I tracked down a problem that others have been experiencing. Seeing a black screen, cable not supported, monitor falling asleep, when booting up their odroid-c2.

Odroid C2 HDMI no display after kernel upgrade
C2 Arch Linux recent kernel update messes up video output
C2-10-03-2016 Ubuntu no DVI video

The code that supposedly fixed the issue as mentioned in the latter topic is what caused the issue. This commit for reference: 8768319

I'm using Arch. So the last unmodified version that worked for me is 3.14.79-5 which is commit 135cacc.
The very next Arch version, 3.14.79-6, commit 8186343, does not work anymore.

I reverted that change on the latest master and rebuilt using the latest Arch PKGBUILD, 3.14.79-12, and the screen is working again.