Bug 349362

I have two U2414H Dell screens. Both have HDMI and Displayport inputs. My laptop has a Mini Displayport output and an HDMI output.
Both screens and laptop support displayport 1.2 and I'm using the cables supplied with the laptop abd both run at 1920x1080 @ 60Hz

Both screens support MST (daisy chaining) although that's not the issue.

When I plug in my laptop to one of the screens (DP1) the desktop locks up, keyboard and mouse become unresponsive. The only way to access the laptop is via SSH from another machine and once i SSH into the machine it's not under any load, the CPU fan is not blowing and I can't see any processes consuming CPU via top.
Meanwhile, the desktop is still unresponsive. It's either totally unresponsive or if I move the mouse, about a minute later the mouse cursor may jump to another position on the screen.

If I tail .xsession-errors via SSH, I see that this file updates very very slowly. It's almost as if the laptop is working in slow motion.

On the odd occasion when I have gotten the displayport screens working (by waiting about 10 minutes), it works  really well and I've even been able to get MST daisy chaining working (ie DP1 is split into DP1-1 and DP1-2 in kscreen and I can move screens around position them correctly). Once I Apply a Kscreen setting, there is an extended wait of about a minute until the settings are applied, screens will flicker  randomly and then it will settle down. If I disable Displayport 1.2 mode on the screens, the same long wait and desktop lock up occurs.

When I use the HDMI output on the laptop, it works straight away with no lock ups or waiting. Interestingly, if I plug in to another Dell monitor (U2711) this works straight away, so it appears to be something related to Displayport connections to these screens.

I've turned DCC/CI off in the screen settings and rebooted the screens, which made the setup process work once.


I've no idea how to debug this, are there any logs that KDE writes to when configuring screens ? If there was something causing high load on the system then it would be easier to debug but the only symptom is that the laptop seems to run in complete slo-mo. I've often had to do a hard reset of the laptop to get a desktop back.


Happy to debug this further and provide information as required. Multiple screen setup in Plasma 5 needs a bit of polish so am happy to help out where possible to make it better.




Reproducible: Always

Steps to Reproduce:
1. Plug in displayport cable to laptop

Actual Results:  
Desktop becomes completely unresponsive. Mouse movements translate to a cursor movement about a minute later, if at all.

Expected Results:  
Screens should show based on last used configuration. MST should work on this kernel, laptop and screen setup. Changing screen layout in the Kscreen settings should immediately apply without an extensive wait.

laptop is running standard Kubuntu 15.04  plasma 5 packages, 5.3.1 from the kubuntu backports.

- HDMI on same screens work
- Displayport on another screen works.