
284 /* We may need long timeouts at least for some chips. */
285 #define REALLY_SLOW_IO
286 i = inb_p(address + 1);
287 if (inb_p(address + 2) != i)
289 if (inb_p(address + 3) != i)
291 if (inb_p(address + 7) != i)
293 #undef REALLY_SLOW_IO
295 /* Let's just hope nothing breaks here */
296 i = inb_p(address + 5) & 0x7f;
297 outb_p(~i & 0x7f,address+5);
298 if ((inb_p(address + 5) & 0x7f) != (~i & 0x7f)) {
305 /* SENSORS ONLY END */
307 /* OK. For now, we presume we have a valid client. We now create the
308 client structure, even though we cannot fill it completely yet.
309 But it allows us to access several i2c functions safely */
311 if (!(data = kzalloc(sizeof(struct foo_data), GFP_KERNEL))) {
316 new_client = &data->client;
317 i2c_set_clientdata(new_client, data);
319 new_client->addr = address;
320 new_client->adapter = adapter;
321 new_client->driver = &foo_driver;
322 new_client->flags = 0;
324 /* Now, we do the remaining detection. If no `force' parameter is used. */
326 /* First, the generic detection (if any), that is skipped if any force
327 parameter was used. */
329 /* The below is of course bogus */
330 if (foo_read(new_client,FOO_REG_GENERIC) != FOO_GENERIC_VALUE)
334 /* SENSORS ONLY START */
336 /* Next, specific detection. This is especially important for `sensors'
339 /* Determine the chip type. Not needed if a `force_CHIPTYPE' parameter
342 i = foo_read(new_client,FOO_REG_CHIPTYPE);
344 kind = chip1; /* As defined in the enum */
345 else if (i == FOO_TYPE_2)
348 printk("foo: Ignoring 'force' parameter for unknown chip at "
349 "adapter %d, address 0x%02x\n",i2c_adapter_id(adapter),address);
354 /* Now set the type and chip names */
356 type_name = "chip1"; /* For /proc entry */
357 client_name = "CHIP 1";
358 } else if (kind == chip2) {
359 type_name = "chip2"; /* For /proc entry */
360 client_name = "CHIP 2";
363 /* Reserve the ISA region */
365 request_region(address,FOO_EXTENT,type_name);
367 /* SENSORS ONLY END */
369 /* Fill in the remaining client fields. */
370 strcpy(new_client->name,client_name);
372 /* SENSORS ONLY BEGIN */
374 /* SENSORS ONLY END */
376 data->valid = 0; /* Only if you use this field */
377 init_MUTEX(&data->update_lock); /* Only if you use this field */
379 /* Any other initializations in data must be done here too. */
381 /* Tell the i2c layer a new client has arrived */
382 if ((err = i2c_attach_client(new_client)))
385 /* SENSORS ONLY BEGIN */
386 /* Register a new directory entry with module sensors. See below for
387 the `template' structure. */
388 if ((i = i2c_register_entry(new_client, type_name,
389 foo_dir_table_template,THIS_MODULE)) < 0) {
395 /* SENSORS ONLY END */
397 /* This function can write default values to the client registers, if
399 foo_init_client(new_client);
402 /* OK, this is not exactly good programming practice, usually. But it is
403 very code-efficient in this case. */
406 i2c_detach_client(new_client);
409 /* SENSORS ONLY START */
411 release_region(address,FOO_EXTENT);
412 /* SENSORS ONLY END */
423 The detach_client call back function is called when a client should be
424 removed. It may actually fail, but only when panicking. This code is
425 much simpler than the attachment code, fortunately!
427 int foo_detach_client(struct i2c_client *client)
431 /* SENSORS ONLY START */
432 /* Deregister with the `i2c-proc' module. */
433 i2c_deregister_entry(((struct lm78_data *)(client->data))->sysctl_id);
434 /* SENSORS ONLY END */
436 /* Try to detach the client from i2c space */
437 if ((err = i2c_detach_client(client)))
440 /* HYBRID SENSORS CHIP ONLY START */
441 if i2c_is_isa_client(client)
442 release_region(client->addr,LM78_EXTENT);
443 /* HYBRID SENSORS CHIP ONLY END */
450 Initializing the module or kernel
451 =================================
453 When the kernel is booted, or when your foo driver module is inserted,
454 you have to do some initializing. Fortunately, just attaching (registering)
455 the driver module is usually enough.
457 /* Keep track of how far we got in the initialization process. If several
458 things have to initialized, and we fail halfway, only those things
459 have to be cleaned up! */
460 static int __initdata foo_initialized = 0;
462 static int __init foo_init(void)
465 printk("foo version %s (%s)\n",FOO_VERSION,FOO_DATE);
467 if ((res = i2c_add_driver(&foo_driver))) {
468 printk("foo: Driver registration failed, module not inserted.\n");
476 void foo_cleanup(void)
478 if (foo_initialized == 1) {
479 if ((res = i2c_del_driver(&foo_driver))) {
480 printk("foo: Driver registration failed, module not removed.\n");
487 /* Substitute your own name and email address */
488 MODULE_AUTHOR("Frodo Looijaard <frodol@dds.nl>"
489 MODULE_DESCRIPTION("Driver for Barf Inc. Foo I2C devices");
491 module_init(foo_init);
492 module_exit(foo_cleanup);
494 Note that some functions are marked by `__init', and some data structures
495 by `__init_data'. Hose functions and structures can be removed after
496 kernel booting (or module loading) is completed.
501 A generic ioctl-like function call back is supported. You will seldom
502 need this. You may even set it to NULL.
504 /* No commands defined */
505 int foo_command(struct i2c_client *client, unsigned int cmd, void *arg)
511 Sending and receiving
512 =====================
514 If you want to communicate with your device, there are several functions
515 to do this. You can find all of them in i2c.h.
517 If you can choose between plain i2c communication and SMBus level
518 communication, please use the last. All adapters understand SMBus level
519 commands, but only some of them understand plain i2c!
522 Plain i2c communication
523 -----------------------
525 extern int i2c_master_send(struct i2c_client *,const char* ,int);
526 extern int i2c_master_recv(struct i2c_client *,char* ,int);
528 These routines read and write some bytes from/to a client. The client
529 contains the i2c address, so you do not have to include it. The second
530 parameter contains the bytes the read/write, the third the length of the
531 buffer. Returned is the actual number of bytes read/written.
533 extern int i2c_transfer(struct i2c_adapter *adap, struct i2c_msg *msg,
536 This sends a series of messages. Each message can be a read or write,
537 and they can be mixed in any way. The transactions are combined: no
538 stop bit is sent between transaction. The i2c_msg structure contains
539 for each message the client address, the number of bytes of the message
540 and the message data itself.
542 You can read the file `i2c-protocol' for more information about the
549 extern s32 i2c_smbus_xfer (struct i2c_adapter * adapter, u16 addr,
550 unsigned short flags,
551 char read_write, u8 command, int size,
552 union i2c_smbus_data * data);
554 This is the generic SMBus function. All functions below are implemented
555 in terms of it. Never use this function directly!
558 extern s32 i2c_smbus_write_quick(struct i2c_client * client, u8 value);
559 extern s32 i2c_smbus_read_byte(struct i2c_client * client);
560 extern s32 i2c_smbus_write_byte(struct i2c_client * client, u8 value);
561 extern s32 i2c_smbus_read_byte_data(struct i2c_client * client, u8 command);
562 extern s32 i2c_smbus_write_byte_data(struct i2c_client * client,
563 u8 command, u8 value);
564 extern s32 i2c_smbus_read_word_data(struct i2c_client * client, u8 command);
565 extern s32 i2c_smbus_write_word_data(struct i2c_client * client,
566 u8 command, u16 value);
567 extern s32 i2c_smbus_write_block_data(struct i2c_client * client,
568 u8 command, u8 length,
570 extern s32 i2c_smbus_read_i2c_block_data(struct i2c_client * client,
571 u8 command, u8 *values);
573 These ones were removed in Linux 2.6.10 because they had no users, but could
574 be added back later if needed:
576 extern s32 i2c_smbus_read_block_data(struct i2c_client * client,
577 u8 command, u8 *values);
578 extern s32 i2c_smbus_write_i2c_block_data(struct i2c_client * client,
579 u8 command, u8 length,
581 extern s32 i2c_smbus_process_call(struct i2c_client * client,
582 u8 command, u16 value);
583 extern s32 i2c_smbus_block_process_call(struct i2c_client *client,
584 u8 command, u8 length,
587 All these transactions return -1 on failure. The 'write' transactions
588 return 0 on success; the 'read' transactions return the read value, except
589 for read_block, which returns the number of values read. The block buffers
590 need not be longer than 32 bytes.
592 You can read the file `smbus-protocol' for more information about the
593 actual SMBus protocol.
596 General purpose routines
597 ========================
599 Below all general purpose routines are listed, that were not mentioned
602 /* This call returns a unique low identifier for each registered adapter,
603 * or -1 if the adapter was not registered.
605 extern int i2c_adapter_id(struct i2c_adapter *adap);
608 The sensors sysctl/proc interface
609 =================================
611 This section only applies if you write `sensors' drivers.
613 Each sensors driver creates a directory in /proc/sys/dev/sensors for each
614 registered client. The directory is called something like foo-i2c-4-65.
615 The sensors module helps you to do this as easily as possible.
620 You will need to define a ctl_table template. This template will automatically
621 be copied to a newly allocated structure and filled in where necessary when
622 you call sensors_register_entry.
624 First, I will give an example definition.
625 static ctl_table foo_dir_table_template[] = {
626 { FOO_SYSCTL_FUNC1, "func1", NULL, 0, 0644, NULL, &i2c_proc_real,
627 &i2c_sysctl_real,NULL,&foo_func },
628 { FOO_SYSCTL_FUNC2, "func2", NULL, 0, 0644, NULL, &i2c_proc_real,
629 &i2c_sysctl_real,NULL,&foo_func },
630 { FOO_SYSCTL_DATA, "data", NULL, 0, 0644, NULL, &i2c_proc_real,
631 &i2c_sysctl_real,NULL,&foo_data },
635 In the above example, three entries are defined. They can either be
636 accessed through the /proc interface, in the /proc/sys/dev/sensors/*
637 directories, as files named func1, func2 and data, or alternatively
638 through the sysctl interface, in the appropriate table, with identifiers
639 FOO_SYSCTL_FUNC1, FOO_SYSCTL_FUNC2 and FOO_SYSCTL_DATA.
641 The third, sixth and ninth parameters should always be NULL, and the
642 fourth should always be 0. The fifth is the mode of the /proc file;
643 0644 is safe, as the file will be owned by root:root.
645 The seventh and eighth parameters should be &i2c_proc_real and
646 &i2c_sysctl_real if you want to export lists of reals (scaled
647 integers). You can also use your own function for them, as usual.
648 Finally, the last parameter is the call-back to gather the data
649 (see below) if you use the *_proc_real functions.
655 The call back functions (foo_func and foo_data in the above example)
656 can be called in several ways; the operation parameter determines
659 * If operation == SENSORS_PROC_REAL_INFO, you must return the
660 magnitude (scaling) in nrels_mag;
661 * If operation == SENSORS_PROC_REAL_READ, you must read information
662 from the chip and return it in results. The number of integers
663 to display should be put in nrels_mag;
664 * If operation == SENSORS_PROC_REAL_WRITE, you must write the
665 supplied information to the chip. nrels_mag will contain the number
666 of integers, results the integers themselves.
668 The *_proc_real functions will display the elements as reals for the
669 /proc interface. If you set the magnitude to 2, and supply 345 for
670 SENSORS_PROC_REAL_READ, it would display 3.45; and if the user would
671 write 45.6 to the /proc file, it would be returned as 4560 for
672 SENSORS_PROC_REAL_WRITE. A magnitude may even be negative!
676 /* FOO_FROM_REG and FOO_TO_REG translate between scaled values and
677 register values. Note the use of the read cache. */
678 void foo_in(struct i2c_client *client, int operation, int ctl_name,
679 int *nrels_mag, long *results)
681 struct foo_data *data = client->data;
682 int nr = ctl_name - FOO_SYSCTL_FUNC1; /* reduce to 0 upwards */
684 if (operation == SENSORS_PROC_REAL_INFO)
686 else if (operation == SENSORS_PROC_REAL_READ) {
687 /* Update the readings cache (if necessary) */
688 foo_update_client(client);
689 /* Get the readings from the cache */
690 results[0] = FOO_FROM_REG(data->foo_func_base[nr]);
691 results[1] = FOO_FROM_REG(data->foo_func_more[nr]);
692 results[2] = FOO_FROM_REG(data->foo_func_readonly[nr]);
694 } else if (operation == SENSORS_PROC_REAL_WRITE) {
695 if (*nrels_mag >= 1) {
696 /* Update the cache */
697 data->foo_base[nr] = FOO_TO_REG(results[0]);
698 /* Update the chip */
699 foo_write_value(client,FOO_REG_FUNC_BASE(nr),data->foo_base[nr]);
701 if (*nrels_mag >= 2) {
702 /* Update the cache */
703 data->foo_more[nr] = FOO_TO_REG(results[1]);
704 /* Update the chip */
705 foo_write_value(client,FOO_REG_FUNC_MORE(nr),data->foo_more[nr]);