Dell U2415 connected to K620 via DP1.2 MST is blank. Driver 384.111 on Ubuntu 16.04

Dell U2415 connected to K620 via DP1.2 MST is blank. Driver 384.111 on Ubuntu 16.04.

Video: https://youtu.be/qZ4LfXFxtYo
Screenshot: https://imgur.com/xB4Zm0i

I have spent many many hours trying to get daisy-chaining working across three Dell UltraSharp U2415 monitors I have.

In the first instance I am only trying to succeed with a mere two monitors. Troubleshooting appears to have been complicated by various overlapping shortcomings in the products, linux and/or DP standard but I have managed to minify these to get something which feels like it should almost be working.

Shortcoming 1: The monitors ship with DP1.1a enabled by default in the settings menu but DP1.2 required for MST. According to Dell, NVIDIA drivers do not like it when the last monitor in the chain has DP1.2 enabled. Mitigation: I have made sure only the intermediary monitor has DP1.2 enabled and that the end-of-chain monitor has DP1.2 disabled.

Shortcoming 2: When one of these Dell monitors has no visuals to display it enters ‘energy saving mode’. When this happens it appears that the driver perceives this as a change in monitor configuaration. When the OS adjusts for this perceived change in configuration the monitor wakes from energy saving mode, the driver reverts back to the previous configuration and a loop condition is created. Mitigation: I have configured the monitors OSD settings menu to its maximum timeout setting (60s), the screen will not enter energy saving mode so long as this menu is open. I can navigate the menu to reset the timeout timer thus can prevent the loop condition.

So what is the current state?

I have all three moniors connected as follows:

  • Monitor A: Connected to K620 DP out. Operating in DP1.2 mode.
  • Monitor B: Connected to Monitor A DP out. Operating in DP1.1a mode but blank.
  • Monitor C: Connected to K620 DVI out. Operating on HDMI input.

In NVIDIA X Server settings all three displays are detected and their layout matches their physica layout on my desk:

[A][B][C]

Rough steps to reproduce:

  • Install Ubuntu 16.04 desktop.
  • Update apt.
  • Install any upgrades.
  • Install HWE stack.
  • sudo ubuntu-drivers autoinstall
  • configure hardware as above
    nvidia-bug-report.log.gz (160 KB)

Can you check with a 390 driver if that helps at least with shortcoming 2?
You can attach files to existing posts, when hovering the mouse pointer over it, a paperclip appears.

The shortcoming #2 behaviour was the same at first but later disappeared. A couple of unbeforeseen symptoms have surfaced. This is what I did:

  • installed and activated the 390.25 drivers (and confirmed correct version active).
  • shortcoming #2 was evident but could still be mitigated by keeping OSD menu open.
  • Used Ctrl + Alt + F1 to switch to TTY1 – monitor B gets signal and displays shell prompt! Only monitors A and C displayed the prompt with the previous driver.
  • stopped lightdm service
  • was trying to remember xserve logging verbosity argument so ran ‘xserve --help’…
  • …apparently xserve ignores --help and just runs anyway
  • started lightdm service, logged in
  • shortcoming #2 resurfaces but this time the loop stops after ~4 iterations.
  • I then switched from GUI on TTY7 to TTY1 and back again just to confirm the behaviour and this error box showed up: https://imgur.com/CkDmMQ9
  • I reopen the NVIDIA X Server Settings window I and monitor [B] monitor has been disabled. From your post I presume this is some sort of graceful degradation feature added to the driver since v384.

I have attached another bug report. Please would you be able advise what I should try next?

Maybe there is some easier way of driving 3-4 monitors from a single card in the 200USD price range?
nvidia-bug-report-390.25.log.gz (116 KB)

Now I can also make [A] the problem monitor and have [B][C] working by doing the following:
  • Unplug B from A

  • Disable DP1.2 on A

  • Enable DP1.2 on A

  • Plug B back into A

  • Press B power button
    Result:
    [■][B][C]

  • A: black

  • B: works

  • C: works

Screenshot: desktop spans all three
Ctrl + Alt + F1: all three get prompt
Ctrl + Alt + F7 (after Ctrl + Alt + F1): Switches back to [A][■][C]


It’s a delicate state; switching out-of and back into TTY7 will cause a revert to [■][B][C]. As will doing ‘detect displays’ in unity-control-center display.

Hi guys, please can you advise on what my options are?