Industrial I/O triggers

Industrial I/O triggers
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struct iio_trigger — industrial I/O trigger device
devm_iio_trigger_alloc — Resource-managed iio_trigger_alloc
devm_iio_trigger_free — Resource-managed iio_trigger_free
IIO trigger sysfs interface
IIO trigger setup
IIO trigger ops

In many situations it is useful for a driver to be able to capture data based on some external event (trigger) as opposed to periodically polling for data. An IIO trigger can be provided by a device driver that also has an IIO device based on hardware generated events (e.g. data ready or threshold exceeded) or provided by a separate driver from an independent interrupt source (e.g. GPIO line connected to some external system, timer interrupt or user space writing a specific file in sysfs). A trigger may initiate data capture for a number of sensors and also it may be completely unrelated to the sensor itself.

IIO trigger sysfs interface

There are two locations in sysfs related to triggers:

/sys/bus/iio/devices/triggerY, this file is created once an IIO trigger is registered with the IIO core and corresponds to trigger with index Y. Because triggers can be very different depending on type there are few standard attributes that we can describe here:
- name, trigger name that can be later used for association with a device.
- sampling_frequency, some timer based triggers use this attribute to specify the frequency for trigger calls.
/sys/bus/iio/devices/iio:deviceX/trigger/, this directory is created once the device supports a triggered buffer. We can associate a trigger with our device by writing the trigger's name in the current_trigger file.

IIO trigger setup

Let's see a simple example of how to setup a trigger to be used by a driver.

      struct iio_trigger_ops trigger_ops = {
          .set_trigger_state = sample_trigger_state,
          .validate_device = sample_validate_device,
      }

      struct iio_trigger *trig;

      /* first, allocate memory for our trigger */
      trig = iio_trigger_alloc(dev, "trig-%s-%d", name, idx);

      /* setup trigger operations field */
      trig->ops = &trigger_ops;

      /* now register the trigger with the IIO core */
      iio_trigger_register(trig);

IIO trigger ops

struct iio_trigger_ops — operations structure for an iio_trigger.

Notice that a trigger has a set of operations attached:

set_trigger_state, switch the trigger on/off on demand.
validate_device, function to validate the device when the current trigger gets changed.