HDMI connected monitors fail to sleep and instead turn back on when amdgpu.dc=1

Submitted by tme

Assigned to Default DRI bug account

Link to original bug (#109246)

Description

Original launchpad thread: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1790861

Overview:
When my computer tries to suspend my monitors after a period of idleness, they'll suspend for a few seconds before waking back up.

Steps to Reproduce:

1. Have a monitor attached over HDMI.
   
2. Set screen energy saving (in Plasma 5) to 1 minute.
   
3. After 1 minute has passed, the monitor will try to enter sleep mode. It will fail and turn back on.
   
4. The monitor will continue to attempt to enter sleep mode every minute until interrupted by the user.
   

Actual Results:
The monitors turn back on a few seconds after entering sleep mode.

Expected Results:
The monitors stay in sleep mode.

Additional Information:
While I have two monitors, this issue is only triggered when one is connected using HDMI. Connecting only one monitor using HDMI will trigger the problem. Connecting either one using DVI does not trigger the problem.
I am unable to test if DisplayPort triggers the problem, as I have no compatible monitors.
Setting the kernel parameter amdgpu.dc=0 does not cause the problem to trigger.

I haven't confirmed if the other affected users in the launchpad thread are using HDMI, so I'm only reporting my experience.