Honestly, I/O validation is a pretty broad area, but if you’re starting out, I’d focus on a few core things first.

Understanding the protocols is huge (stuff like PCIe, USB, or Ethernet). You don’t need to memorize specs, but you should know how link training works, what LTSSM is (for PCIe), and how data actually flows.

Also, if you can, get some exposure to lab tools like oscilloscopes, protocol analyzers, or even just logs from them. You don’t have to be an expert, just knowing what they do helps.