[Bug 87124] New: [Bisected] Linux 3.17+ defaults to low resolution on Thinkpad T440p

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=87124

            Bug ID: 87124
           Summary: [Bisected] Linux 3.17+ defaults to low resolution on
                    Thinkpad T440p
           Product: DRI
           Version: XOrg git
          Hardware: Other
                OS: All
            Status: NEW
          Severity: normal
          Priority: medium
         Component: DRM/Intel
          Assignee: intel-gfx-bugs at lists.freedesktop.org
          Reporter: proski at gnu.org
        QA Contact: intel-gfx-bugs at lists.freedesktop.org
                CC: intel-gfx-bugs at lists.freedesktop.org

Created attachment 110588
  --> https://bugs.freedesktop.org/attachment.cgi?id=110588&action=edit
Output of lspci -vvvnnxxx

I'm using Lenovo Thinkpad T440p with a dock. A monitor with 2560x1600
resolution is connected to a DP port on the dock. The lib panel resolution is
1366x768.

Linux 3.16 starts with the native resolution of the external monitor. Linux
3.17 and newer starts with the resolution of the dock (actually, a few pixels
less than that, 1360x768). It can be set to the native resolution by running

xrandr --output DP4 --auto

Bisection gives the first bad commit: 0e32b39ceed665bfa4a77a4bc307b6652b991632
by 
Dave Airlie - drm/i915: add DP 1.2 MST support (v0.7)

Unlike other bugs introduced by that commit, this bug is not fixed in Linux
3.18 and in the current commit on intel-drm/drm-intel-nightly,
97416b3589be60c5ece051784aa6bf34c31d86bc.

xrandr shows that the new kernels recognize more DP ports. The port connected
to the monitor is seen as DP2 by the old kernels and DP4 by the new kernels.

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