[Bug 97397] New: Kernel 4.7.x does nor initialize external DisplayPort in Dell E7250 making it unusable

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=97397

            Bug ID: 97397
           Summary: Kernel 4.7.x does nor initialize external DisplayPort
                    in Dell E7250 making it unusable
           Product: DRI
           Version: DRI git
          Hardware: x86-64 (AMD64)
                OS: Linux (All)
            Status: NEW
          Severity: major
          Priority: medium
         Component: DRM/Intel
          Assignee: intel-gfx-bugs at lists.freedesktop.org
          Reporter: ed at yen.ipipan.waw.pl
        QA Contact: intel-gfx-bugs at lists.freedesktop.org
                CC: intel-gfx-bugs at lists.freedesktop.org

Created attachment 125882
  --> https://bugs.freedesktop.org/attachment.cgi?id=125882&action=edit
dmesg output

$ uname -a
Linux laptok 4.7.1-1 #1 SMP Wed Aug 17 19:12:42 CEST 2016 x86_64
Intel(R)_Core(TM)_i5-5300U_CPU_ at _2.30GHz PLD Linux

Machine: Dell Latitude E7250

Kernel 4.7.x does not initialize the external DisplayPort in my laptop. During
start it throws in the logs:

[drm:intel_dp_start_link_train [i915]] *ERROR* failed to train DP, aborting

(full log in the attached dmesg output) and does not allow me to use the
external display connected to this output. It does not matter, it the external
screen is connected at boot time or not - it is not visible in the system.

Everything works fine with kernel 4.6.4 - I can connect external display on the
fly (I use DP-to-VGA converter for I usually use this port for presentations
for my students).

The internal eDP screen works fine and without any problems with both 4.6.x and
4.7.x kernels.

This behaviour is 100% reproducible on my hardware.

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