Buffers

The Industrial I/O core offers a way for continuous data capture based on a trigger source. Multiple data channels can be read at once from /dev/iio:deviceX character device node, thus reducing the CPU load.

IIO buffer sysfs interface

An IIO buffer has an associated attributes directory under /sys/bus/iio/iio:deviceX/buffer/*. Here are some of the existing attributes:

  • length, the total number of data samples (capacity) that can be stored by the buffer.

  • enable, activate buffer capture.

IIO buffer setup

The meta information associated with a channel reading placed in a buffer is called a scan element. The important bits configuring scan elements are exposed to userspace applications via the /sys/bus/iio/iio:deviceX/scan_elements/* directory. This file contains attributes of the following form:

  • enable, used for enabling a channel. If and only if its attribute is non zero, then a triggered capture will contain data samples for this channel.

  • type, description of the scan element data storage within the buffer and hence the form in which it is read from user space. Format is [be|le]:[s|u]bits/storagebitsXrepeat[>>shift] . * be or le, specifies big or little endian. * s or u, specifies if signed (2’s complement) or unsigned. * bits, is the number of valid data bits. * storagebits, is the number of bits (after padding) that it occupies in the buffer. * shift, if specified, is the shift that needs to be applied prior to masking out unused bits. * repeat, specifies the number of bits/storagebits repetitions. When the repeat element is 0 or 1, then the repeat value is omitted.

For example, a driver for a 3-axis accelerometer with 12 bit resolution where data is stored in two 8-bits registers as follows:

  7   6   5   4   3   2   1   0
+---+---+---+---+---+---+---+---+
|D3 |D2 |D1 |D0 | X | X | X | X | (LOW byte, address 0x06)
+---+---+---+---+---+---+---+---+

  7   6   5   4   3   2   1   0
+---+---+---+---+---+---+---+---+
|D11|D10|D9 |D8 |D7 |D6 |D5 |D4 | (HIGH byte, address 0x07)
+---+---+---+---+---+---+---+---+

will have the following scan element type for each axis:

$ cat /sys/bus/iio/devices/iio:device0/scan_elements/in_accel_y_type
le:s12/16>>4

A user space application will interpret data samples read from the buffer as two byte little endian signed data, that needs a 4 bits right shift before masking out the 12 valid bits of data.

For implementing buffer support a driver should initialize the following fields in iio_chan_spec definition:

struct iio_chan_spec {
/* other members */
        int scan_index
        struct {
                char sign;
                u8 realbits;
                u8 storagebits;
                u8 shift;
                u8 repeat;
                enum iio_endian endianness;
               } scan_type;
       };

The driver implementing the accelerometer described above will have the following channel definition:

struct iio_chan_spec accel_channels[] = {
        {
                .type = IIO_ACCEL,
                .modified = 1,
                .channel2 = IIO_MOD_X,
                /* other stuff here */
                .scan_index = 0,
                .scan_type = {
                        .sign = 's',
                        .realbits = 12,
                        .storagebits = 16,
                        .shift = 4,
                        .endianness = IIO_LE,
                },
        }
        /* similar for Y (with channel2 = IIO_MOD_Y, scan_index = 1)
         * and Z (with channel2 = IIO_MOD_Z, scan_index = 2) axis
         */
 }

Here scan_index defines the order in which the enabled channels are placed inside the buffer. Channels with a lower scan_index will be placed before channels with a higher index. Each channel needs to have a unique scan_index.

Setting scan_index to -1 can be used to indicate that the specific channel does not support buffered capture. In this case no entries will be created for the channel in the scan_elements directory.

More details

int iio_push_to_buffers_with_timestamp(struct iio_dev *indio_dev, void *data, int64_t timestamp)

push data and timestamp to buffers

Parameters

struct iio_dev * indio_dev

iio_dev structure for device.

void * data

sample data

int64_t timestamp

timestamp for the sample data

Description

Pushes data to the IIO device’s buffers. If timestamps are enabled for the device the function will store the supplied timestamp as the last element in the sample data buffer before pushing it to the device buffers. The sample data buffer needs to be large enough to hold the additional timestamp (usually the buffer should be indio->scan_bytes bytes large).

Returns 0 on success, a negative error code otherwise.

void iio_buffer_set_attrs(struct iio_buffer *buffer, const struct attribute **attrs)

Set buffer specific attributes

Parameters

struct iio_buffer * buffer

The buffer for which we are setting attributes

const struct attribute ** attrs

Pointer to a null terminated list of pointers to attributes

bool iio_validate_scan_mask_onehot(struct iio_dev *indio_dev, const unsigned long *mask)

Validates that exactly one channel is selected

Parameters

struct iio_dev * indio_dev

the iio device

const unsigned long * mask

scan mask to be checked

Description

Return true if exactly one bit is set in the scan mask, false otherwise. It can be used for devices where only one channel can be active for sampling at a time.

int iio_push_to_buffers(struct iio_dev *indio_dev, const void *data)

push to a registered buffer.

Parameters

struct iio_dev * indio_dev

iio_dev structure for device.

const void * data

Full scan.

struct iio_buffer *iio_buffer_get(struct iio_buffer *buffer)

Grab a reference to the buffer

Parameters

struct iio_buffer * buffer

The buffer to grab a reference for, may be NULL

Description

Returns the pointer to the buffer that was passed into the function.

void iio_buffer_put(struct iio_buffer *buffer)

Release the reference to the buffer

Parameters

struct iio_buffer * buffer

The buffer to release the reference for, may be NULL

void iio_device_attach_buffer(struct iio_dev *indio_dev, struct iio_buffer *buffer)

Attach a buffer to a IIO device

Parameters

struct iio_dev * indio_dev

The device the buffer should be attached to

struct iio_buffer * buffer

The buffer to attach to the device

Description

This function attaches a buffer to a IIO device. The buffer stays attached to the device until the device is freed. The function should only be called at most once per device.