Comment 5 for bug 1648122

Attached Xorg.20.log, relevant lines:
[ 136.525] (EE) /dev/dri/card0: failed to set DRM interface version 1.4: Permission denied
...
[ 136.831] (EE) AIGLX: reverting to software rendering

(Note that it can't find other modules, because I removed them during debugging. The same problem occurs on a untampered Mint 18.1 Mate x64 installation; reinstalled on new partition to verify.)

Xorg.0.log shows these lines, which are missing in Xorg.20.log:
[ 105.401] (II) Applying OutputClass "amdgpu-pro" to /dev/dri/card0
[ 105.401] loading driver: amdgpu

The permissions on /dev/dri/card0 seems fine, but this is out of my league:

$ ls -l /dev/dri/card0
crw-rw----+ 1 root video 226, 0 Apr 28 08:34 /dev/dri/card0

$ getfacl /dev/dri/card0
getfacl: Removing leading '/' from absolute path names
# file: dev/dri/card0
# owner: root
# group: video
user::rw-
user:whicheveruserisloggedin:rw-
group::rw-
mask::rw-
other::---

Where 'whicheveruserisloggedin' could be the username of either account, depending on who is using the machine, irrespective of who logged in first. Both users are in the video group.