External graphics cards work on the Raspberry Pi

AMD Radeon HD 7450 Graphics card with Raspberry Pi Compute Module 4

In October 2020, after Raspberry Pi introduced the Compute Module 4, I started out on a journey to get an external graphics card working on the Pi.

At the time, it'd been over a decade since the last time I'd built a PC, and I had a lot to learn about PCI Express, the state of graphics card drivers in Linux, and PCI Express support on various ARM SoCs.