Created attachment 180551 [details] dmesg after booting the computer and turning the screen on manually When booting the kernel the screen of this iMac 21.5" (mid 2010) turns off. I believe this is when entering KMS. If I access the iMac via ssh I see that: /sys/class/drm/card0-eDP-1/status -> "connected" /sys/class/drm/card0-DP-1/status -> "disconnected" I can get the monitor to turn on if I: echo "on" > /sys/class/drm/card0-DP-1/status Everything works perfectly fine after I turn the screen on with that command, including 3D acceleration. If I boot with "nomodeset" the screen doesn't turn off. I don't have an external monitor with DisplayPort that I can connect to this iMac to test if that's where the video output is going, but some google search reveals that some users confirm the output is going to the external DisplayPort output by default.
I think I'm running into the same bug with kernel 4.13 on a 27" iMac from 2009 (model iMac10,1). Turning the monitor on via sysfs doesn't yet seem to work, though, but the sysfs output and other behaviour seem identical.