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IIO Device drivers
This is not intended to provide a comprehensive guide to writing an
IIO device driver. For further information see the drivers within the
subsystem.
The crucial structure for device drivers in iio is iio_dev.
First allocate one using:
struct iio_dev *indio_dev = iio_device_alloc(sizeof(struct chip_state));
where chip_state is a structure of local state data for this instance of
the chip.
That data can be accessed using iio_priv(struct iio_dev *)
Then fill in the following:
- indio_dev->dev.parent
Struct device associated with the underlying hardware.
- indio_dev->name
Name of the device being driven - made available as the name
attribute in sysfs.
- indio_dev->info
pointer to a structure with elements that tend to be fixed for
large sets of different parts supported by a given driver.
This contains:
* info->driver_module:
Set to THIS_MODULE. Used to ensure correct ownership
of various resources allocate by the core.
* info->num_interrupt_lines:
Number of event triggering hardware lines the device has.
* info->event_attrs:
Attributes used to enable / disable hardware events.
* info->attrs:
General device attributes. Typically used for the weird
and the wonderful bits not covered by the channel specification.
* info->read_raw:
Raw data reading function. Used for both raw channel access
and for associate parameters such as offsets and scales.
* info->write_raw:
Raw value writing function. Used for writable device values such
as DAC values and caliboffset.
* info->read_event_config:
Typically only set if there are some interrupt lines. This
is used to read if an on sensor event detector is enabled.
* info->write_event_config:
Enable / disable an on sensor event detector.
* info->read_event_value:
Read value associated with on sensor event detectors. Note that
the meaning of the returned value is dependent on the event
type.
* info->write_event_value:
Write the value associated with on sensor event detectors. E.g.
a threshold above which an interrupt occurs. Note that the
meaning of the value to be set is event type dependant.
- indio_dev->modes:
Specify whether direct access and / or ring buffer access is supported.
- indio_dev->ring:
An optional associated buffer.
- indio_dev->pollfunc:
Poll function related elements. This controls what occurs when a trigger
to which this device is attached sends an event.
- indio_dev->channels:
Specification of device channels. Most attributes etc are built
form this spec.
- indio_dev->num_channels:
How many channels are there?
Once these are set up, a call to iio_device_register(indio_dev),
will register the device with the iio core.
Worth noting here is that, if a ring buffer is to be used, it can be
allocated prior to registering the device with the iio-core, but must
be registered afterwards (otherwise the whole parentage of devices
gets confused)
On remove, iio_device_unregister(indio_dev) will remove the device from
the core, and iio_device_free will clean up.
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