How to customize the dtb (device tree binary) on the Raspberry Pi

Every so often, when you're debugging weird hardware issues on SBCs like the Raspberry Pi, it's useful to get way down into the guts of how the Pi represents its hardware to Linux.

And the Linux kernel uses a method called Device Tree overlays to do it. On the Pi 5 (and other Pis), these overlays are stored as .dtb files inside the /boot/firmware directory, and there's an overlay for every major Raspberry Pi hardware model.

I've had to modify the dtb files in the past to increase the PCIe BAR space for early GPU testing on the Compute Module 4. And recently I've had to mess with how the PCIe address space is set up for testing certain devices on the Raspberry Pi 5.