Documentation/i2c/writing-clients

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