aboutsummaryrefslogtreecommitdiff
path: root/Documentation/i2c/instantiating-devices
diff options
context:
space:
mode:
Diffstat (limited to 'Documentation/i2c/instantiating-devices')
-rw-r--r--Documentation/i2c/instantiating-devices211
1 files changed, 211 insertions, 0 deletions
diff --git a/Documentation/i2c/instantiating-devices b/Documentation/i2c/instantiating-devices
new file mode 100644
index 00000000..22182660
--- /dev/null
+++ b/Documentation/i2c/instantiating-devices
@@ -0,0 +1,211 @@
+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 nxp OHCI driver):
+
+static const unsigned short normal_i2c[] = { 0x2c, 0x2d, I2C_CLIENT_END };
+
+static int usb_hcd_nxp_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.type, "isp1301_nxp", I2C_NAME_SIZE);
+ isp1301_i2c_client = i2c_new_probed_device(i2c_adap, &i2c_info,
+ normal_i2c, NULL);
+ 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.
+
+
+Method 4: Instantiate from user-space
+-------------------------------------
+
+In general, the kernel should know which I2C devices are connected and
+what addresses they live at. However, in certain cases, it does not, so a
+sysfs interface was added to let the user provide the information. This
+interface is made of 2 attribute files which are created in every I2C bus
+directory: new_device and delete_device. Both files are write only and you
+must write the right parameters to them in order to properly instantiate,
+respectively delete, an I2C device.
+
+File new_device takes 2 parameters: the name of the I2C device (a string)
+and the address of the I2C device (a number, typically expressed in
+hexadecimal starting with 0x, but can also be expressed in decimal.)
+
+File delete_device takes a single parameter: the address of the I2C
+device. As no two devices can live at the same address on a given I2C
+segment, the address is sufficient to uniquely identify the device to be
+deleted.
+
+Example:
+# echo eeprom 0x50 > /sys/bus/i2c/devices/i2c-3/new_device
+
+While this interface should only be used when in-kernel device declaration
+can't be done, there is a variety of cases where it can be helpful:
+* The I2C driver usually detects devices (method 3 above) but the bus
+ segment your device lives on doesn't have the proper class bit set and
+ thus detection doesn't trigger.
+* The I2C driver usually detects devices, but your device lives at an
+ unexpected address.
+* The I2C driver usually detects devices, but your device is not detected,
+ either because the detection routine is too strict, or because your
+ device is not officially supported yet but you know it is compatible.
+* You are developing a driver on a test board, where you soldered the I2C
+ device yourself.
+
+This interface is a replacement for the force_* module parameters some I2C
+drivers implement. Being implemented in i2c-core rather than in each
+device driver individually, it is much more efficient, and also has the
+advantage that you do not have to reload the driver to change a setting.
+You can also instantiate the device before the driver is loaded or even
+available, and you don't need to know what driver the device needs.