External monitor erratic blanking when system is inactive

Support Graphics & Display
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1

Hi, I am encountering a weird issue with an external monitor and I wonder if someone has ever faced something similar and may already have a recipe to fix the problem.

I have an aging (haswell) laptop that stays most of the time attached to an external monitor (Philips 27 inches, 2560x1440 pixels, recent), keyboard and mouse. Because it is pre-usb C, the monitor is attached to the laptop HDMI port. The laptop runs KDE plasma as its DE and the latter is configured so that the laptop internal display is always kept inactive. Plasma is also configured to blank the screen after 5 minutes of inactivity.

Now, when the screen is left inactive, the following happens:

  • after 5 minutes the external monitor blanks
  • after approximately 10 more seconds the external monitor reports “no signal”
  • after approximately 5 more seconds the external monitor goes in standby

so far so good, but then

  • after some more minutes, the laptop plays a notification sound, the internal display switches on, then it plays another notification sound the internal display switches off and the the external monitor switches on. For a few seconds a couple of notification popups stay visible saying that a display has been detached and that a display has been attached.

At this point the cycle may repeat, monitor blanks, stand by beep beep, monitor is alive again, or, more likely, the monitor remains unblanked with the lock screen. Funny enough, if at this point I switch off the external monitor, the internal one will never blank either.

Any clue?

My understaning is that after some time the external display has entered stand by, plasma thinks that it has been disconnected and acts consequently. But why? Is it possible to block this behavior? And then? why does the pattern not repeat forever, but blanking remains completely broken after a few cycles? This is also nasty, because when one sees the monitor remaining on he is tempted to switch it off, but at this time, the laptop internal monitor will remain on forever.
Can a KDE bug be at play?

2

You might check your BIOS and make certain that “Wake on LAN” isn’t activated. Sometimes an app checking for updates or sending telemetry can be enough activity to leave the sleep state.

Otherwise, if it is KDE related, then searching through their open bug reports might reveal something.

3

Would like to report some progress in analyzing the issue:

  1. Nothing has recently been changed in the bios, this is a functionality that used to work…
  2. Contrarily to my initial expectation, this is does not seem to be an issue with the monitor dropping some signal when entering stand by: with another laptop, same cabling, also intel graphics (but more recent) the monitor stays in stand by with no problem.

So, it may be:

  • an issue with the current graphics card drivers in the kernel (the other system I tried uses another kernel version) or with the graphics drivers on that specific graphics card (haswell);
  • an incompatibility between the specific version of the graphics card that my laptop uses and the monitor;
  • something failing at the hardware level in my elderly laptop.

In any case, I am already happy that the monitor seems to be OK.