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staging:iio:documentation: Drop most generic docs
These docs are both suffering from being out of date, and from being superceeded by the documentation in Documentation/driver-api/iio Note the inkern.txt drop is left for now as this is an area not well covered by the more recent documentation outside staging. Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com> Link: https://lore.kernel.org/r/20200905174721.216452-5-jic23@kernel.org
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IIO Device drivers
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This is not intended to provide a comprehensive guide to writing an
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IIO device driver. For further information see the drivers within the
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subsystem.
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The crucial structure for device drivers in iio is iio_dev.
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First allocate one using:
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struct iio_dev *indio_dev = iio_device_alloc(parent, sizeof(struct chip_state));
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where chip_state is a structure of local state data for this instance of
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the chip.
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That data can be accessed using iio_priv(struct iio_dev *).
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Then fill in the following:
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- indio_dev->name
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Name of the device being driven - made available as the name
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attribute in sysfs.
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- indio_dev->info
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pointer to a structure with elements that tend to be fixed for
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large sets of different parts supported by a given driver.
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This contains:
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* info->event_attrs:
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Attributes used to enable / disable hardware events.
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* info->attrs:
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General device attributes. Typically used for the weird
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and the wonderful bits not covered by the channel specification.
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* info->read_raw:
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Raw data reading function. Used for both raw channel access
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and for associate parameters such as offsets and scales.
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* info->write_raw:
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Raw value writing function. Used for writable device values such
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as DAC values and calibbias.
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* info->read_event_config:
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Typically only set if there are some interrupt lines. This
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is used to read if an on sensor event detector is enabled.
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* info->write_event_config:
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Enable / disable an on sensor event detector.
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* info->read_event_value:
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Read value associated with on sensor event detectors. Note that
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the meaning of the returned value is dependent on the event
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type.
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* info->write_event_value:
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Write the value associated with on sensor event detectors. E.g.
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a threshold above which an interrupt occurs. Note that the
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meaning of the value to be set is event type dependent.
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- indio_dev->modes:
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Specify whether direct access and / or ring buffer access is supported.
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- indio_dev->buffer:
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An optional associated buffer.
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- indio_dev->pollfunc:
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Poll function related elements. This controls what occurs when a trigger
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to which this device is attached sends an event.
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- indio_dev->channels:
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Specification of device channels. Most attributes etc. are built
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from this spec.
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- indio_dev->num_channels:
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How many channels are there?
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Once these are set up, a call to iio_device_register(indio_dev)
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will register the device with the iio core.
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Worth noting here is that, if a ring buffer is to be used, it can be
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allocated prior to registering the device with the iio-core, but must
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be registered afterwards (otherwise the whole parentage of devices
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gets confused)
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On remove, iio_device_unregister(indio_dev) will remove the device from
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the core, and iio_device_free(indio_dev) will clean up.
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Overview of IIO
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The Industrial I/O subsystem is intended to provide support for devices
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that in some sense are analog to digital converters (ADCs). As many
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actual devices combine some ADCs with digital to analog converters
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(DACs) that functionality is also supported.
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The aim is to fill the gap between the somewhat similar hwmon and
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input subsystems. Hwmon is very much directed at low sample rate
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sensors used in applications such as fan speed control and temperature
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measurement. Input is, as its name suggests focused on input
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devices. In some cases, there is considerable overlap between these and
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IIO.
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A typical device falling into this category would be connected via SPI
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or I2C.
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Functionality of IIO
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* Basic device registration and handling. This is very similar to
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hwmon with simple polled access to device channels via sysfs.
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* Event chrdevs. These are similar to input in that they provide a
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route to user space for hardware triggered events. Such events include
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threshold detectors, free-fall detectors and more complex action
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detection. The events themselves are currently very simple with
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merely an event code and a timestamp. Any data associated with the
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event must be accessed via polling.
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Note: A given device may have one or more event channel. These events are
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turned on or off (if possible) via sysfs interfaces.
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* Hardware buffer support. Some recent sensors have included
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fifo / ring buffers on the sensor chip. These greatly reduce the load
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on the host CPU by buffering relatively large numbers of data samples
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based on an internal sampling clock. Examples include VTI SCA3000
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series and Analog Devices ADXL345 accelerometers. Each buffer supports
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polling to establish when data is available.
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* Trigger and software buffer support. In many data analysis
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applications it is useful to be able to capture data based on some
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external signal (trigger). These triggers might be a data ready
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signal, a gpio line connected to some external system or an on
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processor periodic interrupt. A single trigger may initialize data
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capture or reading from a number of sensors. These triggers are
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used in IIO to fill software buffers acting in a very similar
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fashion to the hardware buffers described above.
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Other documentation:
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device.txt - elements of a typical device driver.
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trigger.txt - elements of a typical trigger driver.
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ring.txt - additional elements required for buffer support.
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sysfs-bus-iio - abi documentation file.
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Buffer support within IIO
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This document is intended as a general overview of the functionality
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a buffer may supply and how it is specified within IIO. For more
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specific information on a given buffer implementation, see the
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comments in the source code. Note that some drivers allow buffer
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implementation to be selected at compile time via Kconfig options.
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A given buffer implementation typically embeds a struct
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iio_ring_buffer and it is a pointer to this that is provided to the
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IIO core. Access to the embedding structure is typically done via
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container_of functions.
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struct iio_ring_buffer contains a struct iio_ring_setup_ops *setup_ops
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which in turn contains the 4 function pointers
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(preenable, postenable, predisable and postdisable).
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These are used to perform device specific steps on either side
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of the core changing its current mode to indicate that the buffer
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is enabled or disabled (along with enabling triggering etc. as appropriate).
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Also in struct iio_ring_buffer is a struct iio_ring_access_funcs.
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The function pointers within here are used to allow the core to handle
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as much buffer functionality as possible. Note almost all of these
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are optional.
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store_to
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If possible, push data to the buffer.
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read_last
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If possible, get the most recent scan from the buffer (without removal).
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This provides polling like functionality whilst the ring buffering is in
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use without a separate read from the device.
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rip_first_n
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The primary buffer reading function. Note that it may well not return
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as much data as requested.
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request_update
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If parameters have changed that require reinitialization or configuration of
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the buffer this will trigger it.
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set_bytes_per_datum
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Set the number of bytes for a complete scan. (All samples + timestamp)
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set_length
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Set the number of complete scans that may be held by the buffer.
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IIO trigger drivers.
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Many triggers are provided by hardware that will also be registered as
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an IIO device. Whilst this can create device specific complexities
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such triggers are registered with the core in the same way as
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stand-alone triggers.
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struct iio_trig *trig = iio_trigger_alloc("<trigger format string>", ...);
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allocates a trigger structure. The key elements to then fill in within
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a driver are:
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trig->set_trigger_state:
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Function that enables / disables the underlying source of the trigger.
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There is also a
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trig->alloc_list which is useful for drivers that allocate multiple
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triggers to keep track of what they have created.
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When these have been set call:
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iio_trigger_register(trig);
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to register the trigger with the core, making it available to trigger
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consumers.
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Trigger Consumers
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Currently triggers are only used for the filling of software
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buffers and as such any device supporting INDIO_BUFFER_TRIGGERED has the
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consumer interface automatically created.
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