MST hub broken on DC drivers on latest 4.11 staging

[Krzysztof Nowicki]

Hi,

I'm using a Carrizo-powered HP laptop with it's docking station that
has an embedded MST hub. I've been successfully using a 4.9 kernel
with an older snapshot of the DC branch having two DP screens
connected to it.

Since I'm having some occasional stability issues I decided it was
time to migrate to 4.11 in hope to get this resolved, especially after
Alex published a 4.11 staging branch with everything integrated.

I have built a kernel from Alex's amd-staging-4.11 branch with stable
kernel 4.11.2 patches rebased on top of it. Unfortunately I had no
success with powering up the screens behind the MST hub. The devices
are being recognized, and I can even see a cloned image on all screens
when in the terminal, but after switching to Xorg only the internal
eDP is on and no additional screens are even listed - just as if some
enablement step was missing to make the screens appear.

I have captured a (quite lengtly) log with drm.debug=0x17
(https://www.dropbox.com/s/wqzgb4i8klt34zr/amdgpu-dc-4.11-20170602.log).

During startup no external displays are connected. At the 401 second I
have connected the first display to the MST hub, while at 585s I have
connected the second one. At 675s both displays have been disconnected
from the hub.

Regards
Chris