[Bug 91791] [SKL-U] System hang when plug in a VGA monitor

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=91791

--- Comment #7 from Gary Wang <gary.c.wang at intel.com> ---
Created attachment 119128
  --> https://bugs.freedesktop.org/attachment.cgi?id=119128&action=edit
System hung when plug-in DP MST-enabled hub in Intel SKL-U CRB

I use DP 1.2 MST-supported HUB (Sunix IT DisplayPort to Dual Graphics Splitter)
to test this issue in SKU-U Intel CRB, two DPs input, one miniDP output.

This DP MST hub works well in my win7 Lenovo X230NB via miniDP port. 

But it also suffer from system hung in Ubuntu 15.04 with
drm-intel-nightly-2015/10/19 when plug-in DP MST Hub to CRB's miniDP port.
...
Feb 11 16:16:03 test kernel: [   96.395534] [drm:intel_dp_set_signal_levels]
Using pre-emphasis level 0
Feb 11 16:16:03 test kernel: [   96.412066] [drm:intel_dp_set_signal_levels]
Using signal levels 00000000
Feb 11 16:16:03 test kernel: [   96.412068] [drm:intel_dp_set_signal_levels]
Using vswing level 0
Feb 11 16:16:03 test kernel: [   96.412069] [drm:intel_dp_set_signal_levels]
Using pre-emphasis level 0
Feb 11 16:16:03 test kernel: [   96.428626]
[drm:intel_dp_link_training_clock_recovery [i915]] *ERROR* too ma
Feb 11 16:16:03 test kernel: [   96.428847]
[drm:intel_dp_link_training_channel_equalization [i915]] *ERROR*
....

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