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+This is a small guide for those who want to write kernel drivers for I2C
+or SMBus devices, using Linux as the protocol host/master (not slave).
+To set up a driver, you need to do several things. Some are optional, and
+some things can be done slightly or completely different. Use this as a
+guide, not as a rule book!
+General remarks
+Try to keep the kernel namespace as clean as possible. The best way to
+do this is to use a unique prefix for all global symbols. This is
+especially important for exported symbols, but it is a good idea to do
+it for non-exported symbols too. We will use the prefix `foo_' in this
+The driver structure
+Usually, you will implement a single driver structure, and instantiate
+all clients from it. Remember, a driver structure contains general access
+routines, and should be zero-initialized except for fields with data you
+provide. A client structure holds device-specific information like the
+driver model device node, and its I2C address.
+static struct i2c_device_id foo_idtable[] = {
+ { "foo", my_id_for_foo },
+ { "bar", my_id_for_bar },
+ { }
+MODULE_DEVICE_TABLE(i2c, foo_idtable);
+static struct i2c_driver foo_driver = {
+ .driver = {
+ .name = "foo",
+ },
+ .id_table = foo_idtable,
+ .probe = foo_probe,
+ .remove = foo_remove,
+ /* if device autodetection is needed: */
+ .detect = foo_detect,
+ .address_list = normal_i2c,
+ .shutdown = foo_shutdown, /* optional */
+ .suspend = foo_suspend, /* optional */
+ .resume = foo_resume, /* optional */
+ .command = foo_command, /* optional, deprecated */
+The name field is the driver name, and must not contain spaces. It
+should match the module name (if the driver can be compiled as a module),
+although you can use MODULE_ALIAS (passing "foo" in this example) to add
+another name for the module. If the driver name doesn't match the module
+name, the module won't be automatically loaded (hotplug/coldplug).
+All other fields are for call-back functions which will be explained
+Extra client data
+Each client structure has a special `data' field that can point to any
+structure at all. You should use this to keep device-specific data.
+ /* store the value */
+ void i2c_set_clientdata(struct i2c_client *client, void *data);
+ /* retrieve the value */
+ void *i2c_get_clientdata(const struct i2c_client *client);
+Note that starting with kernel 2.6.34, you don't have to set the `data' field
+to NULL in remove() or if probe() failed anymore. The i2c-core does this
+automatically on these occasions. Those are also the only times the core will
+touch this field.
+Accessing the client
+Let's say we have a valid client structure. At some time, we will need
+to gather information from the client, or write new information to the
+I have found it useful to define foo_read and foo_write functions for this.
+For some cases, it will be easier to call the i2c functions directly,
+but many chips have some kind of register-value idea that can easily
+be encapsulated.
+The below functions are simple examples, and should not be copied
+int foo_read_value(struct i2c_client *client, u8 reg)
+ if (reg < 0x10) /* byte-sized register */
+ return i2c_smbus_read_byte_data(client, reg);
+ else /* word-sized register */
+ return i2c_smbus_read_word_data(client, reg);
+int foo_write_value(struct i2c_client *client, u8 reg, u16 value)
+ if (reg == 0x10) /* Impossible to write - driver error! */
+ return -EINVAL;
+ else if (reg < 0x10) /* byte-sized register */
+ return i2c_smbus_write_byte_data(client, reg, value);
+ else /* word-sized register */
+ return i2c_smbus_write_word_data(client, reg, value);
+Probing and attaching
+The Linux I2C stack was originally written to support access to hardware
+monitoring chips on PC motherboards, and thus used to embed some assumptions
+that were more appropriate to SMBus (and PCs) than to I2C. One of these
+assumptions was that most adapters and devices drivers support the SMBUS_QUICK
+protocol to probe device presence. Another was that devices and their drivers
+can be sufficiently configured using only such probe primitives.
+As Linux and its I2C stack became more widely used in embedded systems
+and complex components such as DVB adapters, those assumptions became more
+problematic. Drivers for I2C devices that issue interrupts need more (and
+different) configuration information, as do drivers handling chip variants
+that can't be distinguished by protocol probing, or which need some board
+specific information to operate correctly.
+Device/Driver Binding
+System infrastructure, typically board-specific initialization code or
+boot firmware, reports what I2C devices exist. For example, there may be
+a table, in the kernel or from the boot loader, identifying I2C devices
+and linking them to board-specific configuration information about IRQs
+and other wiring artifacts, chip type, and so on. That could be used to
+create i2c_client objects for each I2C device.
+I2C device drivers using this binding model work just like any other
+kind of driver in Linux: they provide a probe() method to bind to
+those devices, and a remove() method to unbind.
+ static int foo_probe(struct i2c_client *client,
+ const struct i2c_device_id *id);
+ static int foo_remove(struct i2c_client *client);
+Remember that the i2c_driver does not create those client handles. The
+handle may be used during foo_probe(). If foo_probe() reports success
+(zero not a negative status code) it may save the handle and use it until
+foo_remove() returns. That binding model is used by most Linux drivers.
+The probe function is called when an entry in the id_table name field
+matches the device's name. It is passed the entry that was matched so
+the driver knows which one in the table matched.
+Device Creation
+If you know for a fact that an I2C device is connected to a given I2C bus,
+you can instantiate that device by simply filling an i2c_board_info
+structure with the device address and driver name, and calling
+i2c_new_device(). This will create the device, then the driver core will
+take care of finding the right driver and will call its probe() method.
+If a driver supports different device types, you can specify the type you
+want using the type field. You can also specify an IRQ and platform data
+if needed.
+Sometimes you know that a device is connected to a given I2C bus, but you
+don't know the exact address it uses. This happens on TV adapters for
+example, where the same driver supports dozens of slightly different
+models, and I2C device addresses change from one model to the next. In
+that case, you can use the i2c_new_probed_device() variant, which is
+similar to i2c_new_device(), except that it takes an additional list of
+possible I2C addresses to probe. A device is created for the first
+responsive address in the list. If you expect more than one device to be
+present in the address range, simply call i2c_new_probed_device() that
+many times.
+The call to i2c_new_device() or i2c_new_probed_device() typically happens
+in the I2C bus driver. You may want to save the returned i2c_client
+reference for later use.
+Device Detection
+Sometimes you do not know in advance which I2C devices are connected to
+a given I2C bus. This is for example the case of hardware monitoring
+devices on a PC's SMBus. In that case, you may want to let your driver
+detect supported devices automatically. This is how the legacy model
+was working, and is now available as an extension to the standard
+driver model.
+You simply have to define a detect callback which will attempt to
+identify supported devices (returning 0 for supported ones and -ENODEV
+for unsupported ones), a list of addresses to probe, and a device type
+(or class) so that only I2C buses which may have that type of device
+connected (and not otherwise enumerated) will be probed. For example,
+a driver for a hardware monitoring chip for which auto-detection is
+needed would set its class to I2C_CLASS_HWMON, and only I2C adapters
+with a class including I2C_CLASS_HWMON would be probed by this driver.
+Note that the absence of matching classes does not prevent the use of
+a device of that type on the given I2C adapter. All it prevents is
+auto-detection; explicit instantiation of devices is still possible.
+Note that this mechanism is purely optional and not suitable for all
+devices. You need some reliable way to identify the supported devices
+(typically using device-specific, dedicated identification registers),
+otherwise misdetections are likely to occur and things can get wrong
+quickly. Keep in mind that the I2C protocol doesn't include any
+standard way to detect the presence of a chip at a given address, let
+alone a standard way to identify devices. Even worse is the lack of
+semantics associated to bus transfers, which means that the same
+transfer can be seen as a read operation by a chip and as a write
+operation by another chip. For these reasons, explicit device
+instantiation should always be preferred to auto-detection where
+Device Deletion
+Each I2C device which has been created using i2c_new_device() or
+i2c_new_probed_device() can be unregistered by calling
+i2c_unregister_device(). If you don't call it explicitly, it will be
+called automatically before the underlying I2C bus itself is removed, as a
+device can't survive its parent in the device driver model.
+Initializing the driver
+When the kernel is booted, or when your foo driver module is inserted,
+you have to do some initializing. Fortunately, just registering the
+driver module is usually enough.
+static int __init foo_init(void)
+ return i2c_add_driver(&foo_driver);
+static void __exit foo_cleanup(void)
+ i2c_del_driver(&foo_driver);
+The module_i2c_driver() macro can be used to reduce above code.
+Note that some functions are marked by `__init'. These functions can
+be removed after kernel booting (or module loading) is completed.
+Likewise, functions marked by `__exit' are dropped by the compiler when
+the code is built into the kernel, as they would never be called.
+Driver Information
+/* Substitute your own name and email address */
+MODULE_AUTHOR("Frodo Looijaard <frodol@dds.nl>"
+MODULE_DESCRIPTION("Driver for Barf Inc. Foo I2C devices");
+/* a few non-GPL license types are also allowed */
+Power Management
+If your I2C device needs special handling when entering a system low
+power state -- like putting a transceiver into a low power mode, or
+activating a system wakeup mechanism -- do that in the suspend() method.
+The resume() method should reverse what the suspend() method does.
+These are standard driver model calls, and they work just like they
+would for any other driver stack. The calls can sleep, and can use
+I2C messaging to the device being suspended or resumed (since their
+parent I2C adapter is active when these calls are issued, and IRQs
+are still enabled).
+System Shutdown
+If your I2C device needs special handling when the system shuts down
+or reboots (including kexec) -- like turning something off -- use a
+shutdown() method.
+Again, this is a standard driver model call, working just like it
+would for any other driver stack: the calls can sleep, and can use
+I2C messaging.
+Command function
+A generic ioctl-like function call back is supported. You will seldom
+need this, and its use is deprecated anyway, so newer design should not
+use it.
+Sending and receiving
+If you want to communicate with your device, there are several functions
+to do this. You can find all of them in <linux/i2c.h>.
+If you can choose between plain I2C communication and SMBus level
+communication, please use the latter. All adapters understand SMBus level
+commands, but only some of them understand plain I2C!
+Plain I2C communication
+ int i2c_master_send(struct i2c_client *client, const char *buf,
+ int count);
+ int i2c_master_recv(struct i2c_client *client, char *buf, int count);
+These routines read and write some bytes from/to a client. The client
+contains the i2c address, so you do not have to include it. The second
+parameter contains the bytes to read/write, the third the number of bytes
+to read/write (must be less than the length of the buffer, also should be
+less than 64k since msg.len is u16.) Returned is the actual number of bytes
+ int i2c_transfer(struct i2c_adapter *adap, struct i2c_msg *msg,
+ int num);
+This sends a series of messages. Each message can be a read or write,
+and they can be mixed in any way. The transactions are combined: no
+stop bit is sent between transaction. The i2c_msg structure contains
+for each message the client address, the number of bytes of the message
+and the message data itself.
+You can read the file `i2c-protocol' for more information about the
+actual I2C protocol.
+SMBus communication
+ s32 i2c_smbus_xfer(struct i2c_adapter *adapter, u16 addr,
+ unsigned short flags, char read_write, u8 command,
+ int size, union i2c_smbus_data *data);
+This is the generic SMBus function. All functions below are implemented
+in terms of it. Never use this function directly!
+ s32 i2c_smbus_read_byte(struct i2c_client *client);
+ s32 i2c_smbus_write_byte(struct i2c_client *client, u8 value);
+ s32 i2c_smbus_read_byte_data(struct i2c_client *client, u8 command);
+ s32 i2c_smbus_write_byte_data(struct i2c_client *client,
+ u8 command, u8 value);
+ s32 i2c_smbus_read_word_data(struct i2c_client *client, u8 command);
+ s32 i2c_smbus_write_word_data(struct i2c_client *client,
+ u8 command, u16 value);
+ s32 i2c_smbus_read_block_data(struct i2c_client *client,
+ u8 command, u8 *values);
+ s32 i2c_smbus_write_block_data(struct i2c_client *client,
+ u8 command, u8 length, const u8 *values);
+ s32 i2c_smbus_read_i2c_block_data(struct i2c_client *client,
+ u8 command, u8 length, u8 *values);
+ s32 i2c_smbus_write_i2c_block_data(struct i2c_client *client,
+ u8 command, u8 length,
+ const u8 *values);
+These ones were removed from i2c-core because they had no users, but could
+be added back later if needed:
+ s32 i2c_smbus_write_quick(struct i2c_client *client, u8 value);
+ s32 i2c_smbus_process_call(struct i2c_client *client,
+ u8 command, u16 value);
+ s32 i2c_smbus_block_process_call(struct i2c_client *client,
+ u8 command, u8 length, u8 *values);
+All these transactions return a negative errno value on failure. The 'write'
+transactions return 0 on success; the 'read' transactions return the read
+value, except for block transactions, which return the number of values
+read. The block buffers need not be longer than 32 bytes.
+You can read the file `smbus-protocol' for more information about the
+actual SMBus protocol.
+General purpose routines
+Below all general purpose routines are listed, that were not mentioned
+ /* Return the adapter number for a specific adapter */
+ int i2c_adapter_id(struct i2c_adapter *adap);