Selecting hardware for embedded Linux

If you are designing or selecting hardware for an embedded Linux project, what do you look out for?

• First, a CPU architecture that is supported by the kernel – unless you plan to add a new architecture yourself of course! Looking at the source code for Linux 5.15 there are 23 architectures each represented by a subdirectory in the arch/ directory. They are all 32-or 64-bit architectures, most with an MMU, but some without. The ones most often found in embedded devices are Arm, RISC-V, PowerPC, MIPS, and x86 each in 32-and 64-bit variants all of which have Memory Management Units (MMUs).
• Most of this book is written with this class of processor in mind. There is another group that doesn’t have an MMU and that runs a subset of Linux known as a microcontroller Linux or uClinux. These processor architectures include ARC (Argonaut RISC Core), Blackfin, MicroBlaze, and Nios. I will mention uClinux from time to time, but I will not go into detail because it is a rather specialized type.
• Second, you will need a reasonable amount of RAM. 16 MB is a good minimum, although it is quite possible to run Linux using half of that. It is even possible to run Linux with 4 MB if you are prepared to go to the trouble of optimizing every part of the system. It may even be possible to get lower, but there comes a point at which it is no longer Linux.
• Third, there is non-volatile storage, usually flash memory. 8 MB is enough for a simple device such as a webcam or basic router. As with RAM, you can create a workable Linux system with less storage if you really want to, but the lower you go the harder it becomes. Linux has extensive support for flash storage devices, including raw NOR and NAND flash chips, and managed flash in the form of SD cards, eMMC chips, USB flash memory, and so on.
• Fourth, a serial port is very useful, preferably a UART-based serial port. It does not have to be fitted on production boards but makes board bring-up, debugging, and development much easier.
• Fifth, you need some means of loading software when starting from scratch. Many microcontroller boards are fitted with a Joint Test Action Group (JTAG) interface for this purpose. Modern SoCs can also load boot code directly from the removable media, especially SD and microSD cards, or serial interfaces such as QSPI or USB.

In addition to these basics, there are interfaces to the specific bits of hardware your device needs to get its job done. Mainline Linux comes with open source drivers for many thousands of different devices, and there are drivers available (of variable quality) from the SoC manufacturer and from the OEMs of third-party chips that may be included in the design.

Remember my comments on the commitment and ability of some manufacturers. As a developer of embedded systems, you will find that you spend quite a lot of time evaluating and adapting third-party code, if you have it, or liaising with the manufacturer if you don’t. Finally, you will have to write the device support for the interfaces that are unique to the device or find someone to do it for you.