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         .owner          = THIS_MODULE,
  29         .name           = "Foo version 2.3 driver",
  30         .attach_adapter = &foo_attach_adapter,
  31         .detach_client  = &foo_detach_client,
  32         .command        = &foo_command /* may be NULL */
  33 }
  34
  35 The name field must match the driver name, including the case. It must not
  36 contain spaces, and may be up to 31 characters long.
  37
  38 All other fields are for call-back functions which will be explained
  39 below.
  40
  41
  42 Extra client data
  43 =================
  44
  45 The client structure has a special `data' field that can point to any
  46 structure at all. You can use this to keep client-specific data. You
  47 do not always need this, but especially for `sensors' drivers, it can
  48 be very useful.
  49
  50 An example structure is below.
  51
  52   struct foo_data {
  53     struct i2c_client client;
  54     struct semaphore lock; /* For ISA access in `sensors' drivers. */
  55     int sysctl_id;         /* To keep the /proc directory entry for
  56                               `sensors' drivers. */
  57     enum chips type;       /* To keep the chips type for `sensors' drivers. */
  58
  59     /* Because the i2c bus is slow, it is often useful to cache the read
  60        information of a chip for some time (for example, 1 or 2 seconds).
  61        It depends of course on the device whether this is really worthwhile
  62        or even sensible. */
  63     struct semaphore update_lock; /* When we are reading lots of information,
  64                                      another process should not update the
  65                                      below information */
  66     char valid;                   /* != 0 if the following fields are valid. */
  67     unsigned long last_updated;   /* In jiffies */
  68     /* Add the read information here too */
  69   };
  70
  71
  72 Accessing the client
  73 ====================
  74
  75 Let's say we have a valid client structure. At some time, we will need
  76 to gather information from the client, or write new information to the
  77 client. How we will export this information to user-space is less
  78 important at this moment (perhaps we do not need to do this at all for
  79 some obscure clients). But we need generic reading and writing routines.
  80
  81 I have found it useful to define foo_read and foo_write function for this.
  82 For some cases, it will be easier to call the i2c functions directly,
  83 but many chips have some kind of register-value idea that can easily
  84 be encapsulated. Also, some chips have both ISA and I2C interfaces, and
  85 it useful to abstract from this (only for `sensors' drivers).
  86
  87 The below functions are simple examples, and should not be copied
  88 literally.
  89
  90   int foo_read_value(struct i2c_client *client, u8 reg)
  91   {
  92     if (reg < 0x10) /* byte-sized register */
  93       return i2c_smbus_read_byte_data(client,reg);
  94     else /* word-sized register */
  95       return i2c_smbus_read_word_data(client,reg);
  96   }
  97
  98   int foo_write_value(struct i2c_client *client, u8 reg, u16 value)
  99   {
 100     if (reg == 0x10) /* Impossible to write - driver error! */ {
 101       return -1;
 102     else if (reg < 0x10) /* byte-sized register */
 103       return i2c_smbus_write_byte_data(client,reg,value);
 104     else /* word-sized register */
 105       return i2c_smbus_write_word_data(client,reg,value);
 106   }
 107
 108 For sensors code, you may have to cope with ISA registers too. Something
 109 like the below often works. Note the locking!
 110
 111   int foo_read_value(struct i2c_client *client, u8 reg)
 112   {
 113     int res;
 114     if (i2c_is_isa_client(client)) {
 115       down(&(((struct foo_data *) (client->data)) -> lock));
 116       outb_p(reg,client->addr + FOO_ADDR_REG_OFFSET);
 117       res = inb_p(client->addr + FOO_DATA_REG_OFFSET);
 118       up(&(((struct foo_data *) (client->data)) -> lock));
 119       return res;
 120     } else
 121       return i2c_smbus_read_byte_data(client,reg);
 122   }
 123
 124 Writing is done the same way.
 125
 126
 127 Probing and attaching
 128 =====================
 129
 130 Most i2c devices can be present on several i2c addresses; for some this
 131 is determined in hardware (by soldering some chip pins to Vcc or Ground),
 132 for others this can be changed in software (by writing to specific client
 133 registers). Some devices are usually on a specific address, but not always;
 134 and some are even more tricky. So you will probably need to scan several
 135 i2c addresses for your clients, and do some sort of detection to see
 136 whether it is actually a device supported by your driver.
 137
 138 To give the user a maximum of possibilities, some default module parameters
 139 are defined to help determine what addresses are scanned. Several macros
 140 are defined in i2c.h to help you support them, as well as a generic
 141 detection algorithm.
 142
 143 You do not have to use this parameter interface; but don't try to use
 144 function i2c_probe() if you don't.
 145
 146 NOTE: If you want to write a `sensors' driver, the interface is slightly
 147       different! See below.
 148
 149
 150
 151 Probing classes
 152 ---------------
 153
 154 All parameters are given as lists of unsigned 16-bit integers. Lists are
 155 terminated by I2C_CLIENT_END.
 156 The following lists are used internally:
 157
 158   normal_i2c: filled in by the module writer.
 159      A list of I2C addresses which should normally be examined.
 160    probe: insmod parameter.
 161      A list of pairs. The first value is a bus number (-1 for any I2C bus),
 162      the second is the address. These addresses are also probed, as if they
 163      were in the 'normal' list.
 164    ignore: insmod parameter.
 165      A list of pairs. The first value is a bus number (-1 for any I2C bus),
 166      the second is the I2C address. These addresses are never probed.
 167      This parameter overrules the 'normal_i2c' list only.
 168    force: insmod parameter.
 169      A list of pairs. The first value is a bus number (-1 for any I2C bus),
 170      the second is the I2C address. A device is blindly assumed to be on
 171      the given address, no probing is done.
 172
 173 Additionally, kind-specific force lists may optionally be defined if
 174 the driver supports several chip kinds. They are grouped in a
 175 NULL-terminated list of pointers named forces, those first element if the
 176 generic force list mentioned above. Each additional list correspond to an
 177 insmod parameter of the form force_<kind>.
 178
 179 Fortunately, as a module writer, you just have to define the `normal_i2c'
 180 parameter. The complete declaration could look like this:
 181
 182   /* Scan 0x37, and 0x48 to 0x4f */
 183   static unsigned short normal_i2c[] = { 0x37, 0x48, 0x49, 0x4a, 0x4b, 0x4c,
 184                                          0x4d, 0x4e, 0x4f, I2C_CLIENT_END };
 185
 186   /* Magic definition of all other variables and things */
 187   I2C_CLIENT_INSMOD;
 188   /* Or, if your driver supports, say, 2 kind of devices: */
 189   I2C_CLIENT_INSMOD_2(foo, bar);
 190
 191 If you use the multi-kind form, an enum will be defined for you:
 192   enum chips { any_chip, foo, bar, ... }
 193 You can then (and certainly should) use it in the driver code.
 194
 195 Note that you *have* to call the defined variable `normal_i2c',
 196 without any prefix!
 197
 198
 199 Attaching to an adapter
 200 -----------------------
 201
 202 Whenever a new adapter is inserted, or for all adapters if the driver is
 203 being registered, the callback attach_adapter() is called. Now is the
 204 time to determine what devices are present on the adapter, and to register
 205 a client for each of them.
 206
 207 The attach_adapter callback is really easy: we just call the generic
 208 detection function. This function will scan the bus for us, using the
 209 information as defined in the lists explained above. If a device is
 210 detected at a specific address, another callback is called.
 211
 212   int foo_attach_adapter(struct i2c_adapter *adapter)
 213   {
 214     return i2c_probe(adapter,&addr_data,&foo_detect_client);
 215   }
 216
 217 Remember, structure `addr_data' is defined by the macros explained above,
 218 so you do not have to define it yourself.
 219
 220 The i2c_probe function will call the foo_detect_client
 221 function only for those i2c addresses that actually have a device on
 222 them (unless a `force' parameter was used). In addition, addresses that
 223 are already in use (by some other registered client) are skipped.
 224
 225
 226 The detect client function
 227 --------------------------
 228
 229 The detect client function is called by i2c_probe. The `kind' parameter
 230 contains -1 for a probed detection, 0 for a forced detection, or a positive
 231 number for a forced detection with a chip type forced.
 232
 233 Below, some things are only needed if this is a `sensors' driver. Those
 234 parts are between /* SENSORS ONLY START */ and /* SENSORS ONLY END */
 235 markers.
 236
 237 Returning an error different from -ENODEV in a detect function will cause
 238 the detection to stop: other addresses and adapters won't be scanned.
 239 This should only be done on fatal or internal errors, such as a memory
 240 shortage or i2c_attach_client failing.
 241
 242 For now, you can ignore the `flags' parameter. It is there for future use.
 243
 244   int foo_detect_client(struct i2c_adapter *adapter, int address,
 245                         unsigned short flags, int kind)
 246   {
 247     int err = 0;
 248     int i;
 249     struct i2c_client *new_client;
 250     struct foo_data *data;
 251     const char *client_name = ""; /* For non-`sensors' drivers, put the real
 252                                      name here! */
 253
 254     /* Let's see whether this adapter can support what we need.
 255        Please substitute the things you need here!
 256        For `sensors' drivers, add `! is_isa &&' to the if statement */
 257     if (!i2c_check_functionality(adapter,I2C_FUNC_SMBUS_WORD_DATA |
 258                                         I2C_FUNC_SMBUS_WRITE_BYTE))
 259        goto ERROR0;
 260
 261     /* SENSORS ONLY START */
 262     const char *type_name = "";
 263     int is_isa = i2c_is_isa_adapter(adapter);
 264
 265     /* Do this only if the chip can additionally be found on the ISA bus
 266        (hybrid chip). */
 267
 268     if (is_isa) {
 269
 270       /* Discard immediately if this ISA range is already used */
 271       /* FIXME: never use check_region(), only request_region() */
 272       if (check_region(address,FOO_EXTENT))
 273         goto ERROR0;
 274
 275       /* Probe whether there is anything on this address.
 276          Some example code is below, but you will have to adapt this
 277          for your own driver */
 278
 279       if (kind < 0) /* Only if no force parameter was used */ {
 280         /* We may need long timeouts at least for some chips. */
 281         #define REALLY_SLOW_IO
 282         i = inb_p(address + 1);
 283         if (inb_p(address + 2) != i)
 284           goto ERROR0;
 285         if (inb_p(address + 3) != i)
 286           goto ERROR0;
 287         if (inb_p(address + 7) != i)
 288           goto ERROR0;
 289         #undef REALLY_SLOW_IO
 290
 291         /* Let's just hope nothing breaks here */
 292         i = inb_p(address + 5) & 0x7f;
 293         outb_p(~i & 0x7f,address+5);
 294         if ((inb_p(address + 5) & 0x7f) != (~i & 0x7f)) {
 295           outb_p(i,address+5);
 296           return 0;
 297         }
 298       }
 299     }
 300
 301     /* SENSORS ONLY END */
 302
 303     /* OK. For now, we presume we have a valid client. We now create the
 304        client structure, even though we cannot fill it completely yet.
 305        But it allows us to access several i2c functions safely */
 306
 307     if (!(data = kzalloc(sizeof(struct foo_data), GFP_KERNEL))) {
 308       err = -ENOMEM;
 309       goto ERROR0;
 310     }
 311
 312     new_client = &data->client;
 313     i2c_set_clientdata(new_client, data);
 314
 315     new_client->addr = address;
 316     new_client->adapter = adapter;
 317     new_client->driver = &foo_driver;
 318     new_client->flags = 0;
 319
 320     /* Now, we do the remaining detection. If no `force' parameter is used. */
 321
 322     /* First, the generic detection (if any), that is skipped if any force
 323        parameter was used. */
 324     if (kind < 0) {
 325       /* The below is of course bogus */
 326       if (foo_read(new_client,FOO_REG_GENERIC) != FOO_GENERIC_VALUE)
 327          goto ERROR1;
 328     }
 329
 330     /* SENSORS ONLY START */
 331
 332     /* Next, specific detection. This is especially important for `sensors'
 333        devices. */
 334
 335     /* Determine the chip type. Not needed if a `force_CHIPTYPE' parameter
 336        was used. */
 337     if (kind <= 0) {
 338       i = foo_read(new_client,FOO_REG_CHIPTYPE);
 339       if (i == FOO_TYPE_1)
 340         kind = chip1; /* As defined in the enum */
 341       else if (i == FOO_TYPE_2)
 342         kind = chip2;
 343       else {
 344         printk("foo: Ignoring 'force' parameter for unknown chip at "
 345                "adapter %d, address 0x%02x\n",i2c_adapter_id(adapter),address);
 346         goto ERROR1;
 347       }
 348     }
 349
 350     /* Now set the type and chip names */
 351     if (kind == chip1) {
 352       type_name = "chip1"; /* For /proc entry */
 353       client_name = "CHIP 1";
 354     } else if (kind == chip2) {
 355       type_name = "chip2"; /* For /proc entry */
 356       client_name = "CHIP 2";
 357     }
 358
 359     /* Reserve the ISA region */
 360     if (is_isa)
 361       request_region(address,FOO_EXTENT,type_name);
 362
 363     /* SENSORS ONLY END */
 364
 365     /* Fill in the remaining client fields. */
 366     strcpy(new_client->name,client_name);
 367
 368     /* SENSORS ONLY BEGIN */
 369     data->type = kind;
 370     /* SENSORS ONLY END */
 371
 372     data->valid = 0; /* Only if you use this field */
 373     init_MUTEX(&data->update_lock); /* Only if you use this field */
 374
 375     /* Any other initializations in data must be done here too. */
 376
 377     /* Tell the i2c layer a new client has arrived */
 378     if ((err = i2c_attach_client(new_client)))
 379       goto ERROR3;
 380
 381     /* SENSORS ONLY BEGIN */
 382     /* Register a new directory entry with module sensors. See below for
 383        the `template' structure. */
 384     if ((i = i2c_register_entry(new_client, type_name,
 385                                     foo_dir_table_template,THIS_MODULE)) < 0) {
 386       err = i;
 387       goto ERROR4;
 388     }
 389     data->sysctl_id = i;
 390
 391     /* SENSORS ONLY END */
 392
 393     /* This function can write default values to the client registers, if
 394        needed. */
 395     foo_init_client(new_client);
 396     return 0;
 397
 398     /* OK, this is not exactly good programming practice, usually. But it is
 399        very code-efficient in this case. */
 400
 401     ERROR4:
 402       i2c_detach_client(new_client);
 403     ERROR3:
 404     ERROR2:
 405     /* SENSORS ONLY START */
 406       if (is_isa)
 407         release_region(address,FOO_EXTENT);
 408     /* SENSORS ONLY END */
 409     ERROR1:
 410       kfree(data);
 411     ERROR0:
 412       return err;
 413   }
 414
 415
 416 Removing the client
 417 ===================
 418
 419 The detach_client call back function is called when a client should be
 420 removed. It may actually fail, but only when panicking. This code is
 421 much simpler than the attachment code, fortunately!
 422
 423   int foo_detach_client(struct i2c_client *client)
 424   {
 425     int err,i;
 426
 427     /* SENSORS ONLY START */
 428     /* Deregister with the `i2c-proc' module. */
 429     i2c_deregister_entry(((struct lm78_data *)(client->data))->sysctl_id);
 430     /* SENSORS ONLY END */
 431
 432     /* Try to detach the client from i2c space */
 433     if ((err = i2c_detach_client(client)))
 434       return err;
 435
 436     /* HYBRID SENSORS CHIP ONLY START */
 437     if i2c_is_isa_client(client)
 438       release_region(client->addr,LM78_EXTENT);
 439     /* HYBRID SENSORS CHIP ONLY END */
 440
 441     kfree(i2c_get_clientdata(client));
 442     return 0;
 443   }
 444
 445
 446 Initializing the module or kernel
 447 =================================
 448
 449 When the kernel is booted, or when your foo driver module is inserted,
 450 you have to do some initializing. Fortunately, just attaching (registering)
 451 the driver module is usually enough.
 452
 453   /* Keep track of how far we got in the initialization process. If several
 454      things have to initialized, and we fail halfway, only those things
 455      have to be cleaned up! */
 456   static int __initdata foo_initialized = 0;
 457
 458   static int __init foo_init(void)
 459   {
 460     int res;
 461     printk("foo version %s (%s)\n",FOO_VERSION,FOO_DATE);
 462
 463     if ((res = i2c_add_driver(&foo_driver))) {
 464       printk("foo: Driver registration failed, module not inserted.\n");
 465       foo_cleanup();
 466       return res;
 467     }
 468     foo_initialized ++;
 469     return 0;
 470   }
 471
 472   void foo_cleanup(void)
 473   {
 474     if (foo_initialized == 1) {
 475       if ((res = i2c_del_driver(&foo_driver))) {
 476         printk("foo: Driver registration failed, module not removed.\n");
 477         return;
 478       }
 479       foo_initialized --;
 480     }
 481   }
 482
 483   /* Substitute your own name and email address */
 484   MODULE_AUTHOR("Frodo Looijaard <frodol@dds.nl>"
 485   MODULE_DESCRIPTION("Driver for Barf Inc. Foo I2C devices");
 486
 487   module_init(foo_init);
 488   module_exit(foo_cleanup);
 489
 490 Note that some functions are marked by `__init', and some data structures
 491 by `__init_data'.  Hose functions and structures can be removed after
 492 kernel booting (or module loading) is completed.
 493
 494 Command function
 495 ================
 496
 497 A generic ioctl-like function call back is supported. You will seldom
 498 need this. You may even set it to NULL.
 499
 500   /* No commands defined */
 501   int foo_command(struct i2c_client *client, unsigned int cmd, void *arg)
 502   {
 503     return 0;
 504   }
 505
 506
 507 Sending and receiving
 508 =====================
 509
 510 If you want to communicate with your device, there are several functions
 511 to do this. You can find all of them in i2c.h.
 512
 513 If you can choose between plain i2c communication and SMBus level
 514 communication, please use the last. All adapters understand SMBus level
 515 commands, but only some of them understand plain i2c!
 516
 517
 518 Plain i2c communication
 519 -----------------------
 520
 521   extern int i2c_master_send(struct i2c_client *,const char* ,int);
 522   extern int i2c_master_recv(struct i2c_client *,char* ,int);
 523
 524 These routines read and write some bytes from/to a client. The client
 525 contains the i2c address, so you do not have to include it. The second
 526 parameter contains the bytes the read/write, the third the length of the
 527 buffer. Returned is the actual number of bytes read/written.
 528
 529   extern int i2c_transfer(struct i2c_adapter *adap, struct i2c_msg *msg,
 530                           int num);
 531
 532 This sends a series of messages. Each message can be a read or write,
 533 and they can be mixed in any way. The transactions are combined: no
 534 stop bit is sent between transaction. The i2c_msg structure contains
 535 for each message the client address, the number of bytes of the message
 536 and the message data itself.
 537
 538 You can read the file `i2c-protocol' for more information about the
 539 actual i2c protocol.
 540
 541
 542 SMBus communication
 543 -------------------
 544
 545   extern s32 i2c_smbus_xfer (struct i2c_adapter * adapter, u16 addr,
 546                              unsigned short flags,
 547                              char read_write, u8 command, int size,
 548                              union i2c_smbus_data * data);
 549
 550   This is the generic SMBus function. All functions below are implemented
 551   in terms of it. Never use this function directly!
 552
 553
 554   extern s32 i2c_smbus_write_quick(struct i2c_client * client, u8 value);
 555   extern s32 i2c_smbus_read_byte(struct i2c_client * client);
 556   extern s32 i2c_smbus_write_byte(struct i2c_client * client, u8 value);
 557   extern s32 i2c_smbus_read_byte_data(struct i2c_client * client, u8 command);
 558   extern s32 i2c_smbus_write_byte_data(struct i2c_client * client,
 559                                        u8 command, u8 value);
 560   extern s32 i2c_smbus_read_word_data(struct i2c_client * client, u8 command);
 561   extern s32 i2c_smbus_write_word_data(struct i2c_client * client,
 562                                        u8 command, u16 value);
 563   extern s32 i2c_smbus_write_block_data(struct i2c_client * client,
 564                                         u8 command, u8 length,
 565                                         u8 *values);
 566   extern s32 i2c_smbus_read_i2c_block_data(struct i2c_client * client,
 567                                            u8 command, u8 *values);
 568
 569 These ones were removed in Linux 2.6.10 because they had no users, but could
 570 be added back later if needed:
 571
 572   extern s32 i2c_smbus_read_block_data(struct i2c_client * client,
 573                                        u8 command, u8 *values);
 574   extern s32 i2c_smbus_write_i2c_block_data(struct i2c_client * client,
 575                                             u8 command, u8 length,
 576                                             u8 *values);
 577   extern s32 i2c_smbus_process_call(struct i2c_client * client,
 578                                     u8 command, u16 value);
 579   extern s32 i2c_smbus_block_process_call(struct i2c_client *client,
 580                                           u8 command, u8 length,
 581                                           u8 *values)
 582
 583 All these transactions return -1 on failure. The 'write' transactions
 584 return 0 on success; the 'read' transactions return the read value, except
 585 for read_block, which returns the number of values read. The block buffers
 586 need not be longer than 32 bytes.
 587
 588 You can read the file `smbus-protocol' for more information about the
 589 actual SMBus protocol.
 590
 591
 592 General purpose routines
 593 ========================
 594
 595 Below all general purpose routines are listed, that were not mentioned
 596 before.
 597
 598   /* This call returns a unique low identifier for each registered adapter,
 599    * or -1 if the adapter was not registered.
 600    */
 601   extern int i2c_adapter_id(struct i2c_adapter *adap);
 602
 603
 604 The sensors sysctl/proc interface
 605 =================================
 606
 607 This section only applies if you write `sensors' drivers.
 608
 609 Each sensors driver creates a directory in /proc/sys/dev/sensors for each
 610 registered client. The directory is called something like foo-i2c-4-65.
 611 The sensors module helps you to do this as easily as possible.
 612
 613 The template
 614 ------------
 615
 616 You will need to define a ctl_table template. This template will automatically
 617 be copied to a newly allocated structure and filled in where necessary when
 618 you call sensors_register_entry.
 619
 620 First, I will give an example definition.
 621   static ctl_table foo_dir_table_template[] = {
 622     { FOO_SYSCTL_FUNC1, "func1", NULL, 0, 0644, NULL, &i2c_proc_real,
 623       &i2c_sysctl_real,NULL,&foo_func },
 624     { FOO_SYSCTL_FUNC2, "func2", NULL, 0, 0644, NULL, &i2c_proc_real,
 625       &i2c_sysctl_real,NULL,&foo_func },
 626     { FOO_SYSCTL_DATA, "data", NULL, 0, 0644, NULL, &i2c_proc_real,
 627       &i2c_sysctl_real,NULL,&foo_data },
 628     { 0 }
 629   };
 630
 631 In the above example, three entries are defined. They can either be
 632 accessed through the /proc interface, in the /proc/sys/dev/sensors/*
 633 directories, as files named func1, func2 and data, or alternatively
 634 through the sysctl interface, in the appropriate table, with identifiers
 635 FOO_SYSCTL_FUNC1, FOO_SYSCTL_FUNC2 and FOO_SYSCTL_DATA.
 636
 637 The third, sixth and ninth parameters should always be NULL, and the
 638 fourth should always be 0. The fifth is the mode of the /proc file;
 639 0644 is safe, as the file will be owned by root:root.
 640
 641 The seventh and eighth parameters should be &i2c_proc_real and
 642 &i2c_sysctl_real if you want to export lists of reals (scaled
 643 integers). You can also use your own function for them, as usual.
 644 Finally, the last parameter is the call-back to gather the data
 645 (see below) if you use the *_proc_real functions.
 646
 647
 648 Gathering the data
 649 ------------------
 650
 651 The call back functions (foo_func and foo_data in the above example)
 652 can be called in several ways; the operation parameter determines
 653 what should be done:
 654
 655   * If operation == SENSORS_PROC_REAL_INFO, you must return the
 656     magnitude (scaling) in nrels_mag;
 657   * If operation == SENSORS_PROC_REAL_READ, you must read information
 658     from the chip and return it in results. The number of integers
 659     to display should be put in nrels_mag;
 660   * If operation == SENSORS_PROC_REAL_WRITE, you must write the
 661     supplied information to the chip. nrels_mag will contain the number
 662     of integers, results the integers themselves.
 663
 664 The *_proc_real functions will display the elements as reals for the
 665 /proc interface. If you set the magnitude to 2, and supply 345 for
 666 SENSORS_PROC_REAL_READ, it would display 3.45; and if the user would
 667 write 45.6 to the /proc file, it would be returned as 4560 for
 668 SENSORS_PROC_REAL_WRITE. A magnitude may even be negative!
 669
 670 An example function:
 671
 672   /* FOO_FROM_REG and FOO_TO_REG translate between scaled values and
 673      register values. Note the use of the read cache. */
 674   void foo_in(struct i2c_client *client, int operation, int ctl_name,
 675               int *nrels_mag, long *results)
 676   {
 677     struct foo_data *data = client->data;
 678     int nr = ctl_name - FOO_SYSCTL_FUNC1; /* reduce to 0 upwards */
 679
 680     if (operation == SENSORS_PROC_REAL_INFO)
 681       *nrels_mag = 2;
 682     else if (operation == SENSORS_PROC_REAL_READ) {
 683       /* Update the readings cache (if necessary) */
 684       foo_update_client(client);
 685       /* Get the readings from the cache */
 686       results[0] = FOO_FROM_REG(data->foo_func_base[nr]);
 687       results[1] = FOO_FROM_REG(data->foo_func_more[nr]);
 688       results[2] = FOO_FROM_REG(data->foo_func_readonly[nr]);
 689       *nrels_mag = 2;
 690     } else if (operation == SENSORS_PROC_REAL_WRITE) {
 691       if (*nrels_mag >= 1) {
 692         /* Update the cache */
 693         data->foo_base[nr] = FOO_TO_REG(results[0]);
 694         /* Update the chip */
 695         foo_write_value(client,FOO_REG_FUNC_BASE(nr),data->foo_base[nr]);
 696       }
 697       if (*nrels_mag >= 2) {
 698         /* Update the cache */
 699         data->foo_more[nr] = FOO_TO_REG(results[1]);
 700         /* Update the chip */
 701         foo_write_value(client,FOO_REG_FUNC_MORE(nr),data->foo_more[nr]);
 702       }
 703     }
 704   }