Context: I have four little PCs doing always-on things, mostly routing. I don't want four monitors, keyboards, mice -- too much clutter and space. So I connect each computer to a single set of peripherals through a KVM switch. The problem: one day, none of the computers could use the display. Clue: journalctl showed that the computers complained: "EDID has corrupt header" Solution: remove power from the KVM briefly. This probably reset it and caused it to re-fetch its copy of EDID information from the monitor. Things worked after this. What I think is going on: - EDID (Extended Display Identification Data) is provided by each monitor to the computer so that the computer knows what the monitor can do. For example, resolutions and refresh rates - a good KVM fetches the EDID information once from the monitor and makes it available to each attached computer, even the ones not currently selected. If it doesn't do this, unselected computers would incorrectly "understand" the monitor. - there was a power flicker in my house. Short enough that not all devices were reset. Perhaps the KVM was partially reset (i.e. got into a bad state)