How to instantiate I2C devices
==============================

Unlike PCI or USB devices, I2C devices are not enumerated at the hardware
level. Instead, the software must know which devices are connected on each
I2C bus segment, and what address these devices are using. For this
reason, the kernel code must instantiate I2C devices explicitly. There are
several ways to achieve this, depending on the context and requirements.


Method 1: Declare the I2C devices by bus number
-----------------------------------------------

This method is appropriate when the I2C bus is a system bus as is the case
for many embedded systems. On such systems, each I2C bus has a number
which is known in advance. It is thus possible to pre-declare the I2C
devices which live on this bus. This is done with an array of struct
i2c_board_info which is registered by calling i2c_register_board_info().

Example (from omap2 h4):

static struct i2c_board_info __initdata h4_i2c_board_info[] = {
	{
		I2C_BOARD_INFO("isp1301_omap", 0x2d),
		.irq		= OMAP_GPIO_IRQ(125),
	},
	{	/* EEPROM on mainboard */
		I2C_BOARD_INFO("24c01", 0x52),
		.platform_data	= &m24c01,
	},
	{	/* EEPROM on cpu card */
		I2C_BOARD_INFO("24c01", 0x57),
		.platform_data	= &m24c01,
	},
};

static void __init omap_h4_init(void)
{
	(...)
	i2c_register_board_info(1, h4_i2c_board_info,
			ARRAY_SIZE(h4_i2c_board_info));
	(...)
}

The above code declares 3 devices on I2C bus 1, including their respective
addresses and custom data needed by their drivers. When the I2C bus in
question is registered, the I2C devices will be instantiated automatically
by i2c-core.

The devices will be automatically unbound and destroyed when the I2C bus
they sit on goes away (if ever.)


Method 2: Instantiate the devices explicitly
--------------------------------------------

This method is appropriate when a larger device uses an I2C bus for
internal communication. A typical case is TV adapters. These can have a
tuner, a video decoder, an audio decoder, etc. usually connected to the
main chip by the means of an I2C bus. You won't know the number of the I2C
bus in advance, so the method 1 described above can't be used. Instead,
you can instantiate your I2C devices explicitly. This is done by filling
a struct i2c_board_info and calling i2c_new_device().

Example (from the sfe4001 network driver):

static struct i2c_board_info sfe4001_hwmon_info = {
	I2C_BOARD_INFO("max6647", 0x4e),
};

int sfe4001_init(struct efx_nic *efx)
{
	(...)
	efx->board_info.hwmon_client =
		i2c_new_device(&efx->i2c_adap, &sfe4001_hwmon_info);

	(...)
}

The above code instantiates 1 I2C device on the I2C bus which is on the
network adapter in question.

A variant of this is when you don't know for sure if an I2C device is
present or not (for example for an optional feature which is not present
on cheap variants of a board but you have no way to tell them apart), or
it may have different addresses from one board to the next (manufacturer
changing its design without notice). In this case, you can call
i2c_new_probed_device() instead of i2c_new_device().

Example (from the pnx4008 OHCI driver):

static const unsigned short normal_i2c[] = { 0x2c, 0x2d, I2C_CLIENT_END };

static int __devinit usb_hcd_pnx4008_probe(struct platform_device *pdev)
{
	(...)
	struct i2c_adapter *i2c_adap;
	struct i2c_board_info i2c_info;

	(...)
	i2c_adap = i2c_get_adapter(2);
	memset(&i2c_info, 0, sizeof(struct i2c_board_info));
	strlcpy(i2c_info.name, "isp1301_pnx", I2C_NAME_SIZE);
	isp1301_i2c_client = i2c_new_probed_device(i2c_adap, &i2c_info,
						   normal_i2c);
	i2c_put_adapter(i2c_adap);
	(...)
}

The above code instantiates up to 1 I2C device on the I2C bus which is on
the OHCI adapter in question. It first tries at address 0x2c, if nothing
is found there it tries address 0x2d, and if still nothing is found, it
simply gives up.

The driver which instantiated the I2C device is responsible for destroying
it on cleanup. This is done by calling i2c_unregister_device() on the
pointer that was earlier returned by i2c_new_device() or
i2c_new_probed_device().


Method 3: Probe an I2C bus for certain devices
----------------------------------------------

Sometimes you do not have enough information about an I2C device, not even
to call i2c_new_probed_device(). The typical case is hardware monitoring
chips on PC mainboards. There are several dozen models, which can live
at 25 different addresses. Given the huge number of mainboards out there,
it is next to impossible to build an exhaustive list of the hardware
monitoring chips being used. Fortunately, most of these chips have
manufacturer and device ID registers, so they can be identified by
probing.

In that case, I2C devices are neither declared nor instantiated
explicitly. Instead, i2c-core will probe for such devices as soon as their
drivers are loaded, and if any is found, an I2C device will be
instantiated automatically. In order to prevent any misbehavior of this
mechanism, the following restrictions apply:
* The I2C device driver must implement the detect() method, which
  identifies a supported device by reading from arbitrary registers.
* Only buses which are likely to have a supported device and agree to be
  probed, will be probed. For example this avoids probing for hardware
  monitoring chips on a TV adapter.

Example:
See lm90_driver and lm90_detect() in drivers/hwmon/lm90.c

I2C devices instantiated as a result of such a successful probe will be
destroyed automatically when the driver which detected them is removed,
or when the underlying I2C bus is itself destroyed, whichever happens
first.

Those of you familiar with the i2c subsystem of 2.4 kernels and early 2.6
kernels will find out that this method 3 is essentially similar to what
was done there. Two significant differences are:
* Probing is only one way to instantiate I2C devices now, while it was the
  only way back then. Where possible, methods 1 and 2 should be preferred.
  Method 3 should only be used when there is no other way, as it can have
  undesirable side effects.
* I2C buses must now explicitly say which I2C driver classes can probe
  them (by the means of the class bitfield), while all I2C buses were
  probed by default back then. The default is an empty class which means
  that no probing happens. The purpose of the class bitfield is to limit
  the aforementioned undesirable side effects.

Once again, method 3 should be avoided wherever possible. Explicit device
instantiation (methods 1 and 2) is much preferred for it is safer and
faster.
