Documentation/i2c/writing-clients

   1 This is a small guide for those who want to write kernel drivers for I2C
   2 or SMBus devices.
   3
   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!
   7
   8
   9 General remarks
  10 ===============
  11
  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.
  17
  18
  19 The driver structure
  20 ====================
  21
  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
  25 address.
  26
  27 static struct i2c_driver foo_driver = {
  28         .driver = {
  29                 .name   = "foo",
  30         },
  31         .attach_adapter = &foo_attach_adapter,
  32         .detach_client  = &foo_detach_client,
  33         .command        = &foo_command /* may be NULL */
  34 }
  35
  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.
  38
  39 All other fields are for call-back functions which will be explained
  40 below.
  41
  42
  43 Extra client data
  44 =================
  45
  46 The client structure has a special `data' field that can point to any
  47 structure at all. You can use this to keep client-specific data. You
  48 do not always need this, but especially for `sensors' drivers, it can
  49 be very useful.
  50
  51 An example structure is below.
  52
  53   struct foo_data {
  54     struct i2c_client client;
  55     struct semaphore lock; /* For ISA access in `sensors' drivers. */
  56     int sysctl_id;         /* To keep the /proc directory entry for
  57                               `sensors' drivers. */
  58     enum chips type;       /* To keep the chips type for `sensors' drivers. */
  59
  60     /* Because the i2c bus is slow, it is often useful to cache the read
  61        information of a chip for some time (for example, 1 or 2 seconds).
  62        It depends of course on the device whether this is really worthwhile
  63        or even sensible. */
  64     struct semaphore update_lock; /* When we are reading lots of information,
  65                                      another process should not update the
  66                                      below information */
  67     char valid;                   /* != 0 if the following fields are valid. */
  68     unsigned long last_updated;   /* In jiffies */
  69     /* Add the read information here too */
  70   };
  71
  72
  73 Accessing the client
  74 ====================
  75
  76 Let's say we have a valid client structure. At some time, we will need
  77 to gather information from the client, or write new information to the
  78 client. How we will export this information to user-space is less
  79 important at this moment (perhaps we do not need to do this at all for
  80 some obscure clients). But we need generic reading and writing routines.
  81
  82 I have found it useful to define foo_read and foo_write function for this.
  83 For some cases, it will be easier to call the i2c functions directly,
  84 but many chips have some kind of register-value idea that can easily
  85 be encapsulated. Also, some chips have both ISA and I2C interfaces, and
  86 it useful to abstract from this (only for `sensors' drivers).
  87
  88 The below functions are simple examples, and should not be copied
  89 literally.
  90
  91   int foo_read_value(struct i2c_client *client, u8 reg)
  92   {
  93     if (reg < 0x10) /* byte-sized register */
  94       return i2c_smbus_read_byte_data(client,reg);
  95     else /* word-sized register */
  96       return i2c_smbus_read_word_data(client,reg);
  97   }
  98
  99   int foo_write_value(struct i2c_client *client, u8 reg, u16 value)
 100   {
 101     if (reg == 0x10) /* Impossible to write - driver error! */ {
 102       return -1;
 103     else if (reg < 0x10) /* byte-sized register */
 104       return i2c_smbus_write_byte_data(client,reg,value);
 105     else /* word-sized register */
 106       return i2c_smbus_write_word_data(client,reg,value);
 107   }
 108
 109 For sensors code, you may have to cope with ISA registers too. Something
 110 like the below often works. Note the locking!
 111
 112   int foo_read_value(struct i2c_client *client, u8 reg)
 113   {
 114     int res;
 115     if (i2c_is_isa_client(client)) {
 116       down(&(((struct foo_data *) (client->data)) -> lock));
 117       outb_p(reg,client->addr + FOO_ADDR_REG_OFFSET);
 118       res = inb_p(client->addr + FOO_DATA_REG_OFFSET);
 119       up(&(((struct foo_data *) (client->data)) -> lock));
 120       return res;
 121     } else
 122       return i2c_smbus_read_byte_data(client,reg);
 123   }
 124
 125 Writing is done the same way.
 126
 127
 128 Probing and attaching
 129 =====================
 130
 131 Most i2c devices can be present on several i2c addresses; for some this
 132 is determined in hardware (by soldering some chip pins to Vcc or Ground),
 133 for others this can be changed in software (by writing to specific client
 134 registers). Some devices are usually on a specific address, but not always;
 135 and some are even more tricky. So you will probably need to scan several
 136 i2c addresses for your clients, and do some sort of detection to see
 137 whether it is actually a device supported by your driver.
 138
 139 To give the user a maximum of possibilities, some default module parameters
 140 are defined to help determine what addresses are scanned. Several macros
 141 are defined in i2c.h to help you support them, as well as a generic
 142 detection algorithm.
 143
 144 You do not have to use this parameter interface; but don't try to use
 145 function i2c_probe() if you don't.
 146
 147 NOTE: If you want to write a `sensors' driver, the interface is slightly
 148       different! See below.
 149
 150
 151
 152 Probing classes
 153 ---------------
 154
 155 All parameters are given as lists of unsigned 16-bit integers. Lists are
 156 terminated by I2C_CLIENT_END.
 157 The following lists are used internally:
 158
 159   normal_i2c: filled in by the module writer.
 160      A list of I2C addresses which should normally be examined.
 161    probe: insmod parameter.
 162      A list of pairs. The first value is a bus number (-1 for any I2C bus),
 163      the second is the address. These addresses are also probed, as if they
 164      were in the 'normal' list.
 165    ignore: insmod parameter.
 166      A list of pairs. The first value is a bus number (-1 for any I2C bus),
 167      the second is the I2C address. These addresses are never probed.
 168      This parameter overrules the 'normal_i2c' list only.
 169    force: 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. A device is blindly assumed to be on
 172      the given address, no probing is done.
 173
 174 Additionally, kind-specific force lists may optionally be defined if
 175 the driver supports several chip kinds. They are grouped in a
 176 NULL-terminated list of pointers named forces, those first element if the
 177 generic force list mentioned above. Each additional list correspond to an
 178 insmod parameter of the form force_<kind>.
 179
 180 Fortunately, as a module writer, you just have to define the `normal_i2c'
 181 parameter. The complete declaration could look like this:
 182
 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 };
 186
 187   /* Magic definition of all other variables and things */
 188   I2C_CLIENT_INSMOD;
 189   /* Or, if your driver supports, say, 2 kind of devices: */
 190   I2C_CLIENT_INSMOD_2(foo, bar);
 191
 192 If you use the multi-kind form, an enum will be defined for you:
 193   enum chips { any_chip, foo, bar, ... }
 194 You can then (and certainly should) use it in the driver code.
 195
 196 Note that you *have* to call the defined variable `normal_i2c',
 197 without any prefix!
 198
 199
 200 Attaching to an adapter
 201 -----------------------
 202
 203 Whenever a new adapter is inserted, or for all adapters if the driver is
 204 being registered, the callback attach_adapter() is called. Now is the
 205 time to determine what devices are present on the adapter, and to register
 206 a client for each of them.
 207
 208 The attach_adapter callback is really easy: we just call the generic
 209 detection function. This function will scan the bus for us, using the
 210 information as defined in the lists explained above. If a device is
 211 detected at a specific address, another callback is called.
 212
 213   int foo_attach_adapter(struct i2c_adapter *adapter)
 214   {
 215     return i2c_probe(adapter,&addr_data,&foo_detect_client);
 216   }
 217
 218 Remember, structure `addr_data' is defined by the macros explained above,
 219 so you do not have to define it yourself.
 220
 221 The i2c_probe function will call the foo_detect_client
 222 function only for those i2c addresses that actually have a device on
 223 them (unless a `force' parameter was used). In addition, addresses that
 224 are already in use (by some other registered client) are skipped.
 225
 226
 227 The detect client function
 228 --------------------------
 229
 230 The detect client function is called by i2c_probe. The `kind' parameter
 231 contains -1 for a probed detection, 0 for a forced detection, or a positive
 232 number for a forced detection with a chip type forced.
 233
 234 Below, some things are only needed if this is a `sensors' driver. Those
 235 parts are between /* SENSORS ONLY START */ and /* SENSORS ONLY END */
 236 markers.
 237
 238 Returning an error different from -ENODEV in a detect function will cause
 239 the detection to stop: other addresses and adapters won't be scanned.
 240 This should only be done on fatal or internal errors, such as a memory
 241 shortage or i2c_attach_client failing.
 242
 243 For now, you can ignore the `flags' parameter. It is there for future use.
 244
 245   int foo_detect_client(struct i2c_adapter *adapter, int address,
 246                         unsigned short flags, int kind)
 247   {
 248     int err = 0;
 249     int i;
 250     struct i2c_client *new_client;
 251     struct foo_data *data;
 252     const char *client_name = ""; /* For non-`sensors' drivers, put the real
 253                                      name here! */
 254
 255     /* Let's see whether this adapter can support what we need.
 256        Please substitute the things you need here!
 257        For `sensors' drivers, add `! is_isa &&' to the if statement */
 258     if (!i2c_check_functionality(adapter,I2C_FUNC_SMBUS_WORD_DATA |
 259                                         I2C_FUNC_SMBUS_WRITE_BYTE))
 260        goto ERROR0;
 261
 262     /* SENSORS ONLY START */
 263     const char *type_name = "";
 264     int is_isa = i2c_is_isa_adapter(adapter);
 265
 266     /* Do this only if the chip can additionally be found on the ISA bus
 267        (hybrid chip). */
 268
 269     if (is_isa) {
 270
 271       /* Discard immediately if this ISA range is already used */
 272       /* FIXME: never use check_region(), only request_region() */
 273       if (check_region(address,FOO_EXTENT))
 274         goto ERROR0;
 275
 276       /* Probe whether there is anything on this address.
 277          Some example code is below, but you will have to adapt this
 278          for your own driver */
 279
 280       if (kind < 0) /* Only if no force parameter was used */ {
 281         /* We may need long timeouts at least for some chips. */
 282         #define REALLY_SLOW_IO
 283         i = inb_p(address + 1);
 284         if (inb_p(address + 2) != i)
 285           goto ERROR0;
 286         if (inb_p(address + 3) != i)
 287           goto ERROR0;
 288         if (inb_p(address + 7) != i)
 289           goto ERROR0;
 290         #undef REALLY_SLOW_IO
 291
 292         /* Let's just hope nothing breaks here */
 293         i = inb_p(address + 5) & 0x7f;
 294         outb_p(~i & 0x7f,address+5);
 295         if ((inb_p(address + 5) & 0x7f) != (~i & 0x7f)) {
 296           outb_p(i,address+5);
 297           return 0;
 298         }
 299       }
 300     }
 301
 302     /* SENSORS ONLY END */
 303
 304     /* OK. For now, we presume we have a valid client. We now create the
 305        client structure, even though we cannot fill it completely yet.
 306        But it allows us to access several i2c functions safely */
 307
 308     if (!(data = kzalloc(sizeof(struct foo_data), GFP_KERNEL))) {
 309       err = -ENOMEM;
 310       goto ERROR0;
 311     }
 312
 313     new_client = &data->client;
 314     i2c_set_clientdata(new_client, data);
 315
 316     new_client->addr = address;
 317     new_client->adapter = adapter;
 318     new_client->driver = &foo_driver;
 319     new_client->flags = 0;
 320
 321     /* Now, we do the remaining detection. If no `force' parameter is used. */
 322
 323     /* First, the generic detection (if any), that is skipped if any force
 324        parameter was used. */
 325     if (kind < 0) {
 326       /* The below is of course bogus */
 327       if (foo_read(new_client,FOO_REG_GENERIC) != FOO_GENERIC_VALUE)
 328          goto ERROR1;
 329     }
 330
 331     /* SENSORS ONLY START */
 332
 333     /* Next, specific detection. This is especially important for `sensors'
 334        devices. */
 335
 336     /* Determine the chip type. Not needed if a `force_CHIPTYPE' parameter
 337        was used. */
 338     if (kind <= 0) {
 339       i = foo_read(new_client,FOO_REG_CHIPTYPE);
 340       if (i == FOO_TYPE_1)
 341         kind = chip1; /* As defined in the enum */
 342       else if (i == FOO_TYPE_2)
 343         kind = chip2;
 344       else {
 345         printk("foo: Ignoring 'force' parameter for unknown chip at "
 346                "adapter %d, address 0x%02x\n",i2c_adapter_id(adapter),address);
 347         goto ERROR1;
 348       }
 349     }
 350
 351     /* Now set the type and chip names */
 352     if (kind == chip1) {
 353       type_name = "chip1"; /* For /proc entry */
 354       client_name = "CHIP 1";
 355     } else if (kind == chip2) {
 356       type_name = "chip2"; /* For /proc entry */
 357       client_name = "CHIP 2";
 358     }
 359
 360     /* Reserve the ISA region */
 361     if (is_isa)
 362       request_region(address,FOO_EXTENT,type_name);
 363
 364     /* SENSORS ONLY END */
 365
 366     /* Fill in the remaining client fields. */
 367     strcpy(new_client->name,client_name);
 368
 369     /* SENSORS ONLY BEGIN */
 370     data->type = kind;
 371     /* SENSORS ONLY END */
 372
 373     data->valid = 0; /* Only if you use this field */
 374     init_MUTEX(&data->update_lock); /* Only if you use this field */
 375
 376     /* Any other initializations in data must be done here too. */
 377
 378     /* Tell the i2c layer a new client has arrived */
 379     if ((err = i2c_attach_client(new_client)))
 380       goto ERROR3;
 381
 382     /* SENSORS ONLY BEGIN */
 383     /* Register a new directory entry with module sensors. See below for
 384        the `template' structure. */
 385     if ((i = i2c_register_entry(new_client, type_name,
 386                                     foo_dir_table_template,THIS_MODULE)) < 0) {
 387       err = i;
 388       goto ERROR4;
 389     }
 390     data->sysctl_id = i;
 391
 392     /* SENSORS ONLY END */
 393
 394     /* This function can write default values to the client registers, if
 395        needed. */
 396     foo_init_client(new_client);
 397     return 0;
 398
 399     /* OK, this is not exactly good programming practice, usually. But it is
 400        very code-efficient in this case. */
 401
 402     ERROR4:
 403       i2c_detach_client(new_client);
 404     ERROR3:
 405     ERROR2:
 406     /* SENSORS ONLY START */
 407       if (is_isa)
 408         release_region(address,FOO_EXTENT);
 409     /* SENSORS ONLY END */
 410     ERROR1:
 411       kfree(data);
 412     ERROR0:
 413       return err;
 414   }
 415
 416
 417 Removing the client
 418 ===================
 419
 420 The detach_client call back function is called when a client should be
 421 removed. It may actually fail, but only when panicking. This code is
 422 much simpler than the attachment code, fortunately!
 423
 424   int foo_detach_client(struct i2c_client *client)
 425   {
 426     int err,i;
 427
 428     /* SENSORS ONLY START */
 429     /* Deregister with the `i2c-proc' module. */
 430     i2c_deregister_entry(((struct lm78_data *)(client->data))->sysctl_id);
 431     /* SENSORS ONLY END */
 432
 433     /* Try to detach the client from i2c space */
 434     if ((err = i2c_detach_client(client)))
 435       return err;
 436
 437     /* HYBRID SENSORS CHIP ONLY START */
 438     if i2c_is_isa_client(client)
 439       release_region(client->addr,LM78_EXTENT);
 440     /* HYBRID SENSORS CHIP ONLY END */
 441
 442     kfree(i2c_get_clientdata(client));
 443     return 0;
 444   }
 445
 446
 447 Initializing the module or kernel
 448 =================================
 449
 450 When the kernel is booted, or when your foo driver module is inserted,
 451 you have to do some initializing. Fortunately, just attaching (registering)
 452 the driver module is usually enough.
 453
 454   /* Keep track of how far we got in the initialization process. If several
 455      things have to initialized, and we fail halfway, only those things
 456      have to be cleaned up! */
 457   static int __initdata foo_initialized = 0;
 458
 459   static int __init foo_init(void)
 460   {
 461     int res;
 462     printk("foo version %s (%s)\n",FOO_VERSION,FOO_DATE);
 463
 464     if ((res = i2c_add_driver(&foo_driver))) {
 465       printk("foo: Driver registration failed, module not inserted.\n");
 466       foo_cleanup();
 467       return res;
 468     }
 469     foo_initialized ++;
 470     return 0;
 471   }
 472
 473   void foo_cleanup(void)
 474   {
 475     if (foo_initialized == 1) {
 476       if ((res = i2c_del_driver(&foo_driver))) {
 477         printk("foo: Driver registration failed, module not removed.\n");
 478         return;
 479       }
 480       foo_initialized --;
 481     }
 482   }
 483
 484   /* Substitute your own name and email address */
 485   MODULE_AUTHOR("Frodo Looijaard <frodol@dds.nl>"
 486   MODULE_DESCRIPTION("Driver for Barf Inc. Foo I2C devices");
 487
 488   module_init(foo_init);
 489   module_exit(foo_cleanup);
 490
 491 Note that some functions are marked by `__init', and some data structures
 492 by `__init_data'.  Hose functions and structures can be removed after
 493 kernel booting (or module loading) is completed.
 494
 495
 496 Command function
 497 ================
 498
 499 A generic ioctl-like function call back is supported. You will seldom
 500 need this, and its use is deprecated anyway, so newer design should not
 501 use it. Set it to NULL.
 502
 503
 504 Sending and receiving
 505 =====================
 506
 507 If you want to communicate with your device, there are several functions
 508 to do this. You can find all of them in i2c.h.
 509
 510 If you can choose between plain i2c communication and SMBus level
 511 communication, please use the last. All adapters understand SMBus level
 512 commands, but only some of them understand plain i2c!
 513
 514
 515 Plain i2c communication
 516 -----------------------
 517
 518   extern int i2c_master_send(struct i2c_client *,const char* ,int);
 519   extern int i2c_master_recv(struct i2c_client *,char* ,int);
 520
 521 These routines read and write some bytes from/to a client. The client
 522 contains the i2c address, so you do not have to include it. The second
 523 parameter contains the bytes the read/write, the third the length of the
 524 buffer. Returned is the actual number of bytes read/written.
 525
 526   extern int i2c_transfer(struct i2c_adapter *adap, struct i2c_msg *msg,
 527                           int num);
 528
 529 This sends a series of messages. Each message can be a read or write,
 530 and they can be mixed in any way. The transactions are combined: no
 531 stop bit is sent between transaction. The i2c_msg structure contains
 532 for each message the client address, the number of bytes of the message
 533 and the message data itself.
 534
 535 You can read the file `i2c-protocol' for more information about the
 536 actual i2c protocol.
 537
 538
 539 SMBus communication
 540 -------------------
 541
 542   extern s32 i2c_smbus_xfer (struct i2c_adapter * adapter, u16 addr,
 543                              unsigned short flags,
 544                              char read_write, u8 command, int size,
 545                              union i2c_smbus_data * data);
 546
 547   This is the generic SMBus function. All functions below are implemented
 548   in terms of it. Never use this function directly!
 549
 550
 551   extern s32 i2c_smbus_write_quick(struct i2c_client * client, u8 value);
 552   extern s32 i2c_smbus_read_byte(struct i2c_client * client);
 553   extern s32 i2c_smbus_write_byte(struct i2c_client * client, u8 value);
 554   extern s32 i2c_smbus_read_byte_data(struct i2c_client * client, u8 command);
 555   extern s32 i2c_smbus_write_byte_data(struct i2c_client * client,
 556                                        u8 command, u8 value);
 557   extern s32 i2c_smbus_read_word_data(struct i2c_client * client, u8 command);
 558   extern s32 i2c_smbus_write_word_data(struct i2c_client * client,
 559                                        u8 command, u16 value);
 560   extern s32 i2c_smbus_write_block_data(struct i2c_client * client,
 561                                         u8 command, u8 length,
 562                                         u8 *values);
 563   extern s32 i2c_smbus_read_i2c_block_data(struct i2c_client * client,
 564                                            u8 command, u8 *values);
 565
 566 These ones were removed in Linux 2.6.10 because they had no users, but could
 567 be added back later if needed:
 568
 569   extern s32 i2c_smbus_read_block_data(struct i2c_client * client,
 570                                        u8 command, u8 *values);
 571   extern s32 i2c_smbus_write_i2c_block_data(struct i2c_client * client,
 572                                             u8 command, u8 length,
 573                                             u8 *values);
 574   extern s32 i2c_smbus_process_call(struct i2c_client * client,
 575                                     u8 command, u16 value);
 576   extern s32 i2c_smbus_block_process_call(struct i2c_client *client,
 577                                           u8 command, u8 length,
 578                                           u8 *values)
 579
 580 All these transactions return -1 on failure. The 'write' transactions
 581 return 0 on success; the 'read' transactions return the read value, except
 582 for read_block, which returns the number of values read. The block buffers
 583 need not be longer than 32 bytes.
 584
 585 You can read the file `smbus-protocol' for more information about the
 586 actual SMBus protocol.
 587
 588
 589 General purpose routines
 590 ========================
 591
 592 Below all general purpose routines are listed, that were not mentioned
 593 before.
 594
 595   /* This call returns a unique low identifier for each registered adapter,
 596    * or -1 if the adapter was not registered.
 597    */
 598   extern int i2c_adapter_id(struct i2c_adapter *adap);
 599
 600
 601 The sensors sysctl/proc interface
 602 =================================
 603
 604 This section only applies if you write `sensors' drivers.
 605
 606 Each sensors driver creates a directory in /proc/sys/dev/sensors for each
 607 registered client. The directory is called something like foo-i2c-4-65.
 608 The sensors module helps you to do this as easily as possible.
 609
 610 The template
 611 ------------
 612
 613 You will need to define a ctl_table template. This template will automatically
 614 be copied to a newly allocated structure and filled in where necessary when
 615 you call sensors_register_entry.
 616
 617 First, I will give an example definition.
 618   static ctl_table foo_dir_table_template[] = {
 619     { FOO_SYSCTL_FUNC1, "func1", NULL, 0, 0644, NULL, &i2c_proc_real,
 620       &i2c_sysctl_real,NULL,&foo_func },
 621     { FOO_SYSCTL_FUNC2, "func2", NULL, 0, 0644, NULL, &i2c_proc_real,
 622       &i2c_sysctl_real,NULL,&foo_func },
 623     { FOO_SYSCTL_DATA, "data", NULL, 0, 0644, NULL, &i2c_proc_real,
 624       &i2c_sysctl_real,NULL,&foo_data },
 625     { 0 }
 626   };
 627
 628 In the above example, three entries are defined. They can either be
 629 accessed through the /proc interface, in the /proc/sys/dev/sensors/*
 630 directories, as files named func1, func2 and data, or alternatively
 631 through the sysctl interface, in the appropriate table, with identifiers
 632 FOO_SYSCTL_FUNC1, FOO_SYSCTL_FUNC2 and FOO_SYSCTL_DATA.
 633
 634 The third, sixth and ninth parameters should always be NULL, and the
 635 fourth should always be 0. The fifth is the mode of the /proc file;
 636 0644 is safe, as the file will be owned by root:root.
 637
 638 The seventh and eighth parameters should be &i2c_proc_real and
 639 &i2c_sysctl_real if you want to export lists of reals (scaled
 640 integers). You can also use your own function for them, as usual.
 641 Finally, the last parameter is the call-back to gather the data
 642 (see below) if you use the *_proc_real functions.
 643
 644
 645 Gathering the data
 646 ------------------
 647
 648 The call back functions (foo_func and foo_data in the above example)
 649 can be called in several ways; the operation parameter determines
 650 what should be done:
 651
 652   * If operation == SENSORS_PROC_REAL_INFO, you must return the
 653     magnitude (scaling) in nrels_mag;
 654   * If operation == SENSORS_PROC_REAL_READ, you must read information
 655     from the chip and return it in results. The number of integers
 656     to display should be put in nrels_mag;
 657   * If operation == SENSORS_PROC_REAL_WRITE, you must write the
 658     supplied information to the chip. nrels_mag will contain the number
 659     of integers, results the integers themselves.
 660
 661 The *_proc_real functions will display the elements as reals for the
 662 /proc interface. If you set the magnitude to 2, and supply 345 for
 663 SENSORS_PROC_REAL_READ, it would display 3.45; and if the user would
 664 write 45.6 to the /proc file, it would be returned as 4560 for
 665 SENSORS_PROC_REAL_WRITE. A magnitude may even be negative!
 666
 667 An example function:
 668
 669   /* FOO_FROM_REG and FOO_TO_REG translate between scaled values and
 670      register values. Note the use of the read cache. */
 671   void foo_in(struct i2c_client *client, int operation, int ctl_name,
 672               int *nrels_mag, long *results)
 673   {
 674     struct foo_data *data = client->data;
 675     int nr = ctl_name - FOO_SYSCTL_FUNC1; /* reduce to 0 upwards */
 676
 677     if (operation == SENSORS_PROC_REAL_INFO)
 678       *nrels_mag = 2;
 679     else if (operation == SENSORS_PROC_REAL_READ) {
 680       /* Update the readings cache (if necessary) */
 681       foo_update_client(client);
 682       /* Get the readings from the cache */
 683       results[0] = FOO_FROM_REG(data->foo_func_base[nr]);
 684       results[1] = FOO_FROM_REG(data->foo_func_more[nr]);
 685       results[2] = FOO_FROM_REG(data->foo_func_readonly[nr]);
 686       *nrels_mag = 2;
 687     } else if (operation == SENSORS_PROC_REAL_WRITE) {
 688       if (*nrels_mag >= 1) {
 689         /* Update the cache */
 690         data->foo_base[nr] = FOO_TO_REG(results[0]);
 691         /* Update the chip */
 692         foo_write_value(client,FOO_REG_FUNC_BASE(nr),data->foo_base[nr]);
 693       }
 694       if (*nrels_mag >= 2) {
 695         /* Update the cache */
 696         data->foo_more[nr] = FOO_TO_REG(results[1]);
 697         /* Update the chip */
 698         foo_write_value(client,FOO_REG_FUNC_MORE(nr),data->foo_more[nr]);
 699       }
 700     }
 701   }