1This is a small guide for those who want to write kernel drivers for I2C 2or SMBus devices, using Linux as the protocol host/master (not slave). 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, and should be zero-initialized except for fields with data you 25provide. A client structure holds device-specific information like the 26driver model device node, and its I2C address. 27 28static struct i2c_driver foo_driver = { 29 .driver = { 30 .name = "foo", 31 }, 32 33 /* iff driver uses driver model ("new style") binding model: */ 34 .probe = foo_probe, 35 .remove = foo_remove, 36 37 /* else, driver uses "legacy" binding model: */ 38 .attach_adapter = foo_attach_adapter, 39 .detach_client = foo_detach_client, 40 41 /* these may be used regardless of the driver binding model */ 42 .shutdown = foo_shutdown, /* optional */ 43 .suspend = foo_suspend, /* optional */ 44 .resume = foo_resume, /* optional */ 45 .command = foo_command, /* optional */ 46} 47 48The name field is the driver name, and must not contain spaces. It 49should match the module name (if the driver can be compiled as a module), 50although you can use MODULE_ALIAS (passing "foo" in this example) to add 51another name for the module. If the driver name doesn't match the module 52name, the module won't be automatically loaded (hotplug/coldplug). 53 54All other fields are for call-back functions which will be explained 55below. 56 57 58Extra client data 59================= 60 61Each client structure has a special `data' field that can point to any 62structure at all. You should use this to keep device-specific data, 63especially in drivers that handle multiple I2C or SMBUS devices. You 64do not always need this, but especially for `sensors' drivers, it can 65be very useful. 66 67 /* store the value */ 68 void i2c_set_clientdata(struct i2c_client *client, void *data); 69 70 /* retrieve the value */ 71 void *i2c_get_clientdata(struct i2c_client *client); 72 73An example structure is below. 74 75 struct foo_data { 76 struct i2c_client client; 77 enum chips type; /* To keep the chips type for `sensors' drivers. */ 78 79 /* Because the i2c bus is slow, it is often useful to cache the read 80 information of a chip for some time (for example, 1 or 2 seconds). 81 It depends of course on the device whether this is really worthwhile 82 or even sensible. */ 83 struct mutex update_lock; /* When we are reading lots of information, 84 another process should not update the 85 below information */ 86 char valid; /* != 0 if the following fields are valid. */ 87 unsigned long last_updated; /* In jiffies */ 88 /* Add the read information here too */ 89 }; 90 91 92Accessing the client 93==================== 94 95Let's say we have a valid client structure. At some time, we will need 96to gather information from the client, or write new information to the 97client. How we will export this information to user-space is less 98important at this moment (perhaps we do not need to do this at all for 99some obscure clients). But we need generic reading and writing routines. 100 101I have found it useful to define foo_read and foo_write function for this. 102For some cases, it will be easier to call the i2c functions directly, 103but many chips have some kind of register-value idea that can easily 104be encapsulated. 105 106The below functions are simple examples, and should not be copied 107literally. 108 109 int foo_read_value(struct i2c_client *client, u8 reg) 110 { 111 if (reg < 0x10) /* byte-sized register */ 112 return i2c_smbus_read_byte_data(client,reg); 113 else /* word-sized register */ 114 return i2c_smbus_read_word_data(client,reg); 115 } 116 117 int foo_write_value(struct i2c_client *client, u8 reg, u16 value) 118 { 119 if (reg == 0x10) /* Impossible to write - driver error! */ { 120 return -1; 121 else if (reg < 0x10) /* byte-sized register */ 122 return i2c_smbus_write_byte_data(client,reg,value); 123 else /* word-sized register */ 124 return i2c_smbus_write_word_data(client,reg,value); 125 } 126 127 128Probing and attaching 129===================== 130 131The Linux I2C stack was originally written to support access to hardware 132monitoring chips on PC motherboards, and thus it embeds some assumptions 133that are more appropriate to SMBus (and PCs) than to I2C. One of these 134assumptions is that most adapters and devices drivers support the SMBUS_QUICK 135protocol to probe device presence. Another is that devices and their drivers 136can be sufficiently configured using only such probe primitives. 137 138As Linux and its I2C stack became more widely used in embedded systems 139and complex components such as DVB adapters, those assumptions became more 140problematic. Drivers for I2C devices that issue interrupts need more (and 141different) configuration information, as do drivers handling chip variants 142that can't be distinguished by protocol probing, or which need some board 143specific information to operate correctly. 144 145Accordingly, the I2C stack now has two models for associating I2C devices 146with their drivers: the original "legacy" model, and a newer one that's 147fully compatible with the Linux 2.6 driver model. These models do not mix, 148since the "legacy" model requires drivers to create "i2c_client" device 149objects after SMBus style probing, while the Linux driver model expects 150drivers to be given such device objects in their probe() routines. 151 152 153Standard Driver Model Binding ("New Style") 154------------------------------------------- 155 156System infrastructure, typically board-specific initialization code or 157boot firmware, reports what I2C devices exist. For example, there may be 158a table, in the kernel or from the boot loader, identifying I2C devices 159and linking them to board-specific configuration information about IRQs 160and other wiring artifacts, chip type, and so on. That could be used to 161create i2c_client objects for each I2C device. 162 163I2C device drivers using this binding model work just like any other 164kind of driver in Linux: they provide a probe() method to bind to 165those devices, and a remove() method to unbind. 166 167 static int foo_probe(struct i2c_client *client); 168 static int foo_remove(struct i2c_client *client); 169 170Remember that the i2c_driver does not create those client handles. The 171handle may be used during foo_probe(). If foo_probe() reports success 172(zero not a negative status code) it may save the handle and use it until 173foo_remove() returns. That binding model is used by most Linux drivers. 174 175Drivers match devices when i2c_client.driver_name and the driver name are 176the same; this approach is used in several other busses that don't have 177device typing support in the hardware. The driver and module name should 178match, so hotplug/coldplug mechanisms will modprobe the driver. 179 180 181Device Creation (Standard driver model) 182--------------------------------------- 183 184If you know for a fact that an I2C device is connected to a given I2C bus, 185you can instantiate that device by simply filling an i2c_board_info 186structure with the device address and driver name, and calling 187i2c_new_device(). This will create the device, then the driver core will 188take care of finding the right driver and will call its probe() method. 189If a driver supports different device types, you can specify the type you 190want using the type field. You can also specify an IRQ and platform data 191if needed. 192 193Sometimes you know that a device is connected to a given I2C bus, but you 194don't know the exact address it uses. This happens on TV adapters for 195example, where the same driver supports dozens of slightly different 196models, and I2C device addresses change from one model to the next. In 197that case, you can use the i2c_new_probed_device() variant, which is 198similar to i2c_new_device(), except that it takes an additional list of 199possible I2C addresses to probe. A device is created for the first 200responsive address in the list. If you expect more than one device to be 201present in the address range, simply call i2c_new_probed_device() that 202many times. 203 204The call to i2c_new_device() or i2c_new_probed_device() typically happens 205in the I2C bus driver. You may want to save the returned i2c_client 206reference for later use. 207 208 209Device Deletion (Standard driver model) 210--------------------------------------- 211 212Each I2C device which has been created using i2c_new_device() or 213i2c_new_probed_device() can be unregistered by calling 214i2c_unregister_device(). If you don't call it explicitly, it will be 215called automatically before the underlying I2C bus itself is removed, as a 216device can't survive its parent in the device driver model. 217 218 219Legacy Driver Binding Model 220--------------------------- 221 222Most i2c devices can be present on several i2c addresses; for some this 223is determined in hardware (by soldering some chip pins to Vcc or Ground), 224for others this can be changed in software (by writing to specific client 225registers). Some devices are usually on a specific address, but not always; 226and some are even more tricky. So you will probably need to scan several 227i2c addresses for your clients, and do some sort of detection to see 228whether it is actually a device supported by your driver. 229 230To give the user a maximum of possibilities, some default module parameters 231are defined to help determine what addresses are scanned. Several macros 232are defined in i2c.h to help you support them, as well as a generic 233detection algorithm. 234 235You do not have to use this parameter interface; but don't try to use 236function i2c_probe() if you don't. 237 238 239Probing classes (Legacy model) 240------------------------------ 241 242All parameters are given as lists of unsigned 16-bit integers. Lists are 243terminated by I2C_CLIENT_END. 244The following lists are used internally: 245 246 normal_i2c: filled in by the module writer. 247 A list of I2C addresses which should normally be examined. 248 probe: insmod parameter. 249 A list of pairs. The first value is a bus number (-1 for any I2C bus), 250 the second is the address. These addresses are also probed, as if they 251 were in the 'normal' list. 252 ignore: insmod parameter. 253 A list of pairs. The first value is a bus number (-1 for any I2C bus), 254 the second is the I2C address. These addresses are never probed. 255 This parameter overrules the 'normal_i2c' list only. 256 force: insmod parameter. 257 A list of pairs. The first value is a bus number (-1 for any I2C bus), 258 the second is the I2C address. A device is blindly assumed to be on 259 the given address, no probing is done. 260 261Additionally, kind-specific force lists may optionally be defined if 262the driver supports several chip kinds. They are grouped in a 263NULL-terminated list of pointers named forces, those first element if the 264generic force list mentioned above. Each additional list correspond to an 265insmod parameter of the form force_<kind>. 266 267Fortunately, as a module writer, you just have to define the `normal_i2c' 268parameter. The complete declaration could look like this: 269 270 /* Scan 0x37, and 0x48 to 0x4f */ 271 static unsigned short normal_i2c[] = { 0x37, 0x48, 0x49, 0x4a, 0x4b, 0x4c, 272 0x4d, 0x4e, 0x4f, I2C_CLIENT_END }; 273 274 /* Magic definition of all other variables and things */ 275 I2C_CLIENT_INSMOD; 276 /* Or, if your driver supports, say, 2 kind of devices: */ 277 I2C_CLIENT_INSMOD_2(foo, bar); 278 279If you use the multi-kind form, an enum will be defined for you: 280 enum chips { any_chip, foo, bar, ... } 281You can then (and certainly should) use it in the driver code. 282 283Note that you *have* to call the defined variable `normal_i2c', 284without any prefix! 285 286 287Attaching to an adapter (Legacy model) 288-------------------------------------- 289 290Whenever a new adapter is inserted, or for all adapters if the driver is 291being registered, the callback attach_adapter() is called. Now is the 292time to determine what devices are present on the adapter, and to register 293a client for each of them. 294 295The attach_adapter callback is really easy: we just call the generic 296detection function. This function will scan the bus for us, using the 297information as defined in the lists explained above. If a device is 298detected at a specific address, another callback is called. 299 300 int foo_attach_adapter(struct i2c_adapter *adapter) 301 { 302 return i2c_probe(adapter,&addr_data,&foo_detect_client); 303 } 304 305Remember, structure `addr_data' is defined by the macros explained above, 306so you do not have to define it yourself. 307 308The i2c_probe function will call the foo_detect_client 309function only for those i2c addresses that actually have a device on 310them (unless a `force' parameter was used). In addition, addresses that 311are already in use (by some other registered client) are skipped. 312 313 314The detect client function (Legacy model) 315----------------------------------------- 316 317The detect client function is called by i2c_probe. The `kind' parameter 318contains -1 for a probed detection, 0 for a forced detection, or a positive 319number for a forced detection with a chip type forced. 320 321Returning an error different from -ENODEV in a detect function will cause 322the detection to stop: other addresses and adapters won't be scanned. 323This should only be done on fatal or internal errors, such as a memory 324shortage or i2c_attach_client failing. 325 326For now, you can ignore the `flags' parameter. It is there for future use. 327 328 int foo_detect_client(struct i2c_adapter *adapter, int address, 329 int kind) 330 { 331 int err = 0; 332 int i; 333 struct i2c_client *client; 334 struct foo_data *data; 335 const char *name = ""; 336 337 /* Let's see whether this adapter can support what we need. 338 Please substitute the things you need here! */ 339 if (!i2c_check_functionality(adapter,I2C_FUNC_SMBUS_WORD_DATA | 340 I2C_FUNC_SMBUS_WRITE_BYTE)) 341 goto ERROR0; 342 343 /* OK. For now, we presume we have a valid client. We now create the 344 client structure, even though we cannot fill it completely yet. 345 But it allows us to access several i2c functions safely */ 346 347 if (!(data = kzalloc(sizeof(struct foo_data), GFP_KERNEL))) { 348 err = -ENOMEM; 349 goto ERROR0; 350 } 351 352 client = &data->client; 353 i2c_set_clientdata(client, data); 354 355 client->addr = address; 356 client->adapter = adapter; 357 client->driver = &foo_driver; 358 359 /* Now, we do the remaining detection. If no `force' parameter is used. */ 360 361 /* First, the generic detection (if any), that is skipped if any force 362 parameter was used. */ 363 if (kind < 0) { 364 /* The below is of course bogus */ 365 if (foo_read(client, FOO_REG_GENERIC) != FOO_GENERIC_VALUE) 366 goto ERROR1; 367 } 368 369 /* Next, specific detection. This is especially important for `sensors' 370 devices. */ 371 372 /* Determine the chip type. Not needed if a `force_CHIPTYPE' parameter 373 was used. */ 374 if (kind <= 0) { 375 i = foo_read(client, FOO_REG_CHIPTYPE); 376 if (i == FOO_TYPE_1) 377 kind = chip1; /* As defined in the enum */ 378 else if (i == FOO_TYPE_2) 379 kind = chip2; 380 else { 381 printk("foo: Ignoring 'force' parameter for unknown chip at " 382 "adapter %d, address 0x%02x\n",i2c_adapter_id(adapter),address); 383 goto ERROR1; 384 } 385 } 386 387 /* Now set the type and chip names */ 388 if (kind == chip1) { 389 name = "chip1"; 390 } else if (kind == chip2) { 391 name = "chip2"; 392 } 393 394 /* Fill in the remaining client fields. */ 395 strlcpy(client->name, name, I2C_NAME_SIZE); 396 data->type = kind; 397 mutex_init(&data->update_lock); /* Only if you use this field */ 398 399 /* Any other initializations in data must be done here too. */ 400 401 /* This function can write default values to the client registers, if 402 needed. */ 403 foo_init_client(client); 404 405 /* Tell the i2c layer a new client has arrived */ 406 if ((err = i2c_attach_client(client))) 407 goto ERROR1; 408 409 return 0; 410 411 /* OK, this is not exactly good programming practice, usually. But it is 412 very code-efficient in this case. */ 413 414 ERROR1: 415 kfree(data); 416 ERROR0: 417 return err; 418 } 419 420 421Removing the client (Legacy model) 422================================== 423 424The detach_client call back function is called when a client should be 425removed. It may actually fail, but only when panicking. This code is 426much simpler than the attachment code, fortunately! 427 428 int foo_detach_client(struct i2c_client *client) 429 { 430 int err; 431 432 /* Try to detach the client from i2c space */ 433 if ((err = i2c_detach_client(client))) 434 return err; 435 436 kfree(i2c_get_clientdata(client)); 437 return 0; 438 } 439 440 441Initializing the module or kernel 442================================= 443 444When the kernel is booted, or when your foo driver module is inserted, 445you have to do some initializing. Fortunately, just attaching (registering) 446the driver module is usually enough. 447 448 static int __init foo_init(void) 449 { 450 int res; 451 452 if ((res = i2c_add_driver(&foo_driver))) { 453 printk("foo: Driver registration failed, module not inserted.\n"); 454 return res; 455 } 456 return 0; 457 } 458 459 static void __exit foo_cleanup(void) 460 { 461 i2c_del_driver(&foo_driver); 462 } 463 464 /* Substitute your own name and email address */ 465 MODULE_AUTHOR("Frodo Looijaard <frodol@dds.nl>" 466 MODULE_DESCRIPTION("Driver for Barf Inc. Foo I2C devices"); 467 468 /* a few non-GPL license types are also allowed */ 469 MODULE_LICENSE("GPL"); 470 471 module_init(foo_init); 472 module_exit(foo_cleanup); 473 474Note that some functions are marked by `__init', and some data structures 475by `__initdata'. These functions and structures can be removed after 476kernel booting (or module loading) is completed. 477 478 479Power Management 480================ 481 482If your I2C device needs special handling when entering a system low 483power state -- like putting a transceiver into a low power mode, or 484activating a system wakeup mechanism -- do that in the suspend() method. 485The resume() method should reverse what the suspend() method does. 486 487These are standard driver model calls, and they work just like they 488would for any other driver stack. The calls can sleep, and can use 489I2C messaging to the device being suspended or resumed (since their 490parent I2C adapter is active when these calls are issued, and IRQs 491are still enabled). 492 493 494System Shutdown 495=============== 496 497If your I2C device needs special handling when the system shuts down 498or reboots (including kexec) -- like turning something off -- use a 499shutdown() method. 500 501Again, this is a standard driver model call, working just like it 502would for any other driver stack: the calls can sleep, and can use 503I2C messaging. 504 505 506Command function 507================ 508 509A generic ioctl-like function call back is supported. You will seldom 510need this, and its use is deprecated anyway, so newer design should not 511use it. Set it to NULL. 512 513 514Sending and receiving 515===================== 516 517If you want to communicate with your device, there are several functions 518to do this. You can find all of them in i2c.h. 519 520If you can choose between plain i2c communication and SMBus level 521communication, please use the last. All adapters understand SMBus level 522commands, but only some of them understand plain i2c! 523 524 525Plain i2c communication 526----------------------- 527 528 extern int i2c_master_send(struct i2c_client *,const char* ,int); 529 extern int i2c_master_recv(struct i2c_client *,char* ,int); 530 531These routines read and write some bytes from/to a client. The client 532contains the i2c address, so you do not have to include it. The second 533parameter contains the bytes the read/write, the third the length of the 534buffer. Returned is the actual number of bytes read/written. 535 536 extern int i2c_transfer(struct i2c_adapter *adap, struct i2c_msg *msg, 537 int num); 538 539This sends a series of messages. Each message can be a read or write, 540and they can be mixed in any way. The transactions are combined: no 541stop bit is sent between transaction. The i2c_msg structure contains 542for each message the client address, the number of bytes of the message 543and the message data itself. 544 545You can read the file `i2c-protocol' for more information about the 546actual i2c protocol. 547 548 549SMBus communication 550------------------- 551 552 extern s32 i2c_smbus_xfer (struct i2c_adapter * adapter, u16 addr, 553 unsigned short flags, 554 char read_write, u8 command, int size, 555 union i2c_smbus_data * data); 556 557 This is the generic SMBus function. All functions below are implemented 558 in terms of it. Never use this function directly! 559 560 561 extern s32 i2c_smbus_write_quick(struct i2c_client * client, u8 value); 562 extern s32 i2c_smbus_read_byte(struct i2c_client * client); 563 extern s32 i2c_smbus_write_byte(struct i2c_client * client, u8 value); 564 extern s32 i2c_smbus_read_byte_data(struct i2c_client * client, u8 command); 565 extern s32 i2c_smbus_write_byte_data(struct i2c_client * client, 566 u8 command, u8 value); 567 extern s32 i2c_smbus_read_word_data(struct i2c_client * client, u8 command); 568 extern s32 i2c_smbus_write_word_data(struct i2c_client * client, 569 u8 command, u16 value); 570 extern s32 i2c_smbus_write_block_data(struct i2c_client * client, 571 u8 command, u8 length, 572 u8 *values); 573 extern s32 i2c_smbus_read_i2c_block_data(struct i2c_client * client, 574 u8 command, u8 *values); 575 576These ones were removed in Linux 2.6.10 because they had no users, but could 577be added back later if needed: 578 579 extern s32 i2c_smbus_read_block_data(struct i2c_client * client, 580 u8 command, u8 *values); 581 extern s32 i2c_smbus_write_i2c_block_data(struct i2c_client * client, 582 u8 command, u8 length, 583 u8 *values); 584 extern s32 i2c_smbus_process_call(struct i2c_client * client, 585 u8 command, u16 value); 586 extern s32 i2c_smbus_block_process_call(struct i2c_client *client, 587 u8 command, u8 length, 588 u8 *values) 589 590All these transactions return -1 on failure. The 'write' transactions 591return 0 on success; the 'read' transactions return the read value, except 592for read_block, which returns the number of values read. The block buffers 593need not be longer than 32 bytes. 594 595You can read the file `smbus-protocol' for more information about the 596actual SMBus protocol. 597 598 599General purpose routines 600======================== 601 602Below all general purpose routines are listed, that were not mentioned 603before. 604 605 /* This call returns a unique low identifier for each registered adapter. 606 */ 607 extern int i2c_adapter_id(struct i2c_adapter *adap); 608