[Bug 65757] New: [snb dp] Screen sometimes remains black after mode change.

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=65757

          Priority: medium
            Bug ID: 65757
          Assignee: intel-gfx-bugs at lists.freedesktop.org
           Summary: [snb dp] Screen sometimes remains black after mode
                    change.
        QA Contact: intel-gfx-bugs at lists.freedesktop.org
          Severity: normal
    Classification: Unclassified
                OS: Linux (All)
          Reporter: michalsrb at gmail.com
          Hardware: Other
            Status: NEW
           Version: unspecified
         Component: DRM/Intel
           Product: DRI

Created attachment 80814
  --> https://bugs.freedesktop.org/attachment.cgi?id=80814&action=edit
drm.debug=0xe dmesg of boot and 4 modetest start/stops leading to black screen

I have Intel Sandybridge GT1 card (8086:0102) with monitor connected over
display port. Sometimes the monitor stays black after mode change.

It happens no matter what caused the mode change. (Observed when i915 module is
loaded, on start/end of X, on resolution change in X or when using modetest
program from console.) The screen lights up again after few additional mode
changes.

It didn't use to happen - I bisected the causing commit:
  2514bc510d0c3aadcc5204056bb440fa36845147
  drm/i915: prefer wide & slow to fast & narrow in DP configs

  It was using 1 lane before the patch and now it is using 2 lanes.

It is still happening with current drm-intel kernel. When I experimentally
limited the lane count to 1, mode changing worked reliably again.

There is no difference in dmesg (with drm.debug=0xe) or in intel_reg_dumper
output after successful mode change and the one leading to blank screen. The
link training passes successfully on first try in both cases.

There is one suspiciously looking message in dmesg:
  [drm] Wrong MCH_SSKPD value: 0x17050407
  [drm] This can cause pipe underruns and display issues.
  [drm] Please upgrade your BIOS to fix this.

Attached dmesg.txt shows boot to console (X is disabled) and 4 modetest program
start/stops, where the mode change caused by last modetest program stop caused
the black screen. It is annotated with lines starting with "Annotation:".

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