[Nouveau] External monitor remains black - nouveau reports "link training failed"

[Fredy Neeser]

Hello list,

I had a working dual-monitor setup with KDE Plasma 5 on a Lenovo W530
"Optimus" laptop (hybrid graphics, nouveau + i915 drivers), but it seems to
fall apart with recent kernels, i.e., the external monitor (connected to
the laptop's docking station via DisplayPort) no longer works.

My setup still works fine with kernels up to and including
4.1.3-201.fc22.x86_64, but  with kernel  4.1.6-200.fc22  (as well as
another update on top of 4.1.6), the external monitor remains black, and
the journal shows several lines

    kernel: nouveau E[   PDISP][0000:01:00.0][0x00000006] 06:0006:0f48:
link training failed

not present with previous kernels -- see also

  https://bugzilla.redhat.com/show_bug.cgi?id=1260053
  Bug 1260053 - External monitor remains black with kernel
4.1.6-200.fc22.x86_64 - nouveau reports "link training failed"


Could this be related to some changes in powering down the DPs (which
requires link training to resume)?

Any ideas for debugging / narrowing down the issue are much appreciated !

Fredy Neeser
IBM Zurich Research Laboratory
Switzerland