Triggered Buffers

Now that we know what buffers and triggers are let’s see how they work together.

IIO triggered buffer setup

A typical triggered buffer setup looks like this:

const struct iio_buffer_setup_ops sensor_buffer_setup_ops = {
  .preenable    = sensor_buffer_preenable,
  .postenable   = sensor_buffer_postenable,
  .postdisable  = sensor_buffer_postdisable,
  .predisable   = sensor_buffer_predisable,
};

irqreturn_t sensor_iio_pollfunc(int irq, void *p)
{
    pf->timestamp = iio_get_time_ns((struct indio_dev *)p);
    return IRQ_WAKE_THREAD;
}

irqreturn_t sensor_trigger_handler(int irq, void *p)
{
    u16 buf[8];
    int i = 0;

    /* read data for each active channel */
    for_each_set_bit(bit, active_scan_mask, masklength)
        buf[i++] = sensor_get_data(bit)

    iio_push_to_buffers_with_timestamp(indio_dev, buf, timestamp);

    iio_trigger_notify_done(trigger);
    return IRQ_HANDLED;
}

/* setup triggered buffer, usually in probe function */
iio_triggered_buffer_setup(indio_dev, sensor_iio_polfunc,
                           sensor_trigger_handler,
                           sensor_buffer_setup_ops);

The important things to notice here are:

  • iio_buffer_setup_ops, the buffer setup functions to be called at predefined points in the buffer configuration sequence (e.g. before enable, after disable). If not specified, the IIO core uses the default iio_triggered_buffer_setup_ops.

  • sensor_iio_pollfunc, the function that will be used as top half of poll function. It should do as little processing as possible, because it runs in interrupt context. The most common operation is recording of the current timestamp and for this reason one can use the IIO core defined iio_pollfunc_store_time() function.

  • sensor_trigger_handler, the function that will be used as bottom half of the poll function. This runs in the context of a kernel thread and all the processing takes place here. It usually reads data from the device and stores it in the internal buffer together with the timestamp recorded in the top half.

More details

int iio_triggered_buffer_setup(struct iio_dev *indio_dev, irqreturn_t (*h)(int irq, void *p), irqreturn_t (*thread)(int irq, void *p), const struct iio_buffer_setup_ops *setup_ops, )

Setup triggered buffer and pollfunc

Parameters

struct iio_dev * indio_dev

IIO device structure

irqreturn_t (*)(int irq, void *p) h

Function which will be used as pollfunc top half

irqreturn_t (*)(int irq, void *p) thread

Function which will be used as pollfunc bottom half

const struct iio_buffer_setup_ops * setup_ops

Buffer setup functions to use for this device. If NULL the default setup functions for triggered buffers will be used.

Description

This function combines some common tasks which will normally be performed when setting up a triggered buffer. It will allocate the buffer and the pollfunc.

Before calling this function the indio_dev structure should already be completely initialized, but not yet registered. In practice this means that this function should be called right before iio_device_register().

To free the resources allocated by this function call iio_triggered_buffer_cleanup().

void iio_triggered_buffer_cleanup(struct iio_dev *indio_dev)

Free resources allocated by iio_triggered_buffer_setup()

Parameters

struct iio_dev * indio_dev

IIO device structure