[Bug 107333] Display Not Detected If Powered Off When amdgpu.dc=1

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=107333

--- Comment #3 from Matias N. Goldberg <dark_sylinc at 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

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