[Bug 112136] New: DP MST with WD15 Dock leads to inconsistent port number

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=112136

            Bug ID: 112136
           Summary: DP MST with WD15 Dock leads to inconsistent port
                    number
           Product: DRI
           Version: unspecified
          Hardware: x86-64 (AMD64)
                OS: Linux (All)
            Status: NEW
          Severity: not set
          Priority: not set
         Component: DRM/Intel
          Assignee: intel-gfx-bugs at lists.freedesktop.org
          Reporter: p.pawit at gmail.com
        QA Contact: intel-gfx-bugs at lists.freedesktop.org
                CC: intel-gfx-bugs at lists.freedesktop.org

Created attachment 145815
  --> https://bugs.freedesktop.org/attachment.cgi?id=145815&action=edit
dmesg with "drm.debug=0x1e log_buf_len=1M"

I am using a Dell Latitude 5491 with the WD15 dockr over USB type-C. The Dock
is a DP MST Dock which 3 ports: HDMI, mDP and VGA. I am using HDMI and mDP. The
displays work correctly, however, when I unplug and replug the displays, they
get new names leading them to reset to the default display configuration until
I also configure displays with that name.

Tested to be broken on 5.3.7 as well as 5.4-rc4.

Attached dmesg and xrandr.

1) Booted up while connected to Dock. Displays show up as DP-2-1 (DP-4 in the
kernel) and DP-5. DP-6 also shows up but is disconnected (this is probably the
VGA work). The output of xrandr is in xrandr_before_verbose.txt
2) Detached dock from system. (See "XXXXXXXXXXXXXX unplug" in dmesg)
3) Replug dock to system. (See "XXXXXXXXXXXXXX replug" in demsg)
4) DP-2-1 and DP-5 show as disconnected. Monitors show as DP-6 and DP-7
instead. DP-8 also shows up disconnected.
5) End of log (See "XXXXXXXXXXXXXX done" in dmesg)

Expected behavior: Two monitors show up as DP-2-1 and DP-2-2 and remain that
way consistently.

The first time I unplug/replug after boot, this happens without fail. However,
the later times sometimes the display port name changes sometimes it doesn't.

Linux Distribution: Mint 19.2

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