Based on kernel version 2.6.26. Page generated on 2008-07-16 21:12 EST.
1 This is a small guide for those who want to write kernel drivers for I2C 2 or SMBus devices, using Linux as the protocol host/master (not slave). 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 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 48 The name field is the driver name, and must not contain spaces. It 49 should match the module name (if the driver can be compiled as a module), 50 although you can use MODULE_ALIAS (passing "foo" in this example) to add 51 another name for the module. If the driver name doesn't match the module 52 name, the module won't be automatically loaded (hotplug/coldplug). 53 54 All other fields are for call-back functions which will be explained 55 below. 56 57 58 Extra client data 59 ================= 60 61 Each client structure has a special `data' field that can point to any 62 structure at all. You should use this to keep device-specific data, 63 especially in drivers that handle multiple I2C or SMBUS devices. You 64 do not always need this, but especially for `sensors' drivers, it can 65 be 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 73 An 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 92 Accessing the client 93 ==================== 94 95 Let's say we have a valid client structure. At some time, we will need 96 to gather information from the client, or write new information to the 97 client. How we will export this information to user-space is less 98 important at this moment (perhaps we do not need to do this at all for 99 some obscure clients). But we need generic reading and writing routines. 100 101 I have found it useful to define foo_read and foo_write function for this. 102 For some cases, it will be easier to call the i2c functions directly, 103 but many chips have some kind of register-value idea that can easily 104 be encapsulated. 105 106 The below functions are simple examples, and should not be copied 107 literally. 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 } 126 127 128 Probing and attaching 129 ===================== 130 131 The Linux I2C stack was originally written to support access to hardware 132 monitoring chips on PC motherboards, and thus it embeds some assumptions 133 that are more appropriate to SMBus (and PCs) than to I2C. One of these 134 assumptions is that most adapters and devices drivers support the SMBUS_QUICK 135 protocol to probe device presence. Another is that devices and their drivers 136 can be sufficiently configured using only such probe primitives. 137 138 As Linux and its I2C stack became more widely used in embedded systems 139 and complex components such as DVB adapters, those assumptions became more 140 problematic. Drivers for I2C devices that issue interrupts need more (and 141 different) configuration information, as do drivers handling chip variants 142 that can't be distinguished by protocol probing, or which need some board 143 specific information to operate correctly. 144 145 Accordingly, the I2C stack now has two models for associating I2C devices 146 with their drivers: the original "legacy" model, and a newer one that's 147 fully compatible with the Linux 2.6 driver model. These models do not mix, 148 since the "legacy" model requires drivers to create "i2c_client" device 149 objects after SMBus style probing, while the Linux driver model expects 150 drivers to be given such device objects in their probe() routines. 151 152 153 Standard Driver Model Binding ("New Style") 154 ------------------------------------------- 155 156 System infrastructure, typically board-specific initialization code or 157 boot firmware, reports what I2C devices exist. For example, there may be 158 a table, in the kernel or from the boot loader, identifying I2C devices 159 and linking them to board-specific configuration information about IRQs 160 and other wiring artifacts, chip type, and so on. That could be used to 161 create i2c_client objects for each I2C device. 162 163 I2C device drivers using this binding model work just like any other 164 kind of driver in Linux: they provide a probe() method to bind to 165 those 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); 169 170 Remember that the i2c_driver does not create those client handles. The 171 handle 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 173 foo_remove() returns. That binding model is used by most Linux drivers. 174 175 Drivers match devices when i2c_client.driver_name and the driver name are 176 the same; this approach is used in several other busses that don't have 177 device typing support in the hardware. The driver and module name should 178 match, so hotplug/coldplug mechanisms will modprobe the driver. 179 180 181 Device Creation (Standard driver model) 182 --------------------------------------- 183 184 If you know for a fact that an I2C device is connected to a given I2C bus, 185 you can instantiate that device by simply filling an i2c_board_info 186 structure with the device address and driver name, and calling 187 i2c_new_device(). This will create the device, then the driver core will 188 take care of finding the right driver and will call its probe() method. 189 If a driver supports different device types, you can specify the type you 190 want using the type field. You can also specify an IRQ and platform data 191 if needed. 192 193 Sometimes you know that a device is connected to a given I2C bus, but you 194 don't know the exact address it uses. This happens on TV adapters for 195 example, where the same driver supports dozens of slightly different 196 models, and I2C device addresses change from one model to the next. In 197 that case, you can use the i2c_new_probed_device() variant, which is 198 similar to i2c_new_device(), except that it takes an additional list of 199 possible I2C addresses to probe. A device is created for the first 200 responsive address in the list. If you expect more than one device to be 201 present in the address range, simply call i2c_new_probed_device() that 202 many times. 203 204 The call to i2c_new_device() or i2c_new_probed_device() typically happens 205 in the I2C bus driver. You may want to save the returned i2c_client 206 reference for later use. 207 208 209 Device Deletion (Standard driver model) 210 --------------------------------------- 211 212 Each I2C device which has been created using i2c_new_device() or 213 i2c_new_probed_device() can be unregistered by calling 214 i2c_unregister_device(). If you don't call it explicitly, it will be 215 called automatically before the underlying I2C bus itself is removed, as a 216 device can't survive its parent in the device driver model. 217 218 219 Legacy Driver Binding Model 220 --------------------------- 221 222 Most i2c devices can be present on several i2c addresses; for some this 223 is determined in hardware (by soldering some chip pins to Vcc or Ground), 224 for others this can be changed in software (by writing to specific client 225 registers). Some devices are usually on a specific address, but not always; 226 and some are even more tricky. So you will probably need to scan several 227 i2c addresses for your clients, and do some sort of detection to see 228 whether it is actually a device supported by your driver. 229 230 To give the user a maximum of possibilities, some default module parameters 231 are defined to help determine what addresses are scanned. Several macros 232 are defined in i2c.h to help you support them, as well as a generic 233 detection algorithm. 234 235 You do not have to use this parameter interface; but don't try to use 236 function i2c_probe() if you don't. 237 238 239 Probing classes (Legacy model) 240 ------------------------------ 241 242 All parameters are given as lists of unsigned 16-bit integers. Lists are 243 terminated by I2C_CLIENT_END. 244 The 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. 260 261 Additionally, kind-specific force lists may optionally be defined if 262 the driver supports several chip kinds. They are grouped in a 263 NULL-terminated list of pointers named forces, those first element if the 264 generic force list mentioned above. Each additional list correspond to an 265 insmod parameter of the form force_<kind>. 266 267 Fortunately, as a module writer, you just have to define the `normal_i2c' 268 parameter. The complete declaration could look like this: 269 270 /* Scan 0x4c to 0x4f */ 271 static const unsigned short normal_i2c[] = { 0x4c, 0x4d, 0x4e, 0x4f, 272 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); 278 279 If you use the multi-kind form, an enum will be defined for you: 280 enum chips { any_chip, foo, bar, ... } 281 You can then (and certainly should) use it in the driver code. 282 283 Note that you *have* to call the defined variable `normal_i2c', 284 without any prefix! 285 286 287 Attaching to an adapter (Legacy model) 288 -------------------------------------- 289 290 Whenever a new adapter is inserted, or for all adapters if the driver is 291 being registered, the callback attach_adapter() is called. Now is the 292 time to determine what devices are present on the adapter, and to register 293 a client for each of them. 294 295 The attach_adapter callback is really easy: we just call the generic 296 detection function. This function will scan the bus for us, using the 297 information as defined in the lists explained above. If a device is 298 detected 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 } 304 305 Remember, structure `addr_data' is defined by the macros explained above, 306 so you do not have to define it yourself. 307 308 The i2c_probe function will call the foo_detect_client 309 function only for those i2c addresses that actually have a device on 310 them (unless a `force' parameter was used). In addition, addresses that 311 are already in use (by some other registered client) are skipped. 312 313 314 The detect client function (Legacy model) 315 ----------------------------------------- 316 317 The detect client function is called by i2c_probe. The `kind' parameter 318 contains -1 for a probed detection, 0 for a forced detection, or a positive 319 number for a forced detection with a chip type forced. 320 321 Returning an error different from -ENODEV in a detect function will cause 322 the detection to stop: other addresses and adapters won't be scanned. 323 This should only be done on fatal or internal errors, such as a memory 324 shortage or i2c_attach_client failing. 325 326 For 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 } 419 420 421 Removing the client (Legacy model) 422 ================================== 423 424 The detach_client call back function is called when a client should be 425 removed. It may actually fail, but only when panicking. This code is 426 much 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 } 439 440 441 Initializing the module or kernel 442 ================================= 443 444 When the kernel is booted, or when your foo driver module is inserted, 445 you have to do some initializing. Fortunately, just attaching (registering) 446 the 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[AT]dds[DOT]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); 473 474 Note that some functions are marked by `__init', and some data structures 475 by `__initdata'. These functions and structures can be removed after 476 kernel booting (or module loading) is completed. 477 478 479 Power Management 480 ================ 481 482 If your I2C device needs special handling when entering a system low 483 power state -- like putting a transceiver into a low power mode, or 484 activating a system wakeup mechanism -- do that in the suspend() method. 485 The resume() method should reverse what the suspend() method does. 486 487 These are standard driver model calls, and they work just like they 488 would for any other driver stack. The calls can sleep, and can use 489 I2C messaging to the device being suspended or resumed (since their 490 parent I2C adapter is active when these calls are issued, and IRQs 491 are still enabled). 492 493 494 System Shutdown 495 =============== 496 497 If your I2C device needs special handling when the system shuts down 498 or reboots (including kexec) -- like turning something off -- use a 499 shutdown() method. 500 501 Again, this is a standard driver model call, working just like it 502 would for any other driver stack: the calls can sleep, and can use 503 I2C messaging. 504 505 506 Command function 507 ================ 508 509 A generic ioctl-like function call back is supported. You will seldom 510 need this, and its use is deprecated anyway, so newer design should not 511 use it. Set it to NULL. 512 513 514 Sending and receiving 515 ===================== 516 517 If you want to communicate with your device, there are several functions 518 to do this. You can find all of them in i2c.h. 519 520 If you can choose between plain i2c communication and SMBus level 521 communication, please use the last. All adapters understand SMBus level 522 commands, but only some of them understand plain i2c! 523 524 525 Plain 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); 530 531 These routines read and write some bytes from/to a client. The client 532 contains the i2c address, so you do not have to include it. The second 533 parameter contains the bytes the read/write, the third the length of the 534 buffer. 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); 538 539 This sends a series of messages. Each message can be a read or write, 540 and they can be mixed in any way. The transactions are combined: no 541 stop bit is sent between transaction. The i2c_msg structure contains 542 for each message the client address, the number of bytes of the message 543 and the message data itself. 544 545 You can read the file `i2c-protocol' for more information about the 546 actual i2c protocol. 547 548 549 SMBus 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 length, u8 *values); 575 576 These ones were removed in Linux 2.6.10 because they had no users, but could 577 be 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) 589 590 All these transactions return -1 on failure. The 'write' transactions 591 return 0 on success; the 'read' transactions return the read value, except 592 for read_block, which returns the number of values read. The block buffers 593 need not be longer than 32 bytes. 594 595 You can read the file `smbus-protocol' for more information about the 596 actual SMBus protocol. 597 598 599 General purpose routines 600 ======================== 601 602 Below all general purpose routines are listed, that were not mentioned 603 before. 604 605 /* This call returns a unique low identifier for each registered adapter. 606 */ 607 extern int i2c_adapter_id(struct i2c_adapter *adap);