IIO System Considerations Tips & Tricks#

IIO context timeout (libiio)#

  • May be triggered by low sample rates and large buffers

  • Long blocking calls (some calibration, profile load, etc.)

Tip

Increase iio_context_set_timeout() timeout parameter or set to 0 to disable the timeout.

Buffer handling, sizes and counts#

Typically set to a frame or chunk size suitable for signal processing (e.g. N x FFT_size)

  • Small buffers -> less latency but more overhead

  • Large buffers -> less overhead but more latency

../../_images/libiio_highspeed_interface.png

Tip

Number of discrete buffers are configurable, default is 4. Can change this using iio_device_set_kernel_buffers_count()

Tip

Capturing starts as soon as the buffer is created! iio_device_create_buffer()

Tip

FIFO like behavior new data is dropped in case it’s not consumed fast enough!

IIO buffer DMA max block size#

Max buffer size is limited by the max_block_size parameter

  • Default 16M

  • Can be adjusted

There are two ways to adjust the max block size

sysfs#

Set via /sys/module/industrialio_buffer_dma/parameters/max_block_size in bytes

~$

echo 67108864 > /sys/module/industrialio_buffer_dma/parameters/max_block_size

~$

cat /sys/module/industrialio_buffer_dma/parameters/max_block_size

67108864

~$

Kernel command line#

Append the following to your kernel command line

industrialio_buffer_dma.max_block_size=size_in_bytes

Linux Contiguous Memory Allocator (or CMA)#

Allocation of big, physically-contiguous memory blocks
Reserve memory early at boot time
Kconfig menu "Device Drivers" -> "Generic Driver Options"-> "Contiguous Memory Allocator"
Kernel command line option cma=size_in_bytes
PlutoSDR default 256M

Tip

Depending on your IIO buffer DMA max block size and number of kernel buffer count you may need to CMA size.

Capturing large contiguous buffers#

  1. Set kernel buffer count to 1

  2. Increase IIO buffer DMA max block size parameter to at least the size of your desired contiguous buffer

  3. Make sure your CMA size is significantly bigger (CMA is used by various other DMA capable devices in your Linux system)

  4. AXI_DMAC controller limits length of the transfers to 2^DMA_LENGTH_WIDTH. The default is 24 so your max transfer size is 16777216 bytes. You may need to increase that and rebuild your design. Please see AXI DMAC (Configuration Parameters section).