If you’ve already read through “Linux Device Drivers”, it may be worth your time to read a more focused Linux driver development textbook. ARM driver development has been popular for some time now and remains relevant today. “Linux Driver Development for Embedded Processors” (ELDD for short) gives a modern look into the development of ARM drivers on Linux. ELDD has a number of selling points:
- Labs targeting multiple ARM processors: NXP iMX7D, Microchip SAMA5D2 and Broadcom BCM2837.
- Excellent Device Tree introduction with many examples.
- Plenty of labs using real hardware.
This article dives into the details starting with processor support.
Processor Options
ELDD gives you the choice of developing for one or more ARM processors: NXP iMX7D, Microchip SAMA5D2, and the Broadcom BCM2837. Unless you’re willing to dish out well over $100, you will end up developing for the BCM2837. The BCM2837 comes in the famous Raspberry Pi. ELDD specifically recommends the Raspberry Pi Model 3B+.
You can grab one these brand new off Amazon for just $60!
ELDD assumes you have the background to perform board bring up and basic Linux administration solo. That said, there is some introductory material in Chapter 1 that walks through how to build, configure, and install the kernel. You will get a description of how to install the kernel on each processor’s platform. You also get a walk through of how to setup an IDE (Eclipse) to build and deploy the lab kernel modules you develop throughout the book.
Despite not having all three boards, it’s nice to see how the lab implementations vary from processor to processor. There are differences in the device tree setup and driver source code. The book does a good job of pointing those differences out where they matter.
The Device Tree
There’s plenty of content out there online explaining what the device tree is and the role it plays in the kernel. Some good resources you can use in conjunction with ELDD are eLinux’s “Device Tree Usage” wiki and Thomas Petazzoni’s 2013 presentation “Device Tree for Dummies”. Petazzoni’s presentation is in particular worth the watch:
What does ELDD have to offer in this area? To be honest, the device tree description and introduction of properties chapter to chapter is a little rough. Additionally, the book has one make a habit of editing the kernel dts files directly. DT overlays get introduced a bit late. All that said, the examples work and serve as bases to build off of. The explanation of how drivers link to nodes given in Chapter 2 is particularly insightful. Highly recommend you read Chapter 2 at least twice!
Hardware Labs
What makes this book shine are the hardware labs. Unlike the “Linux Device Drivers” book which has you making in software devices, ELDD focuses on developing basic drivers for a variety of GPIO, I2C, and SPI devices.
One gripe is that the book doesn’t have an upfront listing of all the hardware required so you can buy it before reading. This issue is now fixed. A list of lab hardware exists on the book’s GitHub repo. Unlike with the processors, most of the supporting hardware is affordable. A tip if you want to save some money. Don’t buy the MIKROE ColorClick and Button R Click devices (a savings of over $50 after shipping). Using a $20 breadboard kit with LEDs, resistors, push buttons, and some jumper wire, you can make the circuits required to replace those items. Beyond the MIKROE products, an STMicroelectronics LED screen is the only other expensive item. All other hardware was available on Amazon and totaled less than $100 shipping included.
One thing to note, the book again assumes the reader has a good bit of knowledge when it comes to reading datasheets and wiring a device to a dev board. Data/signal pins get called out but ground, power, and resistor usage aren’t. Just be wary of this when following along.
The explanations in regards to how the drivers interact with devices are excellent. Every chapter starts with a practical discussion of the available APIs and ends with one or more labs. Each lab starts with an explanation of the Device Tree setup followed by a detailed description of the driver’s key components. A number of chapters introduce the theory or details around the Linux kernel concept associated with the driver. It’s refreshing to read a book that doesn’t assume the reader is a complete OS theory novice. For example, virtual memory isn’t explained in the CS101 sense. Instead you get the VM implementation on Linux for ARM with links to relevant code. The book does this for many different topics including interrupts, synchronization, and deferred work just to name a few.
Conclusion
Linux Driver Development for Embedded Processors is a great option for anyone interested in learning how to develop drivers for ARM devices in the modern day. The book is particularly useful for those who learn through hands on work. The many labs included use real hardware and do a lot to reinforce the previous chapters’ lessons. That said, this book is for an audience with prior experience in programming, electronics, and Linux usage in general. For those completely new to Linux kernel development, read ELDD in parallel to “Linux Kernel Development” by Robert Love. The mixture of theory and practical application complement each other.
You can find the ELDD project with complete source, build instructions, usage, etc. on GitHub under eldd.