pmOS "mainline" kernel meaning/requirements

User @rah2501 asked very interesting question yesterday in IRC chat: what do we actually mean when we say that the kernel is "mainline"?

I've always taken the word "mainline" as the kernel, version of which is even with latest torvalds/linux tag. And that may be not true for some devices running pmOS with "mainline" kernel.

"Mainline" kernel in pmOS community almost always meant the device running mainline kernel at the moment the port was submitted. But eventually, all contributors/maintainers will vanish, disappear, and their "mainline" kernels might be left in a non-upstreamable state.

So what requirements should we add to count that kernel as "mainline"? imo the "mainline" pmOS kernel needs as much requirements as it needs for upstreaming into torvalds/linux tree. It needs to be legally certain, have decent code quality(however this requirement may be not needed because we all have different standards for code quality :p), and be based on unmodified torvalds/linux tag. Then it can be called mainline.