https://bugs.freedesktop.org/show_bug.cgi?id=107333
--- Comment #3 from Matias N. Goldberg dark_sylinc@yahoo.com.ar --- I think I'm having the same problem.
I'll first describe my symptoms (all of them happening with DC=1):
Case 1: 1. Boot System 2. Monitors must be off by the time GRUB inits the kernel. 3. Wait until everything's up 4. Turn on monitor(s) 5. Monitor(s) will go to sleep because they're not receiving any signal
Workaround: Ctrl+Alt+F1 to go into terminal, monitor now goes on. Type: sudo service lightdm restart
Case 2: 1. Suspend the system 2. Turn off the monitors 3. Resume the system 4. Turn on monitor(s) 5. Monitor(s) will go to sleep 6. Trying Ctrl+Alt+F1 then Ctrl+Alt+F7 will show the dreaded "Session is Locked You'll be redirected to the unlock dialog in a few seconds" but after a few seconds the monitors go to sleep again and lightdm never unlocks.
Workaround: Ctrl+Alt+F1 to go into terminal, monitor now goes on. Type: sudo loginctl unlock-sessions
Case 3: 1. Login to XFCE 2. Set "Extend Displays" 3. Turn off the secondary monitors (I have 2). All is OK. The extended desktop still works even though I cannot see it (expected behavior) 4. Turn off the main monitor too. 5. After turning on main monitor, now xfce will be set to "Mirror Displays" instead
Workaround: Super Key + P then select "Extend to the right" again.
Except for case 3 in which a secondary monitor is needed, it doesn't seem like having 2 monitors affects the symptoms.
It's as if the kernel changes something that makes xfce & lightdm go nuts when all monitors go off.
My environment: Xubuntu 18.04 XFCE 4.12 Kernel 4.18.9 libdrm-2.4.94 Device: Radeon RX 560 Series (POLARIS11, DRM 3.26.0, 4.18.9, LLVM 7.0.0) (0x67ff) Version: 18.2.0
Radeon RX 560 2GB Gigabyte Main LG FLATRON E2342 Monitor connected via HDMI Secondary LG FLATRON E2341 Monitor connected via DVI
Booting with amdgpu.dc=0 fixes these issues.
This is the output of xrandr when both monitors are off and amdgpu.dc=0 Screen 0: minimum 320 x 200, current 1920 x 1080, maximum 16384 x 16384 DisplayPort-0 disconnected (normal left inverted right x axis y axis) HDMI-A-0 connected primary 1920x1080+0+0 (normal left inverted right x axis y axis) 510mm x 290mm 1920x1080 60.00*+ 50.00 59.94 1920x1080i 60.00 50.00 59.94 1680x1050 59.88 1400x1050 59.95 1600x900 60.00 1280x1024 60.02 1440x900 59.90 1280x800 59.91 1152x864 75.00 1280x720 60.00 50.00 59.94 1024x768 60.00 800x600 60.32 720x576 50.00 720x480 60.00 59.94 640x480 60.00 59.94 DVI-D-0 connected (normal left inverted right x axis y axis) 1920x1080 60.00 + 1680x1050 59.88 1280x1024 75.02 60.02 1280x960 60.00 1152x864 75.00 1024x768 75.03 60.00 832x624 74.55 800x600 75.00 60.32 56.25 640x480 75.00 59.94
This is the output of xrandr when both monitors are off and amdgpu.dc=1
Screen 0: minimum 320 x 200, current 1920 x 1080, maximum 16384 x 16384 DisplayPort-0 disconnected (normal left inverted right x axis y axis) HDMI-A-0 disconnected primary (normal left inverted right x axis y axis) DVI-D-0 connected (normal left inverted right x axis y axis) 1920x1080 60.00 + 1680x1050 59.88 1280x1024 75.02 60.02 1440x900 60.00 1280x960 60.00 1280x800 60.00 1152x864 75.00 1280x720 60.00 1024x768 75.03 60.00 832x624 74.55 800x600 75.00 60.32 56.25 640x480 75.00 59.94 720x400 70.08
Notice the HDMI monitor is now listed as disconnected.
Perhaps this is a case of "not our bug" and software should accommodate for "hotpluggable monitors". However if that's the case, this is still a problem when the new behavior breaks basic desktop system software on boot and resume-from-suspend; and I'd suggest either DRM should report a state of connected-but-off (if it's not already there, then this is a driver bug), or there should be an extra kernel param to not list powered-off monitors as disconnected, without having to disable amdgpu.dc.
Cheers