1=============================== 2Implementing I2C device drivers 3=============================== 4 5This is a small guide for those who want to write kernel drivers for I2C 6or SMBus devices, using Linux as the protocol host/master (not slave). 7 8To set up a driver, you need to do several things. Some are optional, and 9some things can be done slightly or completely different. Use this as a 10guide, not as a rule book! 11 12 13General remarks 14=============== 15 16Try to keep the kernel namespace as clean as possible. The best way to 17do this is to use a unique prefix for all global symbols. This is 18especially important for exported symbols, but it is a good idea to do 19it for non-exported symbols too. We will use the prefix ``foo_`` in this 20tutorial. 21 22 23The driver structure 24==================== 25 26Usually, you will implement a single driver structure, and instantiate 27all clients from it. Remember, a driver structure contains general access 28routines, and should be zero-initialized except for fields with data you 29provide. A client structure holds device-specific information like the 30driver model device node, and its I2C address. 31 32:: 33 34 static struct i2c_device_id foo_idtable[] = { 35 { "foo", my_id_for_foo }, 36 { "bar", my_id_for_bar }, 37 { } 38 }; 39 40 MODULE_DEVICE_TABLE(i2c, foo_idtable); 41 42 static struct i2c_driver foo_driver = { 43 .driver = { 44 .name = "foo", 45 .pm = &foo_pm_ops, /* optional */ 46 }, 47 48 .id_table = foo_idtable, 49 .probe = foo_probe, 50 .remove = foo_remove, 51 /* if device autodetection is needed: */ 52 .class = I2C_CLASS_SOMETHING, 53 .detect = foo_detect, 54 .address_list = normal_i2c, 55 56 .shutdown = foo_shutdown, /* optional */ 57 .command = foo_command, /* optional, deprecated */ 58 } 59 60The name field is the driver name, and must not contain spaces. It 61should match the module name (if the driver can be compiled as a module), 62although you can use MODULE_ALIAS (passing "foo" in this example) to add 63another name for the module. If the driver name doesn't match the module 64name, the module won't be automatically loaded (hotplug/coldplug). 65 66All other fields are for call-back functions which will be explained 67below. 68 69 70Extra client data 71================= 72 73Each client structure has a special ``data`` field that can point to any 74structure at all. You should use this to keep device-specific data. 75 76:: 77 78 /* store the value */ 79 void i2c_set_clientdata(struct i2c_client *client, void *data); 80 81 /* retrieve the value */ 82 void *i2c_get_clientdata(const struct i2c_client *client); 83 84Note that starting with kernel 2.6.34, you don't have to set the ``data`` field 85to NULL in remove() or if probe() failed anymore. The i2c-core does this 86automatically on these occasions. Those are also the only times the core will 87touch this field. 88 89 90Accessing the client 91==================== 92 93Let's say we have a valid client structure. At some time, we will need 94to gather information from the client, or write new information to the 95client. 96 97I have found it useful to define foo_read and foo_write functions for this. 98For some cases, it will be easier to call the I2C functions directly, 99but many chips have some kind of register-value idea that can easily 100be encapsulated. 101 102The below functions are simple examples, and should not be copied 103literally:: 104 105 int foo_read_value(struct i2c_client *client, u8 reg) 106 { 107 if (reg < 0x10) /* byte-sized register */ 108 return i2c_smbus_read_byte_data(client, reg); 109 else /* word-sized register */ 110 return i2c_smbus_read_word_data(client, reg); 111 } 112 113 int foo_write_value(struct i2c_client *client, u8 reg, u16 value) 114 { 115 if (reg == 0x10) /* Impossible to write - driver error! */ 116 return -EINVAL; 117 else if (reg < 0x10) /* byte-sized register */ 118 return i2c_smbus_write_byte_data(client, reg, value); 119 else /* word-sized register */ 120 return i2c_smbus_write_word_data(client, reg, value); 121 } 122 123 124Probing and attaching 125===================== 126 127The Linux I2C stack was originally written to support access to hardware 128monitoring chips on PC motherboards, and thus used to embed some assumptions 129that were more appropriate to SMBus (and PCs) than to I2C. One of these 130assumptions was that most adapters and devices drivers support the SMBUS_QUICK 131protocol to probe device presence. Another was that devices and their drivers 132can be sufficiently configured using only such probe primitives. 133 134As Linux and its I2C stack became more widely used in embedded systems 135and complex components such as DVB adapters, those assumptions became more 136problematic. Drivers for I2C devices that issue interrupts need more (and 137different) configuration information, as do drivers handling chip variants 138that can't be distinguished by protocol probing, or which need some board 139specific information to operate correctly. 140 141 142Device/Driver Binding 143--------------------- 144 145System infrastructure, typically board-specific initialization code or 146boot firmware, reports what I2C devices exist. For example, there may be 147a table, in the kernel or from the boot loader, identifying I2C devices 148and linking them to board-specific configuration information about IRQs 149and other wiring artifacts, chip type, and so on. That could be used to 150create i2c_client objects for each I2C device. 151 152I2C device drivers using this binding model work just like any other 153kind of driver in Linux: they provide a probe() method to bind to 154those devices, and a remove() method to unbind. 155 156:: 157 158 static int foo_probe(struct i2c_client *client); 159 static void foo_remove(struct i2c_client *client); 160 161Remember that the i2c_driver does not create those client handles. The 162handle may be used during foo_probe(). If foo_probe() reports success 163(zero not a negative status code) it may save the handle and use it until 164foo_remove() returns. That binding model is used by most Linux drivers. 165 166The probe function is called when an entry in the id_table name field 167matches the device's name. If the probe function needs that entry, it 168can retrieve it using 169 170:: 171 172 const struct i2c_device_id *id = i2c_match_id(foo_idtable, client); 173 174 175Device Creation 176--------------- 177 178If you know for a fact that an I2C device is connected to a given I2C bus, 179you can instantiate that device by simply filling an i2c_board_info 180structure with the device address and driver name, and calling 181i2c_new_client_device(). This will create the device, then the driver core 182will take care of finding the right driver and will call its probe() method. 183If a driver supports different device types, you can specify the type you 184want using the type field. You can also specify an IRQ and platform data 185if needed. 186 187Sometimes you know that a device is connected to a given I2C bus, but you 188don't know the exact address it uses. This happens on TV adapters for 189example, where the same driver supports dozens of slightly different 190models, and I2C device addresses change from one model to the next. In 191that case, you can use the i2c_new_scanned_device() variant, which is 192similar to i2c_new_client_device(), except that it takes an additional list 193of possible I2C addresses to probe. A device is created for the first 194responsive address in the list. If you expect more than one device to be 195present in the address range, simply call i2c_new_scanned_device() that 196many times. 197 198The call to i2c_new_client_device() or i2c_new_scanned_device() typically 199happens in the I2C bus driver. You may want to save the returned i2c_client 200reference for later use. 201 202 203Device Detection 204---------------- 205 206Sometimes you do not know in advance which I2C devices are connected to 207a given I2C bus. This is for example the case of hardware monitoring 208devices on a PC's SMBus. In that case, you may want to let your driver 209detect supported devices automatically. This is how the legacy model 210was working, and is now available as an extension to the standard 211driver model. 212 213You simply have to define a detect callback which will attempt to 214identify supported devices (returning 0 for supported ones and -ENODEV 215for unsupported ones), a list of addresses to probe, and a device type 216(or class) so that only I2C buses which may have that type of device 217connected (and not otherwise enumerated) will be probed. For example, 218a driver for a hardware monitoring chip for which auto-detection is 219needed would set its class to I2C_CLASS_HWMON, and only I2C adapters 220with a class including I2C_CLASS_HWMON would be probed by this driver. 221Note that the absence of matching classes does not prevent the use of 222a device of that type on the given I2C adapter. All it prevents is 223auto-detection; explicit instantiation of devices is still possible. 224 225Note that this mechanism is purely optional and not suitable for all 226devices. You need some reliable way to identify the supported devices 227(typically using device-specific, dedicated identification registers), 228otherwise misdetections are likely to occur and things can get wrong 229quickly. Keep in mind that the I2C protocol doesn't include any 230standard way to detect the presence of a chip at a given address, let 231alone a standard way to identify devices. Even worse is the lack of 232semantics associated to bus transfers, which means that the same 233transfer can be seen as a read operation by a chip and as a write 234operation by another chip. For these reasons, explicit device 235instantiation should always be preferred to auto-detection where 236possible. 237 238 239Device Deletion 240--------------- 241 242Each I2C device which has been created using i2c_new_client_device() 243or i2c_new_scanned_device() can be unregistered by calling 244i2c_unregister_device(). If you don't call it explicitly, it will be 245called automatically before the underlying I2C bus itself is removed, 246as a device can't survive its parent in the device driver model. 247 248 249Initializing the driver 250======================= 251 252When the kernel is booted, or when your foo driver module is inserted, 253you have to do some initializing. Fortunately, just registering the 254driver module is usually enough. 255 256:: 257 258 static int __init foo_init(void) 259 { 260 return i2c_add_driver(&foo_driver); 261 } 262 module_init(foo_init); 263 264 static void __exit foo_cleanup(void) 265 { 266 i2c_del_driver(&foo_driver); 267 } 268 module_exit(foo_cleanup); 269 270 The module_i2c_driver() macro can be used to reduce above code. 271 272 module_i2c_driver(foo_driver); 273 274Note that some functions are marked by ``__init``. These functions can 275be removed after kernel booting (or module loading) is completed. 276Likewise, functions marked by ``__exit`` are dropped by the compiler when 277the code is built into the kernel, as they would never be called. 278 279 280Driver Information 281================== 282 283:: 284 285 /* Substitute your own name and email address */ 286 MODULE_AUTHOR("Frodo Looijaard <frodol@dds.nl>" 287 MODULE_DESCRIPTION("Driver for Barf Inc. Foo I2C devices"); 288 289 /* a few non-GPL license types are also allowed */ 290 MODULE_LICENSE("GPL"); 291 292 293Power Management 294================ 295 296If your I2C device needs special handling when entering a system low 297power state -- like putting a transceiver into a low power mode, or 298activating a system wakeup mechanism -- do that by implementing the 299appropriate callbacks for the dev_pm_ops of the driver (like suspend 300and resume). 301 302These are standard driver model calls, and they work just like they 303would for any other driver stack. The calls can sleep, and can use 304I2C messaging to the device being suspended or resumed (since their 305parent I2C adapter is active when these calls are issued, and IRQs 306are still enabled). 307 308 309System Shutdown 310=============== 311 312If your I2C device needs special handling when the system shuts down 313or reboots (including kexec) -- like turning something off -- use a 314shutdown() method. 315 316Again, this is a standard driver model call, working just like it 317would for any other driver stack: the calls can sleep, and can use 318I2C messaging. 319 320 321Command function 322================ 323 324A generic ioctl-like function call back is supported. You will seldom 325need this, and its use is deprecated anyway, so newer design should not 326use it. 327 328 329Sending and receiving 330===================== 331 332If you want to communicate with your device, there are several functions 333to do this. You can find all of them in <linux/i2c.h>. 334 335If you can choose between plain I2C communication and SMBus level 336communication, please use the latter. All adapters understand SMBus level 337commands, but only some of them understand plain I2C! 338 339 340Plain I2C communication 341----------------------- 342 343:: 344 345 int i2c_master_send(struct i2c_client *client, const char *buf, 346 int count); 347 int i2c_master_recv(struct i2c_client *client, char *buf, int count); 348 349These routines read and write some bytes from/to a client. The client 350contains the I2C address, so you do not have to include it. The second 351parameter contains the bytes to read/write, the third the number of bytes 352to read/write (must be less than the length of the buffer, also should be 353less than 64k since msg.len is u16.) Returned is the actual number of bytes 354read/written. 355 356:: 357 358 int i2c_transfer(struct i2c_adapter *adap, struct i2c_msg *msg, 359 int num); 360 361This sends a series of messages. Each message can be a read or write, 362and they can be mixed in any way. The transactions are combined: no 363stop condition is issued between transaction. The i2c_msg structure 364contains for each message the client address, the number of bytes of the 365message and the message data itself. 366 367You can read the file i2c-protocol.rst for more information about the 368actual I2C protocol. 369 370 371SMBus communication 372------------------- 373 374:: 375 376 s32 i2c_smbus_xfer(struct i2c_adapter *adapter, u16 addr, 377 unsigned short flags, char read_write, u8 command, 378 int size, union i2c_smbus_data *data); 379 380This is the generic SMBus function. All functions below are implemented 381in terms of it. Never use this function directly! 382 383:: 384 385 s32 i2c_smbus_read_byte(struct i2c_client *client); 386 s32 i2c_smbus_write_byte(struct i2c_client *client, u8 value); 387 s32 i2c_smbus_read_byte_data(struct i2c_client *client, u8 command); 388 s32 i2c_smbus_write_byte_data(struct i2c_client *client, 389 u8 command, u8 value); 390 s32 i2c_smbus_read_word_data(struct i2c_client *client, u8 command); 391 s32 i2c_smbus_write_word_data(struct i2c_client *client, 392 u8 command, u16 value); 393 s32 i2c_smbus_read_block_data(struct i2c_client *client, 394 u8 command, u8 *values); 395 s32 i2c_smbus_write_block_data(struct i2c_client *client, 396 u8 command, u8 length, const u8 *values); 397 s32 i2c_smbus_read_i2c_block_data(struct i2c_client *client, 398 u8 command, u8 length, u8 *values); 399 s32 i2c_smbus_write_i2c_block_data(struct i2c_client *client, 400 u8 command, u8 length, 401 const u8 *values); 402 403These ones were removed from i2c-core because they had no users, but could 404be added back later if needed:: 405 406 s32 i2c_smbus_write_quick(struct i2c_client *client, u8 value); 407 s32 i2c_smbus_process_call(struct i2c_client *client, 408 u8 command, u16 value); 409 s32 i2c_smbus_block_process_call(struct i2c_client *client, 410 u8 command, u8 length, u8 *values); 411 412All these transactions return a negative errno value on failure. The 'write' 413transactions return 0 on success; the 'read' transactions return the read 414value, except for block transactions, which return the number of values 415read. The block buffers need not be longer than 32 bytes. 416 417You can read the file smbus-protocol.rst for more information about the 418actual SMBus protocol. 419 420 421General purpose routines 422======================== 423 424Below all general purpose routines are listed, that were not mentioned 425before:: 426 427 /* Return the adapter number for a specific adapter */ 428 int i2c_adapter_id(struct i2c_adapter *adap); 429