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/Documentation/i2c/writing-clients

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  1This is a small guide for those who want to write kernel drivers for I2C
  2or SMBus devices.
  3
  4To set up a driver, you need to do several things. Some are optional, and
  5some things can be done slightly or completely different. Use this as a
  6guide, not as a rule book!
  7
  8
  9General remarks
 10===============
 11
 12Try to keep the kernel namespace as clean as possible. The best way to
 13do this is to use a unique prefix for all global symbols. This is 
 14especially important for exported symbols, but it is a good idea to do
 15it for non-exported symbols too. We will use the prefix `foo_' in this
 16tutorial, and `FOO_' for preprocessor variables.
 17
 18
 19The driver structure
 20====================
 21
 22Usually, you will implement a single driver structure, and instantiate
 23all clients from it. Remember, a driver structure contains general access 
 24routines, a client structure specific information like the actual I2C
 25address.
 26
 27static struct i2c_driver foo_driver = {
 28	.owner		= THIS_MODULE,
 29	.name		= "Foo version 2.3 driver",
 30	.id		= I2C_DRIVERID_FOO, /* from i2c-id.h, optional */
 31	.flags		= I2C_DF_NOTIFY,
 32	.attach_adapter	= &foo_attach_adapter,
 33	.detach_client	= &foo_detach_client,
 34	.command	= &foo_command /* may be NULL */
 35}
 36 
 37The name can be chosen freely, and may be upto 40 characters long. Please
 38use something descriptive here.
 39
 40If used, the id should be a unique ID. The range 0xf000 to 0xffff is
 41reserved for local use, and you can use one of those until you start
 42distributing the driver, at which time you should contact the i2c authors
 43to get your own ID(s). Note that most of the time you don't need an ID
 44at all so you can just omit it.
 45
 46Don't worry about the flags field; just put I2C_DF_NOTIFY into it. This
 47means that your driver will be notified when new adapters are found.
 48This is almost always what you want.
 49
 50All other fields are for call-back functions which will be explained 
 51below.
 52
 53There use to be two additional fields in this structure, inc_use et dec_use,
 54for module usage count, but these fields were obsoleted and removed.
 55
 56
 57Extra client data
 58=================
 59
 60The client structure has a special `data' field that can point to any
 61structure at all. You can use this to keep client-specific data. You
 62do not always need this, but especially for `sensors' drivers, it can
 63be very useful.
 64
 65An example structure is below.
 66
 67  struct foo_data {
 68    struct semaphore lock; /* For ISA access in `sensors' drivers. */
 69    int sysctl_id;         /* To keep the /proc directory entry for 
 70                              `sensors' drivers. */
 71    enum chips type;       /* To keep the chips type for `sensors' drivers. */
 72   
 73    /* Because the i2c bus is slow, it is often useful to cache the read
 74       information of a chip for some time (for example, 1 or 2 seconds).
 75       It depends of course on the device whether this is really worthwhile
 76       or even sensible. */
 77    struct semaphore update_lock; /* When we are reading lots of information,
 78                                     another process should not update the
 79                                     below information */
 80    char valid;                   /* != 0 if the following fields are valid. */
 81    unsigned long last_updated;   /* In jiffies */
 82    /* Add the read information here too */
 83  };
 84
 85
 86Accessing the client
 87====================
 88
 89Let's say we have a valid client structure. At some time, we will need
 90to gather information from the client, or write new information to the
 91client. How we will export this information to user-space is less 
 92important at this moment (perhaps we do not need to do this at all for
 93some obscure clients). But we need generic reading and writing routines.
 94
 95I have found it useful to define foo_read and foo_write function for this.
 96For some cases, it will be easier to call the i2c functions directly,
 97but many chips have some kind of register-value idea that can easily
 98be encapsulated. Also, some chips have both ISA and I2C interfaces, and
 99it useful to abstract from this (only for `sensors' drivers).
100
101The below functions are simple examples, and should not be copied
102literally.
103
104  int foo_read_value(struct i2c_client *client, u8 reg)
105  {
106    if (reg < 0x10) /* byte-sized register */
107      return i2c_smbus_read_byte_data(client,reg);
108    else /* word-sized register */
109      return i2c_smbus_read_word_data(client,reg);
110  }
111
112  int foo_write_value(struct i2c_client *client, u8 reg, u16 value)
113  {
114    if (reg == 0x10) /* Impossible to write - driver error! */ {
115      return -1;
116    else if (reg < 0x10) /* byte-sized register */
117      return i2c_smbus_write_byte_data(client,reg,value);
118    else /* word-sized register */
119      return i2c_smbus_write_word_data(client,reg,value);
120  }
121
122For sensors code, you may have to cope with ISA registers too. Something
123like the below often works. Note the locking! 
124
125  int foo_read_value(struct i2c_client *client, u8 reg)
126  {
127    int res;
128    if (i2c_is_isa_client(client)) {
129      down(&(((struct foo_data *) (client->data)) -> lock));
130      outb_p(reg,client->addr + FOO_ADDR_REG_OFFSET);
131      res = inb_p(client->addr + FOO_DATA_REG_OFFSET);
132      up(&(((struct foo_data *) (client->data)) -> lock));
133      return res;
134    } else
135      return i2c_smbus_read_byte_data(client,reg);
136  }
137
138Writing is done the same way.
139
140
141Probing and attaching
142=====================
143
144Most i2c devices can be present on several i2c addresses; for some this
145is determined in hardware (by soldering some chip pins to Vcc or Ground),
146for others this can be changed in software (by writing to specific client
147registers). Some devices are usually on a specific address, but not always;
148and some are even more tricky. So you will probably need to scan several
149i2c addresses for your clients, and do some sort of detection to see
150whether it is actually a device supported by your driver.
151
152To give the user a maximum of possibilities, some default module parameters
153are defined to help determine what addresses are scanned. Several macros
154are defined in i2c.h to help you support them, as well as a generic
155detection algorithm.
156
157You do not have to use this parameter interface; but don't try to use
158function i2c_probe() (or i2c_detect()) if you don't.
159
160NOTE: If you want to write a `sensors' driver, the interface is slightly
161      different! See below.
162
163
164
165Probing classes (i2c)
166---------------------
167
168All parameters are given as lists of unsigned 16-bit integers. Lists are
169terminated by I2C_CLIENT_END.
170The following lists are used internally:
171
172  normal_i2c: filled in by the module writer. 
173     A list of I2C addresses which should normally be examined.
174   normal_i2c_range: filled in by the module writer.
175     A list of pairs of I2C addresses, each pair being an inclusive range of
176     addresses which should normally be examined.
177   probe: insmod parameter. 
178     A list of pairs. The first value is a bus number (-1 for any I2C bus), 
179     the second is the address. These addresses are also probed, as if they 
180     were in the 'normal' list.
181   probe_range: insmod parameter. 
182     A list of triples. The first value is a bus number (-1 for any I2C bus), 
183     the second and third are addresses.  These form an inclusive range of 
184     addresses that are also probed, as if they were in the 'normal' list.
185   ignore: insmod parameter.
186     A list of pairs. The first value is a bus number (-1 for any I2C bus), 
187     the second is the I2C address. These addresses are never probed. 
188     This parameter overrules 'normal' and 'probe', but not the 'force' lists.
189   ignore_range: insmod parameter. 
190     A list of triples. The first value is a bus number (-1 for any I2C bus), 
191     the second and third are addresses. These form an inclusive range of 
192     I2C addresses that are never probed.
193     This parameter overrules 'normal' and 'probe', but not the 'force' lists.
194   force: insmod parameter. 
195     A list of pairs. The first value is a bus number (-1 for any I2C bus),
196     the second is the I2C address. A device is blindly assumed to be on
197     the given address, no probing is done. 
198
199Fortunately, as a module writer, you just have to define the `normal' 
200and/or `normal_range' parameters. The complete declaration could look
201like this:
202
203  /* Scan 0x20 to 0x2f, 0x37, and 0x40 to 0x4f */
204  static unsigned short normal_i2c[] = { 0x37,I2C_CLIENT_END }; 
205  static unsigned short normal_i2c_range[] = { 0x20, 0x2f, 0x40, 0x4f, 
206                                               I2C_CLIENT_END };
207
208  /* Magic definition of all other variables and things */
209  I2C_CLIENT_INSMOD;
210
211Note that you *have* to call the two defined variables `normal_i2c' and
212`normal_i2c_range', without any prefix!
213
214
215Probing classes (sensors)
216-------------------------
217
218If you write a `sensors' driver, you use a slightly different interface.
219As well as I2C addresses, we have to cope with ISA addresses. Also, we
220use a enum of chip types. Don't forget to include `sensors.h'.
221
222The following lists are used internally. They are all lists of integers.
223
224   normal_i2c: filled in by the module writer. Terminated by SENSORS_I2C_END.
225     A list of I2C addresses which should normally be examined.
226   normal_i2c_range: filled in by the module writer. Terminated by 
227     SENSORS_I2C_END
228     A list of pairs of I2C addresses, each pair being an inclusive range of
229     addresses which should normally be examined.
230   normal_isa: filled in by the module writer. Terminated by SENSORS_ISA_END.
231     A list of ISA addresses which should normally be examined.
232   normal_isa_range: filled in by the module writer. Terminated by 
233     SENSORS_ISA_END
234     A list of triples. The first two elements are ISA addresses, being an
235     range of addresses which should normally be examined. The third is the
236     modulo parameter: only addresses which are 0 module this value relative
237     to the first address of the range are actually considered.
238   probe: insmod parameter. Initialize this list with SENSORS_I2C_END values.
239     A list of pairs. The first value is a bus number (SENSORS_ISA_BUS for
240     the ISA bus, -1 for any I2C bus), the second is the address. These
241     addresses are also probed, as if they were in the 'normal' list.
242   probe_range: insmod parameter. Initialize this list with SENSORS_I2C_END 
243     values.
244     A list of triples. The first value is a bus number (SENSORS_ISA_BUS for
245     the ISA bus, -1 for any I2C bus), the second and third are addresses. 
246     These form an inclusive range of addresses that are also probed, as
247     if they were in the 'normal' list.
248   ignore: insmod parameter. Initialize this list with SENSORS_I2C_END values.
249     A list of pairs. The first value is a bus number (SENSORS_ISA_BUS for
250     the ISA bus, -1 for any I2C bus), the second is the I2C address. These
251     addresses are never probed. This parameter overrules 'normal' and 
252     'probe', but not the 'force' lists.
253   ignore_range: insmod parameter. Initialize this list with SENSORS_I2C_END 
254      values.
255     A list of triples. The first value is a bus number (SENSORS_ISA_BUS for
256     the ISA bus, -1 for any I2C bus), the second and third are addresses. 
257     These form an inclusive range of I2C addresses that are never probed.
258     This parameter overrules 'normal' and 'probe', but not the 'force' lists.
259
260Also used is a list of pointers to sensors_force_data structures:
261   force_data: insmod parameters. A list, ending with an element of which
262     the force field is NULL.
263     Each element contains the type of chip and a list of pairs.
264     The first value is a bus number (SENSORS_ISA_BUS for the ISA bus, 
265     -1 for any I2C bus), the second is the address. 
266     These are automatically translated to insmod variables of the form
267     force_foo.
268
269So we have a generic insmod variabled `force', and chip-specific variables
270`force_CHIPNAME'.
271
272Fortunately, as a module writer, you just have to define the `normal' 
273and/or `normal_range' parameters, and define what chip names are used. 
274The complete declaration could look like this:
275  /* Scan i2c addresses 0x20 to 0x2f, 0x37, and 0x40 to 0x4f
276  static unsigned short normal_i2c[] = {0x37,SENSORS_I2C_END};
277  static unsigned short normal_i2c_range[] = {0x20,0x2f,0x40,0x4f,
278                                              SENSORS_I2C_END};
279  /* Scan ISA address 0x290 */
280  static unsigned int normal_isa[] = {0x0290,SENSORS_ISA_END};
281  static unsigned int normal_isa_range[] = {SENSORS_ISA_END};
282
283  /* Define chips foo and bar, as well as all module parameters and things */
284  SENSORS_INSMOD_2(foo,bar);
285
286If you have one chip, you use macro SENSORS_INSMOD_1(chip), if you have 2
287you use macro SENSORS_INSMOD_2(chip1,chip2), etc. If you do not want to
288bother with chip types, you can use SENSORS_INSMOD_0.
289
290A enum is automatically defined as follows:
291  enum chips { any_chip, chip1, chip2, ... }
292
293
294Attaching to an adapter
295-----------------------
296
297Whenever a new adapter is inserted, or for all adapters if the driver is
298being registered, the callback attach_adapter() is called. Now is the
299time to determine what devices are present on the adapter, and to register
300a client for each of them.
301
302The attach_adapter callback is really easy: we just call the generic
303detection function. This function will scan the bus for us, using the
304information as defined in the lists explained above. If a device is
305detected at a specific address, another callback is called.
306
307  int foo_attach_adapter(struct i2c_adapter *adapter)
308  {
309    return i2c_probe(adapter,&addr_data,&foo_detect_client);
310  }
311
312For `sensors' drivers, use the i2c_detect function instead:
313  
314  int foo_attach_adapter(struct i2c_adapter *adapter)
315  { 
316    return i2c_detect(adapter,&addr_data,&foo_detect_client);
317  }
318
319Remember, structure `addr_data' is defined by the macros explained above,
320so you do not have to define it yourself.
321
322The i2c_probe or i2c_detect function will call the foo_detect_client
323function only for those i2c addresses that actually have a device on
324them (unless a `force' parameter was used). In addition, addresses that
325are already in use (by some other registered client) are skipped.
326
327
328The detect client function
329--------------------------
330
331The detect client function is called by i2c_probe or i2c_detect.
332The `kind' parameter contains 0 if this call is due to a `force'
333parameter, and -1 otherwise (for i2c_detect, it contains 0 if
334this call is due to the generic `force' parameter, and the chip type
335number if it is due to a specific `force' parameter).
336
337Below, some things are only needed if this is a `sensors' driver. Those
338parts are between /* SENSORS ONLY START */ and /* SENSORS ONLY END */
339markers. 
340
341This function should only return an error (any value != 0) if there is
342some reason why no more detection should be done anymore. If the
343detection just fails for this address, return 0.
344
345For now, you can ignore the `flags' parameter. It is there for future use.
346
347  int foo_detect_client(struct i2c_adapter *adapter, int address, 
348                        unsigned short flags, int kind)
349  {
350    int err = 0;
351    int i;
352    struct i2c_client *new_client;
353    struct foo_data *data;
354    const char *client_name = ""; /* For non-`sensors' drivers, put the real
355                                     name here! */
356   
357    /* Let's see whether this adapter can support what we need.
358       Please substitute the things you need here! 
359       For `sensors' drivers, add `! is_isa &&' to the if statement */
360    if (!i2c_check_functionality(adapter,I2C_FUNC_SMBUS_WORD_DATA |
361                                        I2C_FUNC_SMBUS_WRITE_BYTE))
362       goto ERROR0;
363
364    /* SENSORS ONLY START */
365    const char *type_name = "";
366    int is_isa = i2c_is_isa_adapter(adapter);
367
368    if (is_isa) {
369
370      /* If this client can't be on the ISA bus at all, we can stop now
371         (call `goto ERROR0'). But for kicks, we will assume it is all
372         right. */
373
374      /* Discard immediately if this ISA range is already used */
375      if (check_region(address,FOO_EXTENT))
376        goto ERROR0;
377
378      /* Probe whether there is anything on this address.
379         Some example code is below, but you will have to adapt this
380         for your own driver */
381
382      if (kind < 0) /* Only if no force parameter was used */ {
383        /* We may need long timeouts at least for some chips. */
384        #define REALLY_SLOW_IO
385        i = inb_p(address + 1);
386        if (inb_p(address + 2) != i)
387          goto ERROR0;
388        if (inb_p(address + 3) != i)
389          goto ERROR0;
390        if (inb_p(address + 7) != i)
391          goto ERROR0;
392        #undef REALLY_SLOW_IO
393
394        /* Let's just hope nothing breaks here */
395        i = inb_p(address + 5) & 0x7f;
396        outb_p(~i & 0x7f,address+5);
397        if ((inb_p(address + 5) & 0x7f) != (~i & 0x7f)) {
398          outb_p(i,address+5);
399          return 0;
400        }
401      }
402    }
403
404    /* SENSORS ONLY END */
405
406    /* OK. For now, we presume we have a valid client. We now create the
407       client structure, even though we cannot fill it completely yet.
408       But it allows us to access several i2c functions safely */
409    
410    /* Note that we reserve some space for foo_data too. If you don't
411       need it, remove it. We do it here to help to lessen memory
412       fragmentation. */
413    if (! (new_client = kmalloc(sizeof(struct i2c_client) + 
414                                sizeof(struct foo_data),
415                                GFP_KERNEL))) {
416      err = -ENOMEM;
417      goto ERROR0;
418    }
419
420    /* This is tricky, but it will set the data to the right value. */
421    client->data = new_client + 1;
422    data = (struct foo_data *) (client->data);
423
424    new_client->addr = address;
425    new_client->data = data;
426    new_client->adapter = adapter;
427    new_client->driver = &foo_driver;
428    new_client->flags = 0;
429
430    /* Now, we do the remaining detection. If no `force' parameter is used. */
431
432    /* First, the generic detection (if any), that is skipped if any force
433       parameter was used. */
434    if (kind < 0) {
435      /* The below is of course bogus */
436      if (foo_read(new_client,FOO_REG_GENERIC) != FOO_GENERIC_VALUE)
437         goto ERROR1;
438    }
439
440    /* SENSORS ONLY START */
441
442    /* Next, specific detection. This is especially important for `sensors'
443       devices. */
444
445    /* Determine the chip type. Not needed if a `force_CHIPTYPE' parameter
446       was used. */
447    if (kind <= 0) {
448      i = foo_read(new_client,FOO_REG_CHIPTYPE);
449      if (i == FOO_TYPE_1) 
450        kind = chip1; /* As defined in the enum */
451      else if (i == FOO_TYPE_2)
452        kind = chip2;
453      else {
454        printk("foo: Ignoring 'force' parameter for unknown chip at "
455               "adapter %d, address 0x%02x\n",i2c_adapter_id(adapter),address);
456        goto ERROR1;
457      }
458    }
459
460    /* Now set the type and chip names */
461    if (kind == chip1) {
462      type_name = "chip1"; /* For /proc entry */
463      client_name = "CHIP 1";
464    } else if (kind == chip2) {
465      type_name = "chip2"; /* For /proc entry */
466      client_name = "CHIP 2";
467    }
468   
469    /* Reserve the ISA region */
470    if (is_isa)
471      request_region(address,FOO_EXTENT,type_name);
472
473    /* SENSORS ONLY END */
474
475    /* Fill in the remaining client fields. */
476    strcpy(new_client->name,client_name);
477
478    /* SENSORS ONLY BEGIN */
479    data->type = kind;
480    /* SENSORS ONLY END */
481
482    data->valid = 0; /* Only if you use this field */
483    init_MUTEX(&data->update_lock); /* Only if you use this field */
484
485    /* Any other initializations in data must be done here too. */
486
487    /* Tell the i2c layer a new client has arrived */
488    if ((err = i2c_attach_client(new_client)))
489      goto ERROR3;
490
491    /* SENSORS ONLY BEGIN */
492    /* Register a new directory entry with module sensors. See below for
493       the `template' structure. */
494    if ((i = i2c_register_entry(new_client, type_name,
495                                    foo_dir_table_template,THIS_MODULE)) < 0) {
496      err = i;
497      goto ERROR4;
498    }
499    data->sysctl_id = i;
500
501    /* SENSORS ONLY END */
502
503    /* This function can write default values to the client registers, if
504       needed. */
505    foo_init_client(new_client);
506    return 0;
507
508    /* OK, this is not exactly good programming practice, usually. But it is
509       very code-efficient in this case. */
510
511    ERROR4:
512      i2c_detach_client(new_client);
513    ERROR3:
514    ERROR2:
515    /* SENSORS ONLY START */
516      if (is_isa)
517        release_region(address,FOO_EXTENT);
518    /* SENSORS ONLY END */
519    ERROR1:
520      kfree(new_client);
521    ERROR0:
522      return err;
523  }
524
525
526Removing the client
527===================
528
529The detach_client call back function is called when a client should be
530removed. It may actually fail, but only when panicking. This code is
531much simpler than the attachment code, fortunately!
532
533  int foo_detach_client(struct i2c_client *client)
534  {
535    int err,i;
536
537    /* SENSORS ONLY START */
538    /* Deregister with the `i2c-proc' module. */
539    i2c_deregister_entry(((struct lm78_data *)(client->data))->sysctl_id);
540    /* SENSORS ONLY END */
541
542    /* Try to detach the client from i2c space */
543    if ((err = i2c_detach_client(client))) {
544      printk("foo.o: Client deregistration failed, client not detached.\n");
545      return err;
546    }
547
548    /* SENSORS ONLY START */
549    if i2c_is_isa_client(client)
550      release_region(client->addr,LM78_EXTENT);
551    /* SENSORS ONLY END */
552
553    kfree(client); /* Frees client data too, if allocated at the same time */
554    return 0;
555  }
556
557
558Initializing the module or kernel
559=================================
560
561When the kernel is booted, or when your foo driver module is inserted, 
562you have to do some initializing. Fortunately, just attaching (registering)
563the driver module is usually enough.
564
565  /* Keep track of how far we got in the initialization process. If several
566     things have to initialized, and we fail halfway, only those things
567     have to be cleaned up! */
568  static int __initdata foo_initialized = 0;
569
570  static int __init foo_init(void)
571  {
572    int res;
573    printk("foo version %s (%s)\n",FOO_VERSION,FOO_DATE);
574    
575    if ((res = i2c_add_driver(&foo_driver))) {
576      printk("foo: Driver registration failed, module not inserted.\n");
577      foo_cleanup();
578      return res;
579    }
580    foo_initialized ++;
581    return 0;
582  }
583
584  void foo_cleanup(void)
585  {
586    if (foo_initialized == 1) {
587      if ((res = i2c_del_driver(&foo_driver))) {
588        printk("foo: Driver registration failed, module not removed.\n");
589        return;
590      }
591      foo_initialized --;
592    }
593  }
594
595  /* Substitute your own name and email address */
596  MODULE_AUTHOR("Frodo Looijaard <frodol@dds.nl>"
597  MODULE_DESCRIPTION("Driver for Barf Inc. Foo I2C devices");
598
599  module_init(foo_init);
600  module_exit(foo_cleanup);
601
602Note that some functions are marked by `__init', and some data structures
603by `__init_data'.  Hose functions and structures can be removed after
604kernel booting (or module loading) is completed.
605
606Command function
607================
608
609A generic ioctl-like function call back is supported. You will seldom
610need this. You may even set it to NULL.
611
612  /* No commands defined */
613  int foo_command(struct i2c_client *client, unsigned int cmd, void *arg)
614  {
615    return 0;
616  }
617
618
619Sending and receiving
620=====================
621
622If you want to communicate with your device, there are several functions
623to do this. You can find all of them in i2c.h.
624
625If you can choose between plain i2c communication and SMBus level
626communication, please use the last. All adapters understand SMBus level
627commands, but only some of them understand plain i2c!
628
629
630Plain i2c communication
631-----------------------
632
633  extern int i2c_master_send(struct i2c_client *,const char* ,int);
634  extern int i2c_master_recv(struct i2c_client *,char* ,int);
635
636These routines read and write some bytes from/to a client. The client
637contains the i2c address, so you do not have to include it. The second
638parameter contains the bytes the read/write, the third the length of the
639buffer. Returned is the actual number of bytes read/written.
640  
641  extern int i2c_transfer(struct i2c_adapter *adap, struct i2c_msg *msg,
642                          int num);
643
644This sends a series of messages. Each message can be a read or write,
645and they can be mixed in any way. The transactions are combined: no
646stop bit is sent between transaction. The i2c_msg structure contains
647for each message the client address, the number of bytes of the message
648and the message data itself.
649
650You can read the file `i2c-protocol' for more information about the
651actual i2c protocol.
652
653
654SMBus communication
655-------------------
656
657  extern s32 i2c_smbus_xfer (struct i2c_adapter * adapter, u16 addr, 
658                             unsigned short flags,
659                             char read_write, u8 command, int size,
660                             union i2c_smbus_data * data);
661
662  This is the generic SMBus function. All functions below are implemented
663  in terms of it. Never use this function directly!
664
665
666  extern s32 i2c_smbus_write_quick(struct i2c_client * client, u8 value);
667  extern s32 i2c_smbus_read_byte(struct i2c_client * client);
668  extern s32 i2c_smbus_write_byte(struct i2c_client * client, u8 value);
669  extern s32 i2c_smbus_read_byte_data(struct i2c_client * client, u8 command);
670  extern s32 i2c_smbus_write_byte_data(struct i2c_client * client,
671                                       u8 command, u8 value);
672  extern s32 i2c_smbus_read_word_data(struct i2c_client * client, u8 command);
673  extern s32 i2c_smbus_write_word_data(struct i2c_client * client,
674                                       u8 command, u16 value);
675  extern s32 i2c_smbus_write_block_data(struct i2c_client * client,
676                                        u8 command, u8 length,
677                                        u8 *values);
678
679These ones were removed in Linux 2.6.10 because they had no users, but could
680be added back later if needed:
681
682  extern s32 i2c_smbus_read_i2c_block_data(struct i2c_client * client,
683                                           u8 command, u8 *values);
684  extern s32 i2c_smbus_read_block_data(struct i2c_client * client,
685                                       u8 command, u8 *values);
686  extern s32 i2c_smbus_write_i2c_block_data(struct i2c_client * client,
687                                            u8 command, u8 length,
688                                            u8 *values);
689  extern s32 i2c_smbus_process_call(struct i2c_client * client,
690                                    u8 command, u16 value);
691  extern s32 i2c_smbus_block_process_call(struct i2c_client *client,
692                                          u8 command, u8 length,
693                                          u8 *values)
694
695All these transactions return -1 on failure. The 'write' transactions 
696return 0 on success; the 'read' transactions return the read value, except 
697for read_block, which returns the number of values read. The block buffers 
698need not be longer than 32 bytes.
699
700You can read the file `smbus-protocol' for more information about the
701actual SMBus protocol.
702
703
704General purpose routines
705========================
706
707Below all general purpose routines are listed, that were not mentioned
708before.
709
710  /* This call returns a unique low identifier for each registered adapter,
711   * or -1 if the adapter was not registered.
712   */
713  extern int i2c_adapter_id(struct i2c_adapter *adap);
714
715
716The sensors sysctl/proc interface
717=================================
718
719This section only applies if you write `sensors' drivers.
720
721Each sensors driver creates a directory in /proc/sys/dev/sensors for each
722registered client. The directory is called something like foo-i2c-4-65.
723The sensors module helps you to do this as easily as possible.
724
725The template
726------------
727
728You will need to define a ctl_table template. This template will automatically
729be copied to a newly allocated structure and filled in where necessary when
730you call sensors_register_entry.
731
732First, I will give an example definition.
733  static ctl_table foo_dir_table_template[] = {
734    { FOO_SYSCTL_FUNC1, "func1", NULL, 0, 0644, NULL, &i2c_proc_real,
735      &i2c_sysctl_real,NULL,&foo_func },
736    { FOO_SYSCTL_FUNC2, "func2", NULL, 0, 0644, NULL, &i2c_proc_real,
737      &i2c_sysctl_real,NULL,&foo_func },
738    { FOO_SYSCTL_DATA, "data", NULL, 0, 0644, NULL, &i2c_proc_real,
739      &i2c_sysctl_real,NULL,&foo_data },
740    { 0 }
741  };
742
743In the above example, three entries are defined. They can either be
744accessed through the /proc interface, in the /proc/sys/dev/sensors/*
745directories, as files named func1, func2 and data, or alternatively 
746through the sysctl interface, in the appropriate table, with identifiers
747FOO_SYSCTL_FUNC1, FOO_SYSCTL_FUNC2 and FOO_SYSCTL_DATA.
748
749The third, sixth and ninth parameters should always be NULL, and the
750fourth should always be 0. The fifth is the mode of the /proc file;
7510644 is safe, as the file will be owned by root:root. 
752
753The seventh and eighth parameters should be &i2c_proc_real and
754&i2c_sysctl_real if you want to export lists of reals (scaled
755integers). You can also use your own function for them, as usual.
756Finally, the last parameter is the call-back to gather the data
757(see below) if you use the *_proc_real functions. 
758
759
760Gathering the data
761------------------
762
763The call back functions (foo_func and foo_data in the above example)
764can be called in several ways; the operation parameter determines
765what should be done:
766
767  * If operation == SENSORS_PROC_REAL_INFO, you must return the
768    magnitude (scaling) in nrels_mag;
769  * If operation == SENSORS_PROC_REAL_READ, you must read information
770    from the chip and return it in results. The number of integers
771    to display should be put in nrels_mag;
772  * If operation == SENSORS_PROC_REAL_WRITE, you must write the
773    supplied information to the chip. nrels_mag will contain the number
774    of integers, results the integers themselves.
775
776The *_proc_real functions will display the elements as reals for the
777/proc interface. If you set the magnitude to 2, and supply 345 for
778SENSORS_PROC_REAL_READ, it would display 3.45; and if the user would
779write 45.6 to the /proc file, it would be returned as 4560 for
780SENSORS_PROC_REAL_WRITE. A magnitude may even be negative!
781
782An example function:
783
784  /* FOO_FROM_REG and FOO_TO_REG translate between scaled values and
785     register values. Note the use of the read cache. */
786  void foo_in(struct i2c_client *client, int operation, int ctl_name, 
787              int *nrels_mag, long *results)
788  {
789    struct foo_data *data = client->data;
790    int nr = ctl_name - FOO_SYSCTL_FUNC1; /* reduce to 0 upwards */
791    
792    if (operation == SENSORS_PROC_REAL_INFO)
793      *nrels_mag = 2;
794    else if (operation == SENSORS_PROC_REAL_READ) {
795      /* Update the readings cache (if necessary) */
796      foo_update_client(client);
797      /* Get the readings from the cache */
798      results[0] = FOO_FROM_REG(data->foo_func_base[nr]);
799      results[1] = FOO_FROM_REG(data->foo_func_more[nr]);
800      results[2] = FOO_FROM_REG(data->foo_func_readonly[nr]);
801      *nrels_mag = 2;
802    } else if (operation == SENSORS_PROC_REAL_WRITE) {
803      if (*nrels_mag >= 1) {
804        /* Update the cache */
805        data->foo_base[nr] = FOO_TO_REG(results[0]);
806        /* Update the chip */
807        foo_write_value(client,FOO_REG_FUNC_BASE(nr),data->foo_base[nr]);
808      }
809      if (*nrels_mag >= 2) {
810        /* Update the cache */
811        data->foo_more[nr] = FOO_TO_REG(results[1]);
812        /* Update the chip */
813        foo_write_value(client,FOO_REG_FUNC_MORE(nr),data->foo_more[nr]);
814      }
815    }
816  }