i2c: Document the different ways to instantiate i2c devices

On popular demand, here comes some documentation about how to
instantiate i2c devices in the new (standard) i2c device driver
binding model.

I have also clarified how the class bitfield lets driver authors
control which buses are probed in the auto-detect case, and warned
more loudly against the abuse of this method.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Acked-by: Michael Lawnick <nospam_lawnick@gmx.de>
Acked-by: Hans Verkuil <hverkuil@xs4all.nl>

1 files changed, 167 insertions, 0 deletions

diff --git a/Documentation/i2c/instantiating-devices b/Documentation/i2c/instantiating-devices
new file mode 100644
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+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.