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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
-tutorial.
-
-
-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: */
- .class = I2C_CLASS_SOMETHING,
- .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
-below.
-
-
-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
-client.
-
-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
-literally.
-
-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
-possible.
-
-
-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);
-}
-
-/* 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 */
-MODULE_LICENSE("GPL");
-
-module_init(foo_init);
-module_exit(foo_cleanup);
-
-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.
-
-
-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
-read/written.
-
- 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_process_call(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_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
-before.
-
- /* Return the adapter number for a specific adapter */
- int i2c_adapter_id(struct i2c_adapter *adap);