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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
27  struct i2c_driver foo_driver
28  { 
29    /* name           */  "Foo version 2.3 and later driver",
30    /* id             */  I2C_DRIVERID_FOO,
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    /* inc_use        */  &foo_inc_use,   /* May be NULL */
36    /* dec_use        */  &foo_dev_use    /* May be NULL */
37  }
38 
39The name can be choosen freely, and may be upto 40 characters long. Please
40use something descriptive here.
41
42The id should be a unique ID. The range 0xf000 to 0xffff is reserved for
43local use, and you can use one of those until you start distributing the
44driver. Before you do that, contact the i2c authors to get your own ID(s).
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
53
54Module usage count
55==================
56
57If your driver can also be compiled as a module, there are moments at
58which the module can not be removed from memory. For example, when you
59are doing a lengthy transaction, or when you create a /proc directory,
60and some process has entered that directory (this last case is the
61main reason why these call-backs were introduced).
62
63To increase or decrease the module usage count, you can use the
64MOD_{INC,DEC}_USE_COUNT macros. They must be called from the module
65which needs to get its usage count changed; that is why each driver
66module has to implement its own callback.
67
68  void foo_inc_use (struct i2c_client *client)
69  {
70  #ifdef MODULE
71    MOD_INC_USE_COUNT;
72  #endif
73  }
74
75  void foo_dec_use (struct i2c_client *client)
76  {
77  #ifdef MODULE
78    MOD_DEC_USE_COUNT;
79  #endif
80  }
81
82Do not call these call-back functions directly; instead, use one of the
83following functions defined in i2c.h:
84  void i2c_inc_use_client(struct i2c_client *);
85  void i2c_dec_use_client(struct i2c_client *);
86
87You should *not* increase the module count just because a device is
88detected and a client created. This would make it impossible to remove
89an adapter driver!
90
91
92Extra client data
93=================
94
95The client structure has a special `data' field that can point to any
96structure at all. You can use this to keep client-specific data. You
97do not always need this, but especially for `sensors' drivers, it can
98be very useful.
99
100An example structure is below.
101
102  struct foo_data {
103    struct semaphore lock; /* For ISA access in `sensors' drivers. */
104    int sysctl_id;         /* To keep the /proc directory entry for
105                              `sensors' drivers. */
106    enum chips type;       /* To keep the chips type for `sensors' drivers. */
107   
108    /* Because the i2c bus is slow, it is often useful to cache the read
109       information of a chip for some time (for example, 1 or 2 seconds).
110       It depends of course on the device whether this is really worthwhile
111       or even sensible. */
112    struct semaphore update_lock; /* When we are reading lots of information,
113                                     another process should not update the
114                                     below information */
115    char valid;                   /* != 0 if the following fields are valid. */
116    unsigned long last_updated;   /* In jiffies */
117    /* Add the read information here too */
118  };
119
120
121Accessing the client
122====================
123
124Let's say we have a valid client structure. At some time, we will need
125to gather information from the client, or write new information to the
126client. How we will export this information to user-space is less
127important at this moment (perhaps we do not need to do this at all for
128some obscure clients). But we need generic reading and writing routines.
129
130I have found it useful to define foo_read and foo_write function for this.
131For some cases, it will be easier to call the i2c functions directly,
132but many chips have some kind of register-value idea that can easily
133be encapsulated. Also, some chips have both ISA and I2C interfaces, and
134it useful to abstract from this (only for `sensors' drivers).
135
136The below functions are simple examples, and should not be copied
137literally.
138
139  int foo_read_value(struct i2c_client *client, u8 reg)
140  {
141    if (reg < 0x10) /* byte-sized register */
142      return i2c_smbus_read_byte_data(client,reg);
143    else /* word-sized register */
144      return i2c_smbus_read_word_data(client,reg);
145  }
146
147  int foo_write_value(struct i2c_client *client, u8 reg, u16 value)
148  {
149    if (reg == 0x10) /* Impossible to write - driver error! */ {
150      return -1;
151    else if (reg < 0x10) /* byte-sized register */
152      return i2c_smbus_write_byte_data(client,reg,value);
153    else /* word-sized register */
154      return i2c_smbus_write_word_data(client,reg,value);
155  }
156
157For sensors code, you may have to cope with ISA registers too. Something
158like the below often works. Note the locking!
159
160  int foo_read_value(struct i2c_client *client, u8 reg)
161  {
162    int res;
163    if (i2c_is_isa_client(client)) {
164      down(&(((struct foo_data *) (client->data)) -> lock));
165      outb_p(reg,client->addr + FOO_ADDR_REG_OFFSET);
166      res = inb_p(client->addr + FOO_DATA_REG_OFFSET);
167      up(&(((struct foo_data *) (client->data)) -> lock));
168      return res;
169    } else
170      return i2c_smbus_read_byte_data(client,reg);
171  }
172
173Writing is done the same way.
174
175
176Probing and attaching
177=====================
178
179Most i2c devices can be present on several i2c addresses; for some this
180is determined in hardware (by soldering some chip pins to Vcc or Ground),
181for others this can be changed in software (by writing to specific client
182registers). Some devices are usually on a specific address, but not always;
183and some are even more tricky. So you will probably need to scan several
184i2c addresses for your clients, and do some sort of detection to see
185whether it is actually a device supported by your driver.
186
187To give the user a maximum of possibilities, some default module parameters
188are defined to help determine what addresses are scanned. Several macros
189are defined in i2c.h to help you support them, as well as a generic
190detection algorithm.
191
192You do not have to use this parameter interface; but don't try to use
193function i2c_probe() (or sensors_detect()) if you don't.
194
195NOTE: If you want to write a `sensors' driver, the interface is slightly
196      different! See below.
197
198
199
200Probing classes (i2c)
201---------------------
202
203All parameters are given as lists of unsigned 16-bit integers. Lists are
204terminated by I2C_CLIENT_END.
205The following lists are used internally:
206
207  normal_i2c: filled in by the module writer.
208     A list of I2C addresses which should normally be examined.
209   normal_i2c_range: filled in by the module writer.
210     A list of pairs of I2C addresses, each pair being an inclusive range of
211     addresses which should normally be examined.
212   probe: insmod parameter.
213     A list of pairs. The first value is a bus number (-1 for any I2C bus),
214     the second is the address. These addresses are also probed, as if they
215     were in the 'normal' list.
216   probe_range: insmod parameter.
217     A list of triples. The first value is a bus number (-1 for any I2C bus),
218     the second and third are addresses.  These form an inclusive range of
219     addresses that are also probed, as if they were in the 'normal' list.
220   ignore: insmod parameter.
221     A list of pairs. The first value is a bus number (-1 for any I2C bus),
222     the second is the I2C address. These addresses are never probed.
223     This parameter overrules 'normal' and 'probe', but not the 'force' lists.
224   ignore_range: insmod parameter.
225     A list of triples. The first value is a bus number (-1 for any I2C bus),
226     the second and third are addresses. These form an inclusive range of
227     I2C addresses that are never probed.
228     This parameter overrules 'normal' and 'probe', but not the 'force' lists.
229   force: insmod parameter.
230     A list of pairs. The first value is a bus number (-1 for any I2C bus),
231     the second is the I2C address. A device is blindly assumed to be on
232     the given address, no probing is done.
233
234Fortunately, as a module writer, you just have to define the `normal'
235and/or `normal_range' parameters. The complete declaration could look
236like this:
237
238  /* Scan 0x20 to 0x2f, 0x37, and 0x40 to 0x4f */
239  static unsigned short normal_i2c[] = { 0x37,I2C_CLIENT_END };
240  static unsigned short normal_i2c_range[] = { 0x20, 0x2f, 0x40, 0x4f,
241                                               I2C_CLIENT_END };
242
243  /* Magic definition of all other variables and things */
244  I2C_CLIENT_INSMOD;
245
246Note that you *have* to call the two defined variables `normal_i2c' and
247`normal_i2c_range', without any prefix!
248
249
250Probing classes (sensors)
251-------------------------
252
253If you write a `sensors' driver, you use a slightly different interface.
254As well as I2C addresses, we have to cope with ISA addresses. Also, we
255use a enum of chip types. Don't forget to include `sensors.h'.
256
257The following lists are used internally. They are all lists of integers.
258
259   normal_i2c: filled in by the module writer. Terminated by SENSORS_I2C_END.
260     A list of I2C addresses which should normally be examined.
261   normal_i2c_range: filled in by the module writer. Terminated by
262     SENSORS_I2C_END
263     A list of pairs of I2C addresses, each pair being an inclusive range of
264     addresses which should normally be examined.
265   normal_isa: filled in by the module writer. Terminated by SENSORS_ISA_END.
266     A list of ISA addresses which should normally be examined.
267   normal_isa_range: filled in by the module writer. Terminated by
268     SENSORS_ISA_END
269     A list of triples. The first two elements are ISA addresses, being an
270     range of addresses which should normally be examined. The third is the
271     modulo parameter: only addresses which are 0 module this value relative
272     to the first address of the range are actually considered.
273   probe: insmod parameter. Initialize this list with SENSORS_I2C_END values.
274     A list of pairs. The first value is a bus number (SENSORS_ISA_BUS for
275     the ISA bus, -1 for any I2C bus), the second is the address. These
276     addresses are also probed, as if they were in the 'normal' list.
277   probe_range: insmod parameter. Initialize this list with SENSORS_I2C_END
278     values.
279     A list of triples. The first value is a bus number (SENSORS_ISA_BUS for
280     the ISA bus, -1 for any I2C bus), the second and third are addresses.
281     These form an inclusive range of addresses that are also probed, as
282     if they were in the 'normal' list.
283   ignore: insmod parameter. Initialize this list with SENSORS_I2C_END values.
284     A list of pairs. The first value is a bus number (SENSORS_ISA_BUS for
285     the ISA bus, -1 for any I2C bus), the second is the I2C address. These
286     addresses are never probed. This parameter overrules 'normal' and
287     'probe', but not the 'force' lists.
288   ignore_range: insmod parameter. Initialize this list with SENSORS_I2C_END
289      values.
290     A list of triples. The first value is a bus number (SENSORS_ISA_BUS for
291     the ISA bus, -1 for any I2C bus), the second and third are addresses.
292     These form an inclusive range of I2C addresses that are never probed.
293     This parameter overrules 'normal' and 'probe', but not the 'force' lists.
294
295Also used is a list of pointers to sensors_force_data structures:
296   force_data: insmod parameters. A list, ending with an element of which
297     the force field is NULL.
298     Each element contains the type of chip and a list of pairs.
299     The first value is a bus number (SENSORS_ISA_BUS for the ISA bus,
300     -1 for any I2C bus), the second is the address.
301     These are automatically translated to insmod variables of the form
302     force_foo.
303
304So we have a generic insmod variabled `force', and chip-specific variables
305`force_CHIPNAME'.
306
307Fortunately, as a module writer, you just have to define the `normal'
308and/or `normal_range' parameters, and define what chip names are used.
309The complete declaration could look like this:
310  /* Scan i2c addresses 0x20 to 0x2f, 0x37, and 0x40 to 0x4f
311  static unsigned short normal_i2c[] = {0x37,SENSORS_I2C_END};
312  static unsigned short normal_i2c_range[] = {0x20,0x2f,0x40,0x4f,
313                                              SENSORS_I2C_END};
314  /* Scan ISA address 0x290 */
315  static unsigned int normal_isa[] = {0x0290,SENSORS_ISA_END};
316  static unsigned int normal_isa_range[] = {SENSORS_ISA_END};
317
318  /* Define chips foo and bar, as well as all module parameters and things */
319  SENSORS_INSMOD_2(foo,bar);
320
321If you have one chip, you use macro SENSORS_INSMOD_1(chip), if you have 2
322you use macro SENSORS_INSMOD_2(chip1,chip2), etc. If you do not want to
323bother with chip types, you can use SENSORS_INSMOD_0.
324
325A enum is automatically defined as follows:
326  enum chips { any_chip, chip1, chip2, ... }
327
328
329Attaching to an adapter
330-----------------------
331
332Whenever a new adapter is inserted, or for all adapters if the driver is
333being registered, the callback attach_adapter() is called. Now is the
334time to determine what devices are present on the adapter, and to register
335a client for each of them.
336
337The attach_adapter callback is really easy: we just call the generic
338detection function. This function will scan the bus for us, using the
339information as defined in the lists explained above. If a device is
340detected at a specific address, another callback is called.
341
342  int foo_attach_adapter(struct i2c_adapter *adapter)
343  {
344    return i2c_probe(adapter,&addr_data,&foo_detect_client);
345  }
346
347For `sensors' drivers, use the sensors_detect function instead:
348 
349  int foo_attach_adapter(struct i2c_adapter *adapter)
350  {
351    return sensors_detect(adapter,&addr_data,&foo_detect_client);
352  }
353
354Remember, structure `addr_data' is defined by the macros explained above,
355so you do not have to define it yourself.
356
357The i2c_probe or sensors_detect function will call the foo_detect_client
358function only for those i2c addresses that actually have a device on
359them (unless a `force' parameter was used). In addition, addresses that
360are already in use (by some other registered client) are skipped.
361
362
363The detect client function
364--------------------------
365
366The detect client function is called by i2c_probe or sensors_detect.
367The `kind' parameter contains 0 if this call is due to a `force'
368parameter, and 0 otherwise (for sensors_detect, it contains 0 if
369this call is due to the generic `force' parameter, and the chip type
370number if it is due to a specific `force' parameter).
371
372Below, some things are only needed if this is a `sensors' driver. Those
373parts are between /* SENSORS ONLY START */ and /* SENSORS ONLY END */
374markers.
375
376This function should only return an error (any value != 0) if there is
377some reason why no more detection should be done anymore. If the
378detection just fails for this address, return 0.
379
380For now, you can ignore the `flags' parameter. It is there for future use.
381
382  /* Unique ID allocation */
383  static int foo_id = 0;
384
385  int foo_detect_client(struct i2c_adapter *adapter, int address,
386                        unsigned short flags, int kind)
387  {
388    int err = 0;
389    int i;
390    struct i2c_client *new_client;
391    struct foo_data *data;
392    const char *client_name = ""; /* For non-`sensors' drivers, put the real
393                                     name here! */
394   
395    /* Let's see whether this adapter can support what we need.
396       Please substitute the things you need here!
397       For `sensors' drivers, add `! is_isa &&' to the if statement */
398    if (i2c_check_functionality(adapter,I2C_FUNC_SMBUS_WORD_DATA |
399                                        I2C_FUNC_SMBUS_WRITE_BYTE))
400       goto ERROR0;
401
402    /* SENSORS ONLY START */
403    const char *type_name = "";
404    int is_isa = i2c_is_isa_adapter(adapter);
405
406    if (is_isa) {
407
408      /* If this client can't be on the ISA bus at all, we can stop now
409         (call `goto ERROR0'). But for kicks, we will assume it is all
410         right. */
411
412      /* Discard immediately if this ISA range is already used */
413      if (check_region(address,FOO_EXTENT))
414        goto ERROR0;
415
416      /* Probe whether there is anything on this address.
417         Some example code is below, but you will have to adapt this
418         for your own driver */
419
420      if (kind < 0) /* Only if no force parameter was used */ {
421        /* We may need long timeouts at least for some chips. */
422        #define REALLY_SLOW_IO
423        i = inb_p(address + 1);
424        if (inb_p(address + 2) != i)
425          goto ERROR0;
426        if (inb_p(address + 3) != i)
427          goto ERROR0;
428        if (inb_p(address + 7) != i)
429          goto ERROR0;
430        #undef REALLY_SLOW_IO
431
432        /* Let's just hope nothing breaks here */
433        i = inb_p(address + 5) & 0x7f;
434        outb_p(~i & 0x7f,address+5);
435        if ((inb_p(address + 5) & 0x7f) != (~i & 0x7f)) {
436          outb_p(i,address+5);
437          return 0;
438        }
439      }
440    }
441
442    /* SENSORS ONLY END */
443
444    /* OK. For now, we presume we have a valid client. We now create the
445       client structure, even though we cannot fill it completely yet.
446       But it allows us to access several i2c functions safely */
447   
448    /* Note that we reserve some space for foo_data too. If you don't
449       need it, remove it. We do it here to help to lessen memory
450       fragmentation. */
451    if (! (new_client = kmalloc(sizeof(struct i2c_client)) +
452                                sizeof(struct foo_data),
453                                GFP_KERNEL)) {
454      err = -ENOMEM;
455      goto ERROR0;
456    }
457
458    /* This is tricky, but it will set the data to the right value. */
459    client->data = new_client + 1;
460    data = (struct foo_data *) (client->data);
461
462    new_client->addr = address;
463    new_client->data = data;
464    new_client->adapter = adapter;
465    new_client->driver = &foo_driver;
466    new_client->flags = 0;
467
468    /* Now, we do the remaining detection. If no `force' parameter is used. */
469
470    /* First, the generic detection (if any), that is skipped if any force
471       parameter was used. */
472    if (kind < 0) {
473      /* The below is of course bogus */
474      if (foo_read(new_client,FOO_REG_GENERIC) != FOO_GENERIC_VALUE)
475         goto ERROR1;
476    }
477
478    /* SENSORS ONLY START */
479
480    /* Next, specific detection. This is especially important for `sensors'
481       devices. */
482
483    /* Determine the chip type. Not needed if a `force_CHIPTYPE' parameter
484       was used. */
485    if (kind <= 0) {
486      i = foo_read(new_client,FOO_REG_CHIPTYPE);
487      if (i == FOO_TYPE_1)
488        kind = chip1; /* As defined in the enum */
489      else if (i == FOO_TYPE_2)
490        kind = chip2;
491      else {
492        printk("foo: Ignoring 'force' parameter for unknown chip at "
493               "adapter %d, address 0x%02x\n",i2c_adapter_id(adapter),address);
494        goto ERROR1;
495      }
496    }
497
498    /* Now set the type and chip names */
499    if (kind == chip1) {
500      type_name = "chip1"; /* For /proc entry */
501      client_name = "CHIP 1";
502    } else if (kind == chip2) {
503      type_name = "chip2"; /* For /proc entry */
504      client_name = "CHIP 2";
505    }
506   
507    /* Reserve the ISA region */
508    if (is_isa)
509      request_region(address,FOO_EXTENT,type_name);
510
511    /* SENSORS ONLY END */
512
513    /* Fill in the remaining client fields. */
514    strcpy(new_client->name,client_name);
515
516    /* SENSORS ONLY BEGIN */
517    data->type = kind;
518    /* SENSORS ONLY END */
519
520    new_client->id = foo_id++; /* Automatically unique */
521    data->valid = 0; /* Only if you use this field */
522    init_MUTEX(&data->update_lock); /* Only if you use this field */
523
524    /* Any other initializations in data must be done here too. */
525
526    /* Tell the i2c layer a new client has arrived */
527    if ((err = i2c_attach_client(new_client)))
528      goto ERROR3;
529
530    /* SENSORS ONLY BEGIN */
531    /* Register a new directory entry with module sensors. See below for
532       the `template' structure. */
533    if ((i = sensors_register_entry(new_client, type_name,
534                                    foo_dir_table_template,THIS_MODULE)) < 0) {
535      err = i;
536      goto ERROR4;
537    }
538    data->sysctl_id = i;
539
540    /* SENSORS ONLY END */
541
542    /* This function can write default values to the client registers, if
543       needed. */
544    foo_init_client(new_client);
545    return 0;
546
547    /* OK, this is not exactly good programming practice, usually. But it is
548       very code-efficient in this case. */
549
550    ERROR4:
551      i2c_detach_client(new_client);
552    ERROR3:
553    ERROR2:
554    /* SENSORS ONLY START */
555      if (is_isa)
556        release_region(address,FOO_EXTENT);
557    /* SENSORS ONLY END */
558    ERROR1:
559      kfree(new_client);
560    ERROR0:
561      return err;
562  }
563
564
565Removing the client
566===================
567
568The detach_client call back function is called when a client should be
569removed. It may actually fail, but only when panicking. This code is
570much simpler than the attachment code, fortunately!
571
572  int foo_detach_client(struct i2c_client *client)
573  {
574    int err,i;
575
576    /* SENSORS ONLY START */
577    /* Deregister with the `sensors' module. */
578    sensors_deregister_entry(((struct lm78_data *)(client->data))->sysctl_id);
579    /* SENSORS ONLY END */
580
581    /* Try to detach the client from i2c space */
582    if ((err = i2c_detach_client(client))) {
583      printk("foo.o: Client deregistration failed, client not detached.\n");
584      return err;
585    }
586
587    /* SENSORS ONLY START */
588    if i2c_is_isa_client(client)
589      release_region(client->addr,LM78_EXTENT);
590    /* SENSORS ONLY END */
591
592    kfree(client); /* Frees client data too, if allocated at the same time */
593    return 0;
594  }
595
596
597Initializing the module or kernel
598=================================
599
600When the kernel is booted, or when your foo driver module is inserted,
601you have to do some initializing. Fortunately, just attaching (registering)
602the driver module is usually enough.
603
604  /* Keep track of how far we got in the initialization process. If several
605     things have to initialized, and we fail halfway, only those things
606     have to be cleaned up! */
607  static int __initdata foo_initialized = 0;
608
609  int __init foo_init(void)
610  {
611    int res;
612    printk("foo version %s (%s)\n",FOO_VERSION,FOO_DATE);
613   
614    if ((res = i2c_add_driver(&foo_driver))) {
615      printk("foo: Driver registration failed, module not inserted.\n");
616      foo_cleanup();
617      return res;
618    }
619    foo_initialized ++;
620    return 0;
621  }
622
623  int __init foo_cleanup(void)
624  {
625    int res;
626    if (foo_initialized == 1) {
627      if ((res = i2c_del_driver(&foo_driver))) {
628        printk("foo: Driver registration failed, module not removed.\n");
629        return res;
630      }
631      foo_initialized --;
632    }
633    return 0;
634  }
635
636  #ifdef MODULE
637
638  /* Substitute your own name and email address */
639  MODULE_AUTHOR("Frodo Looijaard <frodol@dds.nl>"
640  MODULE_DESCRIPTION("Driver for Barf Inc. Foo I2C devices");
641
642  int init_module(void)
643  {
644    return foo_init();
645  }
646
647  int cleanup_module(void)
648  {
649    return foo_cleanup();
650  }
651
652  #endif /* def MODULE */
653
654Note that some functions are marked by `__init', and some data structures
655by `__init_data'. If this driver is compiled as part of the kernel (instead
656of as a module), those functions and structures can be removed after
657kernel booting is completed.
658
659Command function
660================
661
662A generic ioctl-like function call back is supported. You will seldomly
663need this. You may even set it to NULL.
664
665  /* No commands defined */
666  int foo_command(struct i2c_client *client, unsigned int cmd, void *arg)
667  {
668    return 0;
669  }
670
671
672Sending and receiving
673=====================
674
675If you want to communicate with your device, there are several functions
676to do this. You can find all of them in i2c.h.
677
678If you can choose between plain i2c communication and SMBus level
679communication, please use the last. All adapters understand SMBus level
680commands, but only some of them understand plain i2c!
681
682
683Plain i2c communication
684-----------------------
685
686  extern int i2c_master_send(struct i2c_client *,const char* ,int);
687  extern int i2c_master_recv(struct i2c_client *,char* ,int);
688
689These routines read and write some bytes from/to a client. The client
690contains the i2c address, so you do not have to include it. The second
691parameter contains the bytes the read/write, the third the length of the
692buffer. Returned is the actual number of bytes read/written.
693 
694  extern int i2c_transfer(struct i2c_adapter *adap, struct i2c_msg msg[],
695                          int num);
696
697This sends a series of messages. Each message can be a read or write,
698and they can be mixed in any way. The transactions are combined: no
699stop bit is sent between transaction. The i2c_msg structure contains
700for each message the client address, the number of bytes of the message
701and the message data itself.
702
703You can read the file `i2c-protocol' for more information about the
704actual i2c protocol.
705
706
707SMBus communication
708-------------------
709
710  extern s32 i2c_smbus_xfer (struct i2c_adapter * adapter, u16 addr,
711                             unsigned short flags,
712                             char read_write, u8 command, int size,
713                             union i2c_smbus_data * data);
714
715  This is the generic SMBus function. All functions below are implemented
716  in terms of it. Never use this function directly!
717
718
719  extern s32 i2c_smbus_write_quick(struct i2c_client * client, u8 value);
720  extern s32 i2c_smbus_read_byte(struct i2c_client * client);
721  extern s32 i2c_smbus_write_byte(struct i2c_client * client, u8 value);
722  extern s32 i2c_smbus_read_byte_data(struct i2c_client * client, u8 command);
723  extern s32 i2c_smbus_write_byte_data(struct i2c_client * client,
724                                       u8 command, u8 value);
725  extern s32 i2c_smbus_read_word_data(struct i2c_client * client, u8 command);
726  extern s32 i2c_smbus_write_word_data(struct i2c_client * client,
727                                       u8 command, u16 value);
728  extern s32 i2c_smbus_process_call(struct i2c_client * client,
729                                    u8 command, u16 value);
730  extern s32 i2c_smbus_read_block_data(struct i2c_client * client,
731                                       u8 command, u8 *values);
732  extern s32 i2c_smbus_write_block_data(struct i2c_client * client,
733                                        u8 command, u8 length,
734                                        u8 *values);
735
736All these tranactions return -1 on failure. The 'write' transactions
737return 0 on success; the 'read' transactions return the read value, except
738for read_block, which returns the number of values read. The block buffers
739need not be longer than 32 bytes.
740
741You can read the file `smbus-protocol' for more information about the
742actual SMBus protocol.
743
744
745General purpose routines
746========================
747
748Below all general purpose routines are listed, that were not mentioned
749before.
750
751  /* This call returns a unique low identifier for each registered adapter,
752   * or -1 if the adapter was not regisitered.
753   */
754  extern int i2c_adapter_id(struct i2c_adapter *adap);
755
756
757The sensors sysctl/proc interface
758=================================
759
760This section only applies if you write `sensors' drivers.
761
762Each sensors driver creates a directory in /proc/sys/dev/sensors for each
763registered client. The directory is called something like foo-i2c-4-65.
764The sensors module helps you to do this as easily as possible.
765
766The template
767------------
768
769You will need to define a ctl_table template. This template will automatically
770be copied to a newly allocated structure and filled in where necessary when
771you call sensors_register_entry.
772
773First, I will give an example definition.
774  static ctl_table foo_dir_table_template[] = {
775    { FOO_SYSCTL_FUNC1, "func1", NULL, 0, 0644, NULL, &sensors_proc_real,
776      &sensors_sysctl_real,NULL,&foo_func },
777    { FOO_SYSCTL_FUNC2, "func2", NULL, 0, 0644, NULL, &sensors_proc_real,
778      &sensors_sysctl_real,NULL,&foo_func },
779    { FOO_SYSCTL_DATA, "data", NULL, 0, 0644, NULL, &sensors_proc_real,
780      &sensors_sysctl_real,NULL,&foo_data },
781    { 0 }
782  };
783
784In the above example, three entries are defined. They can either be
785accessed through the /proc interface, in the /proc/sys/dev/sensors/*
786directories, as files named func1, func2 and data, or alternatively
787through the sysctl interface, in the appropriate table, with identifiers
788FOO_SYSCTL_FUNC1, FOO_SYSCTL_FUNC2 and FOO_SYSCTL_DATA.
789
790The third, sixth and ninth parameters should always be NULL, and the
791fourth should always be 0. The fifth is the mode of the /proc file;
7920644 is safe, as the file will be owned by root:root.
793
794The seventh and eigth parameters should be &sensors_proc_real and
795&sensors_sysctl_real if you want to export lists of reals (scaled
796integers). You can also use your own function for them, as usual.
797Finally, the last parameter is the call-back to gather the data
798(see below) if you use the *_proc_real functions.
799
800
801Gathering the data
802------------------
803
804The call back functions (foo_func and foo_data in the above example)
805can be called in several ways; the operation parameter determines
806what should be done:
807
808  * If operation == SENSORS_PROC_REAL_INFO, you must return the
809    magnitude (scaling) in nrels_mag;
810  * If operation == SENSORS_PROC_REAL_READ, you must read information
811    from the chip and return it in results. The number of integers
812    to display should be put in nrels_mag;
813  * If operation == SENSORS_PROC_REAL_WRITE, you must write the
814    supplied information to the chip. nrels_mag will contain the number
815    of integers, results the integers themselves.
816
817The *_proc_real functions will display the elements as reals for the
818/proc interface. If you set the magnitude to 2, and supply 345 for
819SENSORS_PROC_REAL_READ, it would display 3.45; and if the user would
820write 45.6 to the /proc file, it would be returned as 4560 for
821SENSORS_PROC_REAL_WRITE. A magnitude may even be negative!
822
823An example function:
824
825  /* FOO_FROM_REG and FOO_TO_REG translate between scaled values and
826     register values. Note the use of the read cache. */
827  void foo_in(struct i2c_client *client, int operation, int ctl_name,
828              int *nrels_mag, long *results)
829  {
830    struct foo_data *data = client->data;
831    int nr = ctl_name - FOO_SYSCTL_FUNC1; /* reduce to 0 upwards */
832   
833    if (operation == SENSORS_PROC_REAL_INFO)
834      *nrels_mag = 2;
835    else if (operation == SENSORS_PROC_REAL_READ) {
836      /* Update the readings cache (if necessary) */
837      foo_update_client(client);
838      /* Get the readings from the cache */
839      results[0] = FOO_FROM_REG(data->foo_func_base[nr]);
840      results[1] = FOO_FROM_REG(data->foo_func_more[nr]);
841      results[2] = FOO_FROM_REG(data->foo_func_readonly[nr]);
842      *nrels_mag = 2;
843    } else if (operation == SENSORS_PROC_REAL_WRITE) {
844      if (*nrels_mag >= 1) {
845        /* Update the cache */
846        data->foo_base[nr] = FOO_TO_REG(results[0]);
847        /* Update the chip */
848        foo_write_value(client,FOO_REG_FUNC_BASE(nr),data->foo_base[nr]);
849      }
850      if (*nrels_mag >= 2) {
851        /* Update the cache */
852        data->foo_more[nr] = FOO_TO_REG(results[1]);
853        /* Update the chip */
854        foo_write_value(client,FOO_REG_FUNC_MORE(nr),data->foo_more[nr]);
855      }
856    }
857  }
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