The iio_simple_dummy Anatomy

Introduction

In the MAC0470/5856 – Linux Kernel Development course at IME-USP (and the FLUSP activities), the best way to understand a new subsystem is by dissecting a simple, well-commented example driver. For the Industrial I/O (IIO) subsystem, that driver is iio_simple_dummy.

This post is my personal study notes and explanations after going through the excellent FLUSP tutorial “The iio_simple_dummy Anatomy” by Rodrigo Siqueira. I followed the same systematic approach he recommends: find the initialization points, understand the core elements (especially channels), dissect the read/write functions, and then connect everything.

Why iio_simple_dummy is the Perfect Learning Driver

• It is 100% software (no real hardware needed)
• Heavily commented in the kernel source (drivers/iio/dummy/iio_simple_dummy.c)
• Demonstrates almost every important IIO concept: channels, attributes, raw/scale/offset values, buffers, and events
• Lets you focus on the subsystem instead of bus-specific details (I2C/SPI/etc.)

Architecture Overview

flowchart TD

Host["Host (Debian) → Load iio_simple_dummy module"]

Host --> iio["iio_sw_device (configfs)"]

iio --> device["iio_device (with iio_dummy_channels[])"]

device --> info["iio_info (read_raw / write_raw)"]

info --> private["private data (iio_dummy_state)"]

private --> sysfs["sysfs"]
private --> chardev["character device (for userspace)"]

The dummy driver registers itself as a software IIO device, exposing simulated sensors to userspace.

What I Learned

1. Channels are everything — almost all IIO magic lives in the iio_chan_spec array. Master this struct and you understand 80% of any IIO driver.
2. Separate vs shared masks are elegant: they keep the sysfs interface clean and avoid duplication.
3. Dummy drivers remove hardware noise — you can focus purely on the subsystem before touching real I2C/SPI sensors.
4. Read/write_raw are the bridge between kernel and userspace. They are simple once you see the mask + chan pattern.
5. The kernel community puts huge effort into documentation — the comments in iio_simple_dummy.c and the FLUSP tutorial are gold.