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, and should be zero-initialized except for fields with data you
  25 provide.  A client structure holds device-specific information like the
  26 driver model device node, and its I2C address.
  27
  28 static struct i2c_driver foo_driver = {
  29         .driver = {
  30                 .name   = "foo",
  31         },
  32         .attach_adapter = foo_attach_adapter,
  33         .detach_client  = foo_detach_client,
  34         .shutdown       = foo_shutdown, /* optional */
  35         .suspend        = foo_suspend,  /* optional */
  36         .resume         = foo_resume,   /* optional */
  37         .command        = foo_command,  /* optional */
  38 }
  39
  40 The name field is the driver name, and must not contain spaces.  It
  41 should match the module name (if the driver can be compiled as a module),
  42 although you can use MODULE_ALIAS (passing "foo" in this example) to add
  43 another name for the module.
  44
  45 All other fields are for call-back functions which will be explained
  46 below.
  47
  48
  49 Extra client data
  50 =================
  51
  52 Each client structure has a special `data' field that can point to any
  53 structure at all.  You should use this to keep device-specific data,
  54 especially in drivers that handle multiple I2C or SMBUS devices.  You
  55 do not always need this, but especially for `sensors' drivers, it can
  56 be very useful.
  57
  58         /* store the value */
  59         void i2c_set_clientdata(struct i2c_client *client, void *data);
  60
  61         /* retrieve the value */
  62         void *i2c_get_clientdata(struct i2c_client *client);
  63
  64 An example structure is below.
  65
  66   struct foo_data {
  67     struct i2c_client client;
  68     struct semaphore lock; /* For ISA access in `sensors' drivers. */
  69     int sysctl_id;         /* To keep the /proc directory entry for
  70                               `sensors' drivers. */
  71     enum chips type;       /* To keep the chips type for `sensors' drivers. */
  72
  73     /* Because the i2c bus is slow, it is often useful to cache the read
  74        information of a chip for some time (for example, 1 or 2 seconds).
  75        It depends of course on the device whether this is really worthwhile
  76        or even sensible. */
  77     struct semaphore update_lock; /* When we are reading lots of information,
  78                                      another process should not update the
  79                                      below information */
  80     char valid;                   /* != 0 if the following fields are valid. */
  81     unsigned long last_updated;   /* In jiffies */
  82     /* Add the read information here too */
  83   };
  84
  85
  86 Accessing the client
  87 ====================
  88
  89 Let's say we have a valid client structure. At some time, we will need
  90 to gather information from the client, or write new information to the
  91 client. How we will export this information to user-space is less
  92 important at this moment (perhaps we do not need to do this at all for
  93 some obscure clients). But we need generic reading and writing routines.
  94
  95 I have found it useful to define foo_read and foo_write function for this.
  96 For some cases, it will be easier to call the i2c functions directly,
  97 but many chips have some kind of register-value idea that can easily
  98 be encapsulated. Also, some chips have both ISA and I2C interfaces, and
  99 it useful to abstract from this (only for `sensors' drivers).
 100
 101 The below functions are simple examples, and should not be copied
 102 literally.
 103
 104   int foo_read_value(struct i2c_client *client, u8 reg)
 105   {
 106     if (reg < 0x10) /* byte-sized register */
 107       return i2c_smbus_read_byte_data(client,reg);
 108     else /* word-sized register */
 109       return i2c_smbus_read_word_data(client,reg);
 110   }
 111
 112   int foo_write_value(struct i2c_client *client, u8 reg, u16 value)
 113   {
 114     if (reg == 0x10) /* Impossible to write - driver error! */ {
 115       return -1;
 116     else if (reg < 0x10) /* byte-sized register */
 117       return i2c_smbus_write_byte_data(client,reg,value);
 118     else /* word-sized register */
 119       return i2c_smbus_write_word_data(client,reg,value);
 120   }
 121
 122 For sensors code, you may have to cope with ISA registers too. Something
 123 like the below often works. Note the locking!
 124
 125   int foo_read_value(struct i2c_client *client, u8 reg)
 126   {
 127     int res;
 128     if (i2c_is_isa_client(client)) {
 129       down(&(((struct foo_data *) (client->data)) -> lock));
 130       outb_p(reg,client->addr + FOO_ADDR_REG_OFFSET);
 131       res = inb_p(client->addr + FOO_DATA_REG_OFFSET);
 132       up(&(((struct foo_data *) (client->data)) -> lock));
 133       return res;
 134     } else
 135       return i2c_smbus_read_byte_data(client,reg);
 136   }
 137
 138 Writing is done the same way.
 139
 140
 141 Probing and attaching
 142 =====================
 143
 144 Most i2c devices can be present on several i2c addresses; for some this
 145 is determined in hardware (by soldering some chip pins to Vcc or Ground),
 146 for others this can be changed in software (by writing to specific client
 147 registers). Some devices are usually on a specific address, but not always;
 148 and some are even more tricky. So you will probably need to scan several
 149 i2c addresses for your clients, and do some sort of detection to see
 150 whether it is actually a device supported by your driver.
 151
 152 To give the user a maximum of possibilities, some default module parameters
 153 are defined to help determine what addresses are scanned. Several macros
 154 are defined in i2c.h to help you support them, as well as a generic
 155 detection algorithm.
 156
 157 You do not have to use this parameter interface; but don't try to use
 158 function i2c_probe() if you don't.
 159
 160 NOTE: If you want to write a `sensors' driver, the interface is slightly
 161       different! See below.
 162
 163
 164
 165 Probing classes
 166 ---------------
 167
 168 All parameters are given as lists of unsigned 16-bit integers. Lists are
 169 terminated by I2C_CLIENT_END.
 170 The following lists are used internally:
 171
 172   normal_i2c: filled in by the module writer.
 173      A list of I2C addresses which should normally be examined.
 174    probe: insmod parameter.
 175      A list of pairs. The first value is a bus number (-1 for any I2C bus),
 176      the second is the address. These addresses are also probed, as if they
 177      were in the 'normal' list.
 178    ignore: insmod parameter.
 179      A list of pairs. The first value is a bus number (-1 for any I2C bus),
 180      the second is the I2C address. These addresses are never probed.
 181      This parameter overrules the 'normal_i2c' list only.
 182    force: insmod parameter.
 183      A list of pairs. The first value is a bus number (-1 for any I2C bus),
 184      the second is the I2C address. A device is blindly assumed to be on
 185      the given address, no probing is done.
 186
 187 Additionally, kind-specific force lists may optionally be defined if
 188 the driver supports several chip kinds. They are grouped in a
 189 NULL-terminated list of pointers named forces, those first element if the
 190 generic force list mentioned above. Each additional list correspond to an
 191 insmod parameter of the form force_<kind>.
 192
 193 Fortunately, as a module writer, you just have to define the `normal_i2c'
 194 parameter. The complete declaration could look like this:
 195
 196   /* Scan 0x37, and 0x48 to 0x4f */
 197   static unsigned short normal_i2c[] = { 0x37, 0x48, 0x49, 0x4a, 0x4b, 0x4c,
 198                                          0x4d, 0x4e, 0x4f, I2C_CLIENT_END };
 199
 200   /* Magic definition of all other variables and things */
 201   I2C_CLIENT_INSMOD;
 202   /* Or, if your driver supports, say, 2 kind of devices: */
 203   I2C_CLIENT_INSMOD_2(foo, bar);
 204
 205 If you use the multi-kind form, an enum will be defined for you:
 206   enum chips { any_chip, foo, bar, ... }
 207 You can then (and certainly should) use it in the driver code.
 208
 209 Note that you *have* to call the defined variable `normal_i2c',
 210 without any prefix!
 211
 212
 213 Attaching to an adapter
 214 -----------------------
 215
 216 Whenever a new adapter is inserted, or for all adapters if the driver is
 217 being registered, the callback attach_adapter() is called. Now is the
 218 time to determine what devices are present on the adapter, and to register
 219 a client for each of them.
 220
 221 The attach_adapter callback is really easy: we just call the generic
 222 detection function. This function will scan the bus for us, using the
 223 information as defined in the lists explained above. If a device is
 224 detected at a specific address, another callback is called.
 225
 226   int foo_attach_adapter(struct i2c_adapter *adapter)
 227   {
 228     return i2c_probe(adapter,&addr_data,&foo_detect_client);
 229   }
 230
 231 Remember, structure `addr_data' is defined by the macros explained above,
 232 so you do not have to define it yourself.
 233
 234 The i2c_probe function will call the foo_detect_client
 235 function only for those i2c addresses that actually have a device on
 236 them (unless a `force' parameter was used). In addition, addresses that
 237 are already in use (by some other registered client) are skipped.
 238
 239
 240 The detect client function
 241 --------------------------
 242
 243 The detect client function is called by i2c_probe. The `kind' parameter
 244 contains -1 for a probed detection, 0 for a forced detection, or a positive
 245 number for a forced detection with a chip type forced.
 246
 247 Below, some things are only needed if this is a `sensors' driver. Those
 248 parts are between /* SENSORS ONLY START */ and /* SENSORS ONLY END */
 249 markers.
 250
 251 Returning an error different from -ENODEV in a detect function will cause
 252 the detection to stop: other addresses and adapters won't be scanned.
 253 This should only be done on fatal or internal errors, such as a memory
 254 shortage or i2c_attach_client failing.
 255
 256 For now, you can ignore the `flags' parameter. It is there for future use.
 257
 258   int foo_detect_client(struct i2c_adapter *adapter, int address,
 259                         unsigned short flags, int kind)
 260   {
 261     int err = 0;
 262     int i;
 263     struct i2c_client *new_client;
 264     struct foo_data *data;
 265     const char *client_name = ""; /* For non-`sensors' drivers, put the real
 266                                      name here! */
 267
 268     /* Let's see whether this adapter can support what we need.
 269        Please substitute the things you need here!
 270        For `sensors' drivers, add `! is_isa &&' to the if statement */
 271     if (!i2c_check_functionality(adapter,I2C_FUNC_SMBUS_WORD_DATA |
 272                                         I2C_FUNC_SMBUS_WRITE_BYTE))
 273        goto ERROR0;
 274
 275     /* SENSORS ONLY START */
 276     const char *type_name = "";
 277     int is_isa = i2c_is_isa_adapter(adapter);
 278
 279     /* Do this only if the chip can additionally be found on the ISA bus
 280        (hybrid chip). */
 281
 282     if (is_isa) {
 283
 284       /* Discard immediately if this ISA range is already used */
 285       /* FIXME: never use check_region(), only request_region() */
 286       if (check_region(address,FOO_EXTENT))
 287         goto ERROR0;
 288
 289       /* Probe whether there is anything on this address.
 290          Some example code is below, but you will have to adapt this
 291          for your own driver */
 292
 293       if (kind < 0) /* Only if no force parameter was used */ {
 294         /* We may need long timeouts at least for some chips. */
 295         #define REALLY_SLOW_IO
 296         i = inb_p(address + 1);
 297         if (inb_p(address + 2) != i)
 298           goto ERROR0;
 299         if (inb_p(address + 3) != i)
 300           goto ERROR0;
 301         if (inb_p(address + 7) != i)
 302           goto ERROR0;
 303         #undef REALLY_SLOW_IO
 304
 305         /* Let's just hope nothing breaks here */
 306         i = inb_p(address + 5) & 0x7f;
 307         outb_p(~i & 0x7f,address+5);
 308         if ((inb_p(address + 5) & 0x7f) != (~i & 0x7f)) {
 309           outb_p(i,address+5);
 310           return 0;
 311         }
 312       }
 313     }
 314
 315     /* SENSORS ONLY END */
 316
 317     /* OK. For now, we presume we have a valid client. We now create the
 318        client structure, even though we cannot fill it completely yet.
 319        But it allows us to access several i2c functions safely */
 320
 321     if (!(data = kzalloc(sizeof(struct foo_data), GFP_KERNEL))) {
 322       err = -ENOMEM;
 323       goto ERROR0;
 324     }
 325
 326     new_client = &data->client;
 327     i2c_set_clientdata(new_client, data);
 328
 329     new_client->addr = address;
 330     new_client->adapter = adapter;
 331     new_client->driver = &foo_driver;
 332     new_client->flags = 0;
 333
 334     /* Now, we do the remaining detection. If no `force' parameter is used. */
 335
 336     /* First, the generic detection (if any), that is skipped if any force
 337        parameter was used. */
 338     if (kind < 0) {
 339       /* The below is of course bogus */
 340       if (foo_read(new_client,FOO_REG_GENERIC) != FOO_GENERIC_VALUE)
 341          goto ERROR1;
 342     }
 343
 344     /* SENSORS ONLY START */
 345
 346     /* Next, specific detection. This is especially important for `sensors'
 347        devices. */
 348
 349     /* Determine the chip type. Not needed if a `force_CHIPTYPE' parameter
 350        was used. */
 351     if (kind <= 0) {
 352       i = foo_read(new_client,FOO_REG_CHIPTYPE);
 353       if (i == FOO_TYPE_1)
 354         kind = chip1; /* As defined in the enum */
 355       else if (i == FOO_TYPE_2)
 356         kind = chip2;
 357       else {
 358         printk("foo: Ignoring 'force' parameter for unknown chip at "
 359                "adapter %d, address 0x%02x\n",i2c_adapter_id(adapter),address);
 360         goto ERROR1;
 361       }
 362     }
 363
 364     /* Now set the type and chip names */
 365     if (kind == chip1) {
 366       type_name = "chip1"; /* For /proc entry */
 367       client_name = "CHIP 1";
 368     } else if (kind == chip2) {
 369       type_name = "chip2"; /* For /proc entry */
 370       client_name = "CHIP 2";
 371     }
 372
 373     /* Reserve the ISA region */
 374     if (is_isa)
 375       request_region(address,FOO_EXTENT,type_name);
 376
 377     /* SENSORS ONLY END */
 378
 379     /* Fill in the remaining client fields. */
 380     strcpy(new_client->name,client_name);
 381
 382     /* SENSORS ONLY BEGIN */
 383     data->type = kind;
 384     /* SENSORS ONLY END */
 385
 386     data->valid = 0; /* Only if you use this field */
 387     init_MUTEX(&data->update_lock); /* Only if you use this field */
 388
 389     /* Any other initializations in data must be done here too. */
 390
 391     /* Tell the i2c layer a new client has arrived */
 392     if ((err = i2c_attach_client(new_client)))
 393       goto ERROR3;
 394
 395     /* SENSORS ONLY BEGIN */
 396     /* Register a new directory entry with module sensors. See below for
 397        the `template' structure. */
 398     if ((i = i2c_register_entry(new_client, type_name,
 399                                     foo_dir_table_template,THIS_MODULE)) < 0) {
 400       err = i;
 401       goto ERROR4;
 402     }
 403     data->sysctl_id = i;
 404
 405     /* SENSORS ONLY END */
 406
 407     /* This function can write default values to the client registers, if
 408        needed. */
 409     foo_init_client(new_client);
 410     return 0;
 411
 412     /* OK, this is not exactly good programming practice, usually. But it is
 413        very code-efficient in this case. */
 414
 415     ERROR4:
 416       i2c_detach_client(new_client);
 417     ERROR3:
 418     ERROR2:
 419     /* SENSORS ONLY START */
 420       if (is_isa)
 421         release_region(address,FOO_EXTENT);
 422     /* SENSORS ONLY END */
 423     ERROR1:
 424       kfree(data);
 425     ERROR0:
 426       return err;
 427   }
 428
 429
 430 Removing the client
 431 ===================
 432
 433 The detach_client call back function is called when a client should be
 434 removed. It may actually fail, but only when panicking. This code is
 435 much simpler than the attachment code, fortunately!
 436
 437   int foo_detach_client(struct i2c_client *client)
 438   {
 439     int err,i;
 440
 441     /* SENSORS ONLY START */
 442     /* Deregister with the `i2c-proc' module. */
 443     i2c_deregister_entry(((struct lm78_data *)(client->data))->sysctl_id);
 444     /* SENSORS ONLY END */
 445
 446     /* Try to detach the client from i2c space */
 447     if ((err = i2c_detach_client(client)))
 448       return err;
 449
 450     /* HYBRID SENSORS CHIP ONLY START */
 451     if i2c_is_isa_client(client)
 452       release_region(client->addr,LM78_EXTENT);
 453     /* HYBRID SENSORS CHIP ONLY END */
 454
 455     kfree(i2c_get_clientdata(client));
 456     return 0;
 457   }
 458
 459
 460 Initializing the module or kernel
 461 =================================
 462
 463 When the kernel is booted, or when your foo driver module is inserted,
 464 you have to do some initializing. Fortunately, just attaching (registering)
 465 the driver module is usually enough.
 466
 467   /* Keep track of how far we got in the initialization process. If several
 468      things have to initialized, and we fail halfway, only those things
 469      have to be cleaned up! */
 470   static int __initdata foo_initialized = 0;
 471
 472   static int __init foo_init(void)
 473   {
 474     int res;
 475     printk("foo version %s (%s)\n",FOO_VERSION,FOO_DATE);
 476
 477     if ((res = i2c_add_driver(&foo_driver))) {
 478       printk("foo: Driver registration failed, module not inserted.\n");
 479       foo_cleanup();
 480       return res;
 481     }
 482     foo_initialized ++;
 483     return 0;
 484   }
 485
 486   void foo_cleanup(void)
 487   {
 488     if (foo_initialized == 1) {
 489       if ((res = i2c_del_driver(&foo_driver))) {
 490         printk("foo: Driver registration failed, module not removed.\n");
 491         return;
 492       }
 493       foo_initialized --;
 494     }
 495   }
 496
 497   /* Substitute your own name and email address */
 498   MODULE_AUTHOR("Frodo Looijaard <frodol@dds.nl>"
 499   MODULE_DESCRIPTION("Driver for Barf Inc. Foo I2C devices");
 500
 501   module_init(foo_init);
 502   module_exit(foo_cleanup);
 503
 504 Note that some functions are marked by `__init', and some data structures
 505 by `__init_data'.  Hose functions and structures can be removed after
 506 kernel booting (or module loading) is completed.
 507
 508
 509 Power Management
 510 ================
 511
 512 If your I2C device needs special handling when entering a system low
 513 power state -- like putting a transceiver into a low power mode, or
 514 activating a system wakeup mechanism -- do that in the suspend() method.
 515 The resume() method should reverse what the suspend() method does.
 516
 517 These are standard driver model calls, and they work just like they
 518 would for any other driver stack.  The calls can sleep, and can use
 519 I2C messaging to the device being suspended or resumed (since their
 520 parent I2C adapter is active when these calls are issued, and IRQs
 521 are still enabled).
 522
 523
 524 System Shutdown
 525 ===============
 526
 527 If your I2C device needs special handling when the system shuts down
 528 or reboots (including kexec) -- like turning something off -- use a
 529 shutdown() method.
 530
 531 Again, this is a standard driver model call, working just like it
 532 would for any other driver stack:  the calls can sleep, and can use
 533 I2C messaging.
 534
 535
 536 Command function
 537 ================
 538
 539 A generic ioctl-like function call back is supported. You will seldom
 540 need this, and its use is deprecated anyway, so newer design should not
 541 use it. Set it to NULL.
 542
 543
 544 Sending and receiving
 545 =====================
 546
 547 If you want to communicate with your device, there are several functions
 548 to do this. You can find all of them in i2c.h.
 549
 550 If you can choose between plain i2c communication and SMBus level
 551 communication, please use the last. All adapters understand SMBus level
 552 commands, but only some of them understand plain i2c!
 553
 554
 555 Plain i2c communication
 556 -----------------------
 557
 558   extern int i2c_master_send(struct i2c_client *,const char* ,int);
 559   extern int i2c_master_recv(struct i2c_client *,char* ,int);
 560
 561 These routines read and write some bytes from/to a client. The client
 562 contains the i2c address, so you do not have to include it. The second
 563 parameter contains the bytes the read/write, the third the length of the
 564 buffer. Returned is the actual number of bytes read/written.
 565
 566   extern int i2c_transfer(struct i2c_adapter *adap, struct i2c_msg *msg,
 567                           int num);
 568
 569 This sends a series of messages. Each message can be a read or write,
 570 and they can be mixed in any way. The transactions are combined: no
 571 stop bit is sent between transaction. The i2c_msg structure contains
 572 for each message the client address, the number of bytes of the message
 573 and the message data itself.
 574
 575 You can read the file `i2c-protocol' for more information about the
 576 actual i2c protocol.
 577
 578
 579 SMBus communication
 580 -------------------
 581
 582   extern s32 i2c_smbus_xfer (struct i2c_adapter * adapter, u16 addr,
 583                              unsigned short flags,
 584                              char read_write, u8 command, int size,
 585                              union i2c_smbus_data * data);
 586
 587   This is the generic SMBus function. All functions below are implemented
 588   in terms of it. Never use this function directly!
 589
 590
 591   extern s32 i2c_smbus_write_quick(struct i2c_client * client, u8 value);
 592   extern s32 i2c_smbus_read_byte(struct i2c_client * client);
 593   extern s32 i2c_smbus_write_byte(struct i2c_client * client, u8 value);
 594   extern s32 i2c_smbus_read_byte_data(struct i2c_client * client, u8 command);
 595   extern s32 i2c_smbus_write_byte_data(struct i2c_client * client,
 596                                        u8 command, u8 value);
 597   extern s32 i2c_smbus_read_word_data(struct i2c_client * client, u8 command);
 598   extern s32 i2c_smbus_write_word_data(struct i2c_client * client,
 599                                        u8 command, u16 value);
 600   extern s32 i2c_smbus_write_block_data(struct i2c_client * client,
 601                                         u8 command, u8 length,
 602                                         u8 *values);
 603   extern s32 i2c_smbus_read_i2c_block_data(struct i2c_client * client,
 604                                            u8 command, u8 *values);
 605
 606 These ones were removed in Linux 2.6.10 because they had no users, but could
 607 be added back later if needed:
 608
 609   extern s32 i2c_smbus_read_block_data(struct i2c_client * client,
 610                                        u8 command, u8 *values);
 611   extern s32 i2c_smbus_write_i2c_block_data(struct i2c_client * client,
 612                                             u8 command, u8 length,
 613                                             u8 *values);
 614   extern s32 i2c_smbus_process_call(struct i2c_client * client,
 615                                     u8 command, u16 value);
 616   extern s32 i2c_smbus_block_process_call(struct i2c_client *client,
 617                                           u8 command, u8 length,
 618                                           u8 *values)
 619
 620 All these transactions return -1 on failure. The 'write' transactions
 621 return 0 on success; the 'read' transactions return the read value, except
 622 for read_block, which returns the number of values read. The block buffers
 623 need not be longer than 32 bytes.
 624
 625 You can read the file `smbus-protocol' for more information about the
 626 actual SMBus protocol.
 627
 628
 629 General purpose routines
 630 ========================
 631
 632 Below all general purpose routines are listed, that were not mentioned
 633 before.
 634
 635   /* This call returns a unique low identifier for each registered adapter,
 636    * or -1 if the adapter was not registered.
 637    */
 638   extern int i2c_adapter_id(struct i2c_adapter *adap);
 639
 640
 641 The sensors sysctl/proc interface
 642 =================================
 643
 644 This section only applies if you write `sensors' drivers.
 645
 646 Each sensors driver creates a directory in /proc/sys/dev/sensors for each
 647 registered client. The directory is called something like foo-i2c-4-65.
 648 The sensors module helps you to do this as easily as possible.
 649
 650 The template
 651 ------------
 652
 653 You will need to define a ctl_table template. This template will automatically
 654 be copied to a newly allocated structure and filled in where necessary when
 655 you call sensors_register_entry.
 656
 657 First, I will give an example definition.
 658   static ctl_table foo_dir_table_template[] = {
 659     { FOO_SYSCTL_FUNC1, "func1", NULL, 0, 0644, NULL, &i2c_proc_real,
 660       &i2c_sysctl_real,NULL,&foo_func },
 661     { FOO_SYSCTL_FUNC2, "func2", NULL, 0, 0644, NULL, &i2c_proc_real,
 662       &i2c_sysctl_real,NULL,&foo_func },
 663     { FOO_SYSCTL_DATA, "data", NULL, 0, 0644, NULL, &i2c_proc_real,
 664       &i2c_sysctl_real,NULL,&foo_data },
 665     { 0 }
 666   };
 667
 668 In the above example, three entries are defined. They can either be
 669 accessed through the /proc interface, in the /proc/sys/dev/sensors/*
 670 directories, as files named func1, func2 and data, or alternatively
 671 through the sysctl interface, in the appropriate table, with identifiers
 672 FOO_SYSCTL_FUNC1, FOO_SYSCTL_FUNC2 and FOO_SYSCTL_DATA.
 673
 674 The third, sixth and ninth parameters should always be NULL, and the
 675 fourth should always be 0. The fifth is the mode of the /proc file;
 676 0644 is safe, as the file will be owned by root:root.
 677
 678 The seventh and eighth parameters should be &i2c_proc_real and
 679 &i2c_sysctl_real if you want to export lists of reals (scaled
 680 integers). You can also use your own function for them, as usual.
 681 Finally, the last parameter is the call-back to gather the data
 682 (see below) if you use the *_proc_real functions.
 683
 684
 685 Gathering the data
 686 ------------------
 687
 688 The call back functions (foo_func and foo_data in the above example)
 689 can be called in several ways; the operation parameter determines
 690 what should be done:
 691
 692   * If operation == SENSORS_PROC_REAL_INFO, you must return the
 693     magnitude (scaling) in nrels_mag;
 694   * If operation == SENSORS_PROC_REAL_READ, you must read information
 695     from the chip and return it in results. The number of integers
 696     to display should be put in nrels_mag;
 697   * If operation == SENSORS_PROC_REAL_WRITE, you must write the
 698     supplied information to the chip. nrels_mag will contain the number
 699     of integers, results the integers themselves.
 700
 701 The *_proc_real functions will display the elements as reals for the
 702 /proc interface. If you set the magnitude to 2, and supply 345 for
 703 SENSORS_PROC_REAL_READ, it would display 3.45; and if the user would
 704 write 45.6 to the /proc file, it would be returned as 4560 for
 705 SENSORS_PROC_REAL_WRITE. A magnitude may even be negative!
 706
 707 An example function:
 708
 709   /* FOO_FROM_REG and FOO_TO_REG translate between scaled values and
 710      register values. Note the use of the read cache. */
 711   void foo_in(struct i2c_client *client, int operation, int ctl_name,
 712               int *nrels_mag, long *results)
 713   {
 714     struct foo_data *data = client->data;
 715     int nr = ctl_name - FOO_SYSCTL_FUNC1; /* reduce to 0 upwards */
 716
 717     if (operation == SENSORS_PROC_REAL_INFO)
 718       *nrels_mag = 2;
 719     else if (operation == SENSORS_PROC_REAL_READ) {
 720       /* Update the readings cache (if necessary) */
 721       foo_update_client(client);
 722       /* Get the readings from the cache */
 723       results[0] = FOO_FROM_REG(data->foo_func_base[nr]);
 724       results[1] = FOO_FROM_REG(data->foo_func_more[nr]);
 725       results[2] = FOO_FROM_REG(data->foo_func_readonly[nr]);
 726       *nrels_mag = 2;
 727     } else if (operation == SENSORS_PROC_REAL_WRITE) {
 728       if (*nrels_mag >= 1) {
 729         /* Update the cache */
 730         data->foo_base[nr] = FOO_TO_REG(results[0]);
 731         /* Update the chip */
 732         foo_write_value(client,FOO_REG_FUNC_BASE(nr),data->foo_base[nr]);
 733       }
 734       if (*nrels_mag >= 2) {
 735         /* Update the cache */
 736         data->foo_more[nr] = FOO_TO_REG(results[1]);
 737         /* Update the chip */
 738         foo_write_value(client,FOO_REG_FUNC_MORE(nr),data->foo_more[nr]);
 739       }
 740     }
 741   }