Nouveau - Sep 2015 - External monitor remains black - nouveau reports "link training failed"

Hi 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.

My setup 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"


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

Fredy Neeser
IBM Zurich Research Laboratory
Switzerland