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diff --git a/Documentation/driver-api/iio/core.rst b/Documentation/driver-api/iio/core.rst new file mode 100644 index 000000000..715cf2948 --- /dev/null +++ b/Documentation/driver-api/iio/core.rst @@ -0,0 +1,182 @@ +============= +Core elements +============= + +The Industrial I/O core offers both a unified framework for writing drivers for +many different types of embedded sensors and a standard interface to user space +applications manipulating sensors. The implementation can be found under +:file:`drivers/iio/industrialio-*` + +Industrial I/O Devices +---------------------- + +* struct iio_dev - industrial I/O device +* iio_device_alloc() - allocate an :c:type:`iio_dev` from a driver +* iio_device_free() - free an :c:type:`iio_dev` from a driver +* iio_device_register() - register a device with the IIO subsystem +* iio_device_unregister() - unregister a device from the IIO + subsystem + +An IIO device usually corresponds to a single hardware sensor and it +provides all the information needed by a driver handling a device. +Let's first have a look at the functionality embedded in an IIO device +then we will show how a device driver makes use of an IIO device. + +There are two ways for a user space application to interact with an IIO driver. + +1. :file:`/sys/bus/iio/iio:device{X}/`, this represents a hardware sensor + and groups together the data channels of the same chip. +2. :file:`/dev/iio:device{X}`, character device node interface used for + buffered data transfer and for events information retrieval. + +A typical IIO driver will register itself as an :doc:`I2C <../i2c>` or +:doc:`SPI <../spi>` driver and will create two routines, probe and remove. + +At probe: + +1. Call iio_device_alloc(), which allocates memory for an IIO device. +2. Initialize IIO device fields with driver specific information (e.g. + device name, device channels). +3. Call iio_device_register(), this registers the device with the + IIO core. After this call the device is ready to accept requests from user + space applications. + +At remove, we free the resources allocated in probe in reverse order: + +1. iio_device_unregister(), unregister the device from the IIO core. +2. iio_device_free(), free the memory allocated for the IIO device. + +IIO device sysfs interface +========================== + +Attributes are sysfs files used to expose chip info and also allowing +applications to set various configuration parameters. For device with +index X, attributes can be found under /sys/bus/iio/iio:deviceX/ directory. +Common attributes are: + +* :file:`name`, description of the physical chip. +* :file:`dev`, shows the major:minor pair associated with + :file:`/dev/iio:deviceX` node. +* :file:`sampling_frequency_available`, available discrete set of sampling + frequency values for device. +* Available standard attributes for IIO devices are described in the + :file:`Documentation/ABI/testing/sysfs-bus-iio` file in the Linux kernel + sources. + +IIO device channels +=================== + +struct iio_chan_spec - specification of a single channel + +An IIO device channel is a representation of a data channel. An IIO device can +have one or multiple channels. For example: + +* a thermometer sensor has one channel representing the temperature measurement. +* a light sensor with two channels indicating the measurements in the visible + and infrared spectrum. +* an accelerometer can have up to 3 channels representing acceleration on X, Y + and Z axes. + +An IIO channel is described by the struct iio_chan_spec. +A thermometer driver for the temperature sensor in the example above would +have to describe its channel as follows:: + + static const struct iio_chan_spec temp_channel[] = { + { + .type = IIO_TEMP, + .info_mask_separate = BIT(IIO_CHAN_INFO_PROCESSED), + }, + }; + +Channel sysfs attributes exposed to userspace are specified in the form of +bitmasks. Depending on their shared info, attributes can be set in one of the +following masks: + +* **info_mask_separate**, attributes will be specific to + this channel +* **info_mask_shared_by_type**, attributes are shared by all channels of the + same type +* **info_mask_shared_by_dir**, attributes are shared by all channels of the same + direction +* **info_mask_shared_by_all**, attributes are shared by all channels + +When there are multiple data channels per channel type we have two ways to +distinguish between them: + +* set **.modified** field of :c:type:`iio_chan_spec` to 1. Modifiers are + specified using **.channel2** field of the same :c:type:`iio_chan_spec` + structure and are used to indicate a physically unique characteristic of the + channel such as its direction or spectral response. For example, a light + sensor can have two channels, one for infrared light and one for both + infrared and visible light. +* set **.indexed** field of :c:type:`iio_chan_spec` to 1. In this case the + channel is simply another instance with an index specified by the **.channel** + field. + +Here is how we can make use of the channel's modifiers:: + + static const struct iio_chan_spec light_channels[] = { + { + .type = IIO_INTENSITY, + .modified = 1, + .channel2 = IIO_MOD_LIGHT_IR, + .info_mask_separate = BIT(IIO_CHAN_INFO_RAW), + .info_mask_shared = BIT(IIO_CHAN_INFO_SAMP_FREQ), + }, + { + .type = IIO_INTENSITY, + .modified = 1, + .channel2 = IIO_MOD_LIGHT_BOTH, + .info_mask_separate = BIT(IIO_CHAN_INFO_RAW), + .info_mask_shared = BIT(IIO_CHAN_INFO_SAMP_FREQ), + }, + { + .type = IIO_LIGHT, + .info_mask_separate = BIT(IIO_CHAN_INFO_PROCESSED), + .info_mask_shared = BIT(IIO_CHAN_INFO_SAMP_FREQ), + }, + } + +This channel's definition will generate two separate sysfs files for raw data +retrieval: + +* :file:`/sys/bus/iio/iio:device{X}/in_intensity_ir_raw` +* :file:`/sys/bus/iio/iio:device{X}/in_intensity_both_raw` + +one file for processed data: + +* :file:`/sys/bus/iio/iio:device{X}/in_illuminance_input` + +and one shared sysfs file for sampling frequency: + +* :file:`/sys/bus/iio/iio:device{X}/sampling_frequency`. + +Here is how we can make use of the channel's indexing:: + + static const struct iio_chan_spec light_channels[] = { + { + .type = IIO_VOLTAGE, + .indexed = 1, + .channel = 0, + .info_mask_separate = BIT(IIO_CHAN_INFO_RAW), + }, + { + .type = IIO_VOLTAGE, + .indexed = 1, + .channel = 1, + .info_mask_separate = BIT(IIO_CHAN_INFO_RAW), + }, + } + +This will generate two separate attributes files for raw data retrieval: + +* :file:`/sys/bus/iio/devices/iio:device{X}/in_voltage0_raw`, representing + voltage measurement for channel 0. +* :file:`/sys/bus/iio/devices/iio:device{X}/in_voltage1_raw`, representing + voltage measurement for channel 1. + +More details +============ +.. kernel-doc:: include/linux/iio/iio.h +.. kernel-doc:: drivers/iio/industrialio-core.c + :export: |