linux/Documentation/i2c/writing-clients
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   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.
  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_device_id foo_idtable[] = {
  29        { "foo", my_id_for_foo },
  30        { "bar", my_id_for_bar },
  31        { }
  32};
  33
  34MODULE_DEVICE_TABLE(i2c, foo_idtable);
  35
  36static struct i2c_driver foo_driver = {
  37        .driver = {
  38                .name   = "foo",
  39        },
  40
  41        .id_table       = foo_idtable,
  42        .probe          = foo_probe,
  43        .remove         = foo_remove,
  44        /* if device autodetection is needed: */
  45        .class          = I2C_CLASS_SOMETHING,
  46        .detect         = foo_detect,
  47        .address_list   = normal_i2c,
  48
  49        .shutdown       = foo_shutdown, /* optional */
  50        .suspend        = foo_suspend,  /* optional */
  51        .resume         = foo_resume,   /* optional */
  52        .command        = foo_command,  /* optional, deprecated */
  53}
  54
  55The name field is the driver name, and must not contain spaces.  It
  56should match the module name (if the driver can be compiled as a module),
  57although you can use MODULE_ALIAS (passing "foo" in this example) to add
  58another name for the module.  If the driver name doesn't match the module
  59name, the module won't be automatically loaded (hotplug/coldplug).
  60
  61All other fields are for call-back functions which will be explained
  62below.
  63
  64
  65Extra client data
  66=================
  67
  68Each client structure has a special `data' field that can point to any
  69structure at all.  You should use this to keep device-specific data.
  70
  71        /* store the value */
  72        void i2c_set_clientdata(struct i2c_client *client, void *data);
  73
  74        /* retrieve the value */
  75        void *i2c_get_clientdata(const struct i2c_client *client);
  76
  77Note that starting with kernel 2.6.34, you don't have to set the `data' field
  78to NULL in remove() or if probe() failed anymore. The i2c-core does this
  79automatically on these occasions. Those are also the only times the core will
  80touch this field.
  81
  82
  83Accessing the client
  84====================
  85
  86Let's say we have a valid client structure. At some time, we will need
  87to gather information from the client, or write new information to the
  88client.
  89
  90I have found it useful to define foo_read and foo_write functions for this.
  91For some cases, it will be easier to call the i2c functions directly,
  92but many chips have some kind of register-value idea that can easily
  93be encapsulated.
  94
  95The below functions are simple examples, and should not be copied
  96literally.
  97
  98int foo_read_value(struct i2c_client *client, u8 reg)
  99{
 100        if (reg < 0x10) /* byte-sized register */
 101                return i2c_smbus_read_byte_data(client, reg);
 102        else            /* word-sized register */
 103                return i2c_smbus_read_word_data(client, reg);
 104}
 105
 106int foo_write_value(struct i2c_client *client, u8 reg, u16 value)
 107{
 108        if (reg == 0x10)        /* Impossible to write - driver error! */
 109                return -EINVAL;
 110        else if (reg < 0x10)    /* byte-sized register */
 111                return i2c_smbus_write_byte_data(client, reg, value);
 112        else                    /* word-sized register */
 113                return i2c_smbus_write_word_data(client, reg, value);
 114}
 115
 116
 117Probing and attaching
 118=====================
 119
 120The Linux I2C stack was originally written to support access to hardware
 121monitoring chips on PC motherboards, and thus used to embed some assumptions
 122that were more appropriate to SMBus (and PCs) than to I2C.  One of these
 123assumptions was that most adapters and devices drivers support the SMBUS_QUICK
 124protocol to probe device presence.  Another was that devices and their drivers
 125can be sufficiently configured using only such probe primitives.
 126
 127As Linux and its I2C stack became more widely used in embedded systems
 128and complex components such as DVB adapters, those assumptions became more
 129problematic.  Drivers for I2C devices that issue interrupts need more (and
 130different) configuration information, as do drivers handling chip variants
 131that can't be distinguished by protocol probing, or which need some board
 132specific information to operate correctly.
 133
 134
 135Device/Driver Binding
 136---------------------
 137
 138System infrastructure, typically board-specific initialization code or
 139boot firmware, reports what I2C devices exist.  For example, there may be
 140a table, in the kernel or from the boot loader, identifying I2C devices
 141and linking them to board-specific configuration information about IRQs
 142and other wiring artifacts, chip type, and so on.  That could be used to
 143create i2c_client objects for each I2C device.
 144
 145I2C device drivers using this binding model work just like any other
 146kind of driver in Linux:  they provide a probe() method to bind to
 147those devices, and a remove() method to unbind.
 148
 149        static int foo_probe(struct i2c_client *client,
 150                             const struct i2c_device_id *id);
 151        static int foo_remove(struct i2c_client *client);
 152
 153Remember that the i2c_driver does not create those client handles.  The
 154handle may be used during foo_probe().  If foo_probe() reports success
 155(zero not a negative status code) it may save the handle and use it until
 156foo_remove() returns.  That binding model is used by most Linux drivers.
 157
 158The probe function is called when an entry in the id_table name field
 159matches the device's name. It is passed the entry that was matched so
 160the driver knows which one in the table matched.
 161
 162
 163Device Creation
 164---------------
 165
 166If you know for a fact that an I2C device is connected to a given I2C bus,
 167you can instantiate that device by simply filling an i2c_board_info
 168structure with the device address and driver name, and calling
 169i2c_new_device().  This will create the device, then the driver core will
 170take care of finding the right driver and will call its probe() method.
 171If a driver supports different device types, you can specify the type you
 172want using the type field.  You can also specify an IRQ and platform data
 173if needed.
 174
 175Sometimes you know that a device is connected to a given I2C bus, but you
 176don't know the exact address it uses.  This happens on TV adapters for
 177example, where the same driver supports dozens of slightly different
 178models, and I2C device addresses change from one model to the next.  In
 179that case, you can use the i2c_new_probed_device() variant, which is
 180similar to i2c_new_device(), except that it takes an additional list of
 181possible I2C addresses to probe.  A device is created for the first
 182responsive address in the list.  If you expect more than one device to be
 183present in the address range, simply call i2c_new_probed_device() that
 184many times.
 185
 186The call to i2c_new_device() or i2c_new_probed_device() typically happens
 187in the I2C bus driver. You may want to save the returned i2c_client
 188reference for later use.
 189
 190
 191Device Detection
 192----------------
 193
 194Sometimes you do not know in advance which I2C devices are connected to
 195a given I2C bus.  This is for example the case of hardware monitoring
 196devices on a PC's SMBus.  In that case, you may want to let your driver
 197detect supported devices automatically.  This is how the legacy model
 198was working, and is now available as an extension to the standard
 199driver model.
 200
 201You simply have to define a detect callback which will attempt to
 202identify supported devices (returning 0 for supported ones and -ENODEV
 203for unsupported ones), a list of addresses to probe, and a device type
 204(or class) so that only I2C buses which may have that type of device
 205connected (and not otherwise enumerated) will be probed.  For example,
 206a driver for a hardware monitoring chip for which auto-detection is
 207needed would set its class to I2C_CLASS_HWMON, and only I2C adapters
 208with a class including I2C_CLASS_HWMON would be probed by this driver.
 209Note that the absence of matching classes does not prevent the use of
 210a device of that type on the given I2C adapter.  All it prevents is
 211auto-detection; explicit instantiation of devices is still possible.
 212
 213Note that this mechanism is purely optional and not suitable for all
 214devices.  You need some reliable way to identify the supported devices
 215(typically using device-specific, dedicated identification registers),
 216otherwise misdetections are likely to occur and things can get wrong
 217quickly.  Keep in mind that the I2C protocol doesn't include any
 218standard way to detect the presence of a chip at a given address, let
 219alone a standard way to identify devices.  Even worse is the lack of
 220semantics associated to bus transfers, which means that the same
 221transfer can be seen as a read operation by a chip and as a write
 222operation by another chip.  For these reasons, explicit device
 223instantiation should always be preferred to auto-detection where
 224possible.
 225
 226
 227Device Deletion
 228---------------
 229
 230Each I2C device which has been created using i2c_new_device() or
 231i2c_new_probed_device() can be unregistered by calling
 232i2c_unregister_device().  If you don't call it explicitly, it will be
 233called automatically before the underlying I2C bus itself is removed, as a
 234device can't survive its parent in the device driver model.
 235
 236
 237Initializing the driver
 238=======================
 239
 240When the kernel is booted, or when your foo driver module is inserted,
 241you have to do some initializing. Fortunately, just registering the
 242driver module is usually enough.
 243
 244static int __init foo_init(void)
 245{
 246        return i2c_add_driver(&foo_driver);
 247}
 248module_init(foo_init);
 249
 250static void __exit foo_cleanup(void)
 251{
 252        i2c_del_driver(&foo_driver);
 253}
 254module_exit(foo_cleanup);
 255
 256The module_i2c_driver() macro can be used to reduce above code.
 257
 258module_i2c_driver(foo_driver);
 259
 260Note that some functions are marked by `__init'.  These functions can
 261be removed after kernel booting (or module loading) is completed.
 262Likewise, functions marked by `__exit' are dropped by the compiler when
 263the code is built into the kernel, as they would never be called.
 264
 265
 266Driver Information
 267==================
 268
 269/* Substitute your own name and email address */
 270MODULE_AUTHOR("Frodo Looijaard <frodol@dds.nl>"
 271MODULE_DESCRIPTION("Driver for Barf Inc. Foo I2C devices");
 272
 273/* a few non-GPL license types are also allowed */
 274MODULE_LICENSE("GPL");
 275
 276
 277Power Management
 278================
 279
 280If your I2C device needs special handling when entering a system low
 281power state -- like putting a transceiver into a low power mode, or
 282activating a system wakeup mechanism -- do that in the suspend() method.
 283The resume() method should reverse what the suspend() method does.
 284
 285These are standard driver model calls, and they work just like they
 286would for any other driver stack.  The calls can sleep, and can use
 287I2C messaging to the device being suspended or resumed (since their
 288parent I2C adapter is active when these calls are issued, and IRQs
 289are still enabled).
 290
 291
 292System Shutdown
 293===============
 294
 295If your I2C device needs special handling when the system shuts down
 296or reboots (including kexec) -- like turning something off -- use a
 297shutdown() method.
 298
 299Again, this is a standard driver model call, working just like it
 300would for any other driver stack:  the calls can sleep, and can use
 301I2C messaging.
 302
 303
 304Command function
 305================
 306
 307A generic ioctl-like function call back is supported. You will seldom
 308need this, and its use is deprecated anyway, so newer design should not
 309use it.
 310
 311
 312Sending and receiving
 313=====================
 314
 315If you want to communicate with your device, there are several functions
 316to do this. You can find all of them in <linux/i2c.h>.
 317
 318If you can choose between plain I2C communication and SMBus level
 319communication, please use the latter. All adapters understand SMBus level
 320commands, but only some of them understand plain I2C!
 321
 322
 323Plain I2C communication
 324-----------------------
 325
 326        int i2c_master_send(struct i2c_client *client, const char *buf,
 327                            int count);
 328        int i2c_master_recv(struct i2c_client *client, char *buf, int count);
 329
 330These routines read and write some bytes from/to a client. The client
 331contains the i2c address, so you do not have to include it. The second
 332parameter contains the bytes to read/write, the third the number of bytes
 333to read/write (must be less than the length of the buffer, also should be
 334less than 64k since msg.len is u16.) Returned is the actual number of bytes
 335read/written.
 336
 337        int i2c_transfer(struct i2c_adapter *adap, struct i2c_msg *msg,
 338                         int num);
 339
 340This sends a series of messages. Each message can be a read or write,
 341and they can be mixed in any way. The transactions are combined: no
 342stop bit is sent between transaction. The i2c_msg structure contains
 343for each message the client address, the number of bytes of the message
 344and the message data itself.
 345
 346You can read the file `i2c-protocol' for more information about the
 347actual I2C protocol.
 348
 349
 350SMBus communication
 351-------------------
 352
 353        s32 i2c_smbus_xfer(struct i2c_adapter *adapter, u16 addr,
 354                           unsigned short flags, char read_write, u8 command,
 355                           int size, union i2c_smbus_data *data);
 356
 357This is the generic SMBus function. All functions below are implemented
 358in terms of it. Never use this function directly!
 359
 360        s32 i2c_smbus_read_byte(struct i2c_client *client);
 361        s32 i2c_smbus_write_byte(struct i2c_client *client, u8 value);
 362        s32 i2c_smbus_read_byte_data(struct i2c_client *client, u8 command);
 363        s32 i2c_smbus_write_byte_data(struct i2c_client *client,
 364                                      u8 command, u8 value);
 365        s32 i2c_smbus_read_word_data(struct i2c_client *client, u8 command);
 366        s32 i2c_smbus_write_word_data(struct i2c_client *client,
 367                                      u8 command, u16 value);
 368        s32 i2c_smbus_read_block_data(struct i2c_client *client,
 369                                      u8 command, u8 *values);
 370        s32 i2c_smbus_write_block_data(struct i2c_client *client,
 371                                       u8 command, u8 length, const u8 *values);
 372        s32 i2c_smbus_read_i2c_block_data(struct i2c_client *client,
 373                                          u8 command, u8 length, u8 *values);
 374        s32 i2c_smbus_write_i2c_block_data(struct i2c_client *client,
 375                                           u8 command, u8 length,
 376                                           const u8 *values);
 377
 378These ones were removed from i2c-core because they had no users, but could
 379be added back later if needed:
 380
 381        s32 i2c_smbus_write_quick(struct i2c_client *client, u8 value);
 382        s32 i2c_smbus_process_call(struct i2c_client *client,
 383                                   u8 command, u16 value);
 384        s32 i2c_smbus_block_process_call(struct i2c_client *client,
 385                                         u8 command, u8 length, u8 *values);
 386
 387All these transactions return a negative errno value on failure. The 'write'
 388transactions return 0 on success; the 'read' transactions return the read
 389value, except for block transactions, which return the number of values
 390read. The block buffers need not be longer than 32 bytes.
 391
 392You can read the file `smbus-protocol' for more information about the
 393actual SMBus protocol.
 394
 395
 396General purpose routines
 397========================
 398
 399Below all general purpose routines are listed, that were not mentioned
 400before.
 401
 402        /* Return the adapter number for a specific adapter */
 403        int i2c_adapter_id(struct i2c_adapter *adap);
 404
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