[Bug 104001] New: DP monitor on dock of HP laptop doesn't work with 4.2 kernel

[bugzilla-daemon at bugzilla.kernel.org]

https://bugzilla.kernel.org/show_bug.cgi?id=104001

            Bug ID: 104001
           Summary: DP monitor on dock of HP laptop doesn't work with 4.2
                    kernel
           Product: Drivers
           Version: 2.5
    Kernel Version: 4.2
          Hardware: All
                OS: Linux
              Tree: Mainline
            Status: NEW
          Severity: normal
          Priority: P1
         Component: Video(DRI - Intel)
          Assignee: intel-gfx-bugs at lists.freedesktop.org
          Reporter: tiwai at suse.de
                CC: intel-gfx-bugs at lists.freedesktop.org
        Regression: No

The DP monitor connected to a dock port of HP EliteBook 940 G1 doesn't work any
longer since 4.2 kernel.  It worked with 4.1.

When connected, it results in repeated errors of 
  [drm:intel_dp_start_link_train [i915]] *ERROR* too many voltage retries, give
up

eventually got many other errors and warnings.

The git bisection pointed to the commit
8504c74c7ae48b4b8ed1f1c0acf67482a7f45c93
  drm/i915: Preserve ddi_pll_sel when allocating new pipe_config

and actually reverting this commit makes DP working again.

IIRC, it's MST.

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