[Bug 104451] New: [SKL] Black screen on 4.15-rc6 (DP link training error) [regression]

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=104451

            Bug ID: 104451
           Summary: [SKL] Black screen on 4.15-rc6 (DP link training
                    error) [regression]
           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: direx at betriebsdirektor.de
        QA Contact: intel-gfx-bugs at lists.freedesktop.org
                CC: intel-gfx-bugs at lists.freedesktop.org

Created attachment 136486
  --> https://bugs.freedesktop.org/attachment.cgi?id=136486&action=edit
DP link training errors in dmesg with drm.debug=0x1e

I've just booted 4.15-rc6 and I am getting a black external screen (Thinkpad
L460 with Ultra Dock and external DP monitor). Stable Kernels (4.14 and older)
work just fine.

The dmesg is flooded with link training errors, so I was unable to capture a
full system log through SSH. The attached dmesg lines are the last link
training messages with drm.debug=0x1e. Thousands of these messages appear every
second.

The messages appear right when modesetting kicks in, before Wayland is
starting.

This is a major issue, as this is a regression which cause DP displays to fail.


My Configuration:

- Thinkpad L460 with Ultra Dock (contains a DP MST hub)
- External DP monitor connected to Ultra Dock
- Arch Linux
- Kernel 4.15-rc6
- Intel Core i5-6200U

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