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diff --git a/Documentation/i2c/instantiating-devices.rst b/Documentation/i2c/instantiating-devices.rst new file mode 100644 index 000000000..3ea056a95 --- /dev/null +++ b/Documentation/i2c/instantiating-devices.rst @@ -0,0 +1,272 @@ +============================== +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 statically +-------------------------------------------- + +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 information is provided to the kernel in a different way on different +architectures: device tree, ACPI or board files. + +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). + + +Declare the I2C devices via devicetree +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +On platforms using devicetree, the declaration of I2C devices is done in +subnodes of the master controller. + +Example: + +.. code-block:: dts + + i2c1: i2c@400a0000 { + /* ... master properties skipped ... */ + clock-frequency = <100000>; + + flash@50 { + compatible = "atmel,24c256"; + reg = <0x50>; + }; + + pca9532: gpio@60 { + compatible = "nxp,pca9532"; + gpio-controller; + #gpio-cells = <2>; + reg = <0x60>; + }; + }; + +Here, two devices are attached to the bus using a speed of 100kHz. For +additional properties which might be needed to set up the device, please refer +to its devicetree documentation in Documentation/devicetree/bindings/. + + +Declare the I2C devices via ACPI +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +ACPI can also describe I2C devices. There is special documentation for this +which is currently located at Documentation/firmware-guide/acpi/enumeration.rst. + + +Declare the I2C devices in board files +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +In many embedded architectures, devicetree has replaced the old hardware +description based on board files, but the latter are still used in old +code. Instantiating I2C devices via board files is done with an array of +struct i2c_board_info which is registered by calling +i2c_register_board_info(). + +Example (from omap2 h4): + +.. code-block:: c + + static struct i2c_board_info h4_i2c_board_info[] __initdata = { + { + 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. + + +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_client_device(). + +Example (from the sfe4001 network driver): + +.. code-block:: c + + 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_client_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_scanned_device() instead of i2c_new_client_device(). + +Example (from the nxp OHCI driver): + +.. code-block:: c + + 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)); + strscpy(i2c_info.type, "isp1301_nxp", sizeof(i2c_info.type)); + isp1301_i2c_client = i2c_new_scanned_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_client_device() or +i2c_new_scanned_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_scanned_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. |