With a freshly installed x11-misc/sddm-0.13.0-r3, staring sddm-greeter failed with this error message in the logs:
Failed to create OpenGL context for format QSurfaceFormat ...
Following the tip in https://forums.gentoo.org/viewtopic-p-7849988.html worked: Adding the sddm user to the video group resolved the problem and sddm showed up correctly:
# usermod -a -G video sddm
IMHO this should be done already by the ebuild at install time, when the `sddm` user is created.
The ebuild has already done this for some time, but it only works for a new install - it can't modify an existing user. Perhaps we can add a warning to the ebuild if this is the case.
Shouldn't systemd or consolekit be taking care of this by setting ACLs on /dev/dri/*?