I’ve got two fantastic posts about Linux today, from the same author – Chris Siebenmann. First, the history behind kernel mode setting in Linux.
In the older days of Linux, the kernel didn’t know very much about graphics (at least on PCs). Instead, setting up and handling graphics hardware was the domain of the X server; the kernel gave it access to PCI (or AGP) resources, and the X server directly stored values and read things out. Part of what the X server did was set the graphics mode (ie, the modeline resolution, depth, and scan frequencies), initially from explicit modelines and then over time from EDID information and other things you didn’t have to configure (which was great). This was user space mode setting. There were a variety of reasons to do this at the time (cf) but it had various drawbacks, including requiring the X server to have significant privileges (cf Fedora removing them).
You can see where this is going.