[DELL][BIONIC] Freeze after docking station disconnect

[Gregor Vindry]

Problem description
-------------------
When I disconnect my Dell Latitude 5501 laptop from its Dell WD19 docking station, the screens layout doesn't switch back to single-screen, and the system either:

* freezes immediately (desktop background still displayed, but no mouse, no keyboard)
* doesn't freeze. In that case, if I open a terminal and sudo poweroff, the screen turns black, then the system freezes.

In both cases, the only recovery method consists in a hard reset.

Hardware setup description
--------------------------
Laptop is connected to docking station via thunderbolt port.

The WD19 docking station is connected to 2 external screens via display ports + USB3 cables (to connect to the embedded USB3 ports on the screens).

One of the screens is connected to a keyboard and a mouse via USB.

Technical info
--------------
See attached file kern.log for a disconnect (see at Mar 23 15:35:56).

The problem does not happen on older kernel 4.15.0-1073-oem, so it looks like a regression introduced between 4.15.0-1074-oem and 4.15.0-1076-oem.

Laptop BIOS version is up-to-date (v1.6.1).

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: linux-image-4.15.0-1076-oem 4.15.0-1076.86
ProcVersionSignature: Ubuntu 4.15.0-1076.86-oem 4.15.18
Uname: Linux 4.15.0-1076-oem x86_64
ApportVersion: 2.20.9-0ubuntu7.12
Architecture: amd64
AudioDevicesInUse:
 USER PID ACCESS COMMAND
 /dev/snd/controlC0: gvindry 2492 F.... pulseaudio
CurrentDesktop: ubuntu:GNOME
Date: Mon Mar 23 15:42:42 2020
DistributionChannelDescriptor:
 # This is the distribution channel descriptor for the OEM CDs
 # For more information see http://wiki.ubuntu.com/DistributionChannelDescriptor
 canonical-oem-somerville-bionic-amd64-20180608-47+brookhollow+X77
InstallationDate: Installed on 2019-12-28 (86 days ago)
InstallationMedia: Ubuntu 18.04 "Bionic" - Build amd64 LIVE Binary 20180608-09:38
MachineType: Dell Inc. Latitude 5501
ProcEnviron:
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=fr_FR.UTF-8
 SHELL=/bin/bash
ProcFB: 0 inteldrmfb
ProcKernelCmdLine: BOOT_IMAGE=/boot/vmlinuz-4.15.0-1076-oem root=UUID=d829dfff-8fb3-46f6-8175-92de7a872776 ro mem_sleep_default=deep quiet splash vt.handoff=1
RelatedPackageVersions:
 linux-restricted-modules-4.15.0-1076-oem N/A
 linux-backports-modules-4.15.0-1076-oem N/A
 linux-firmware 1.173.16
SourcePackage: linux-oem
UpgradeStatus: No upgrade log present (probably fresh install)
WifiSyslog:

dmi.bios.date: 11/14/2019
dmi.bios.vendor: Dell Inc.
dmi.bios.version: 1.6.1
dmi.board.name: 0Y8H01
dmi.board.vendor: Dell Inc.
dmi.board.version: A00
dmi.chassis.type: 10
dmi.chassis.vendor: Dell Inc.
dmi.modalias: dmi:bvnDellInc.:bvr1.6.1:bd11/14/2019:svnDellInc.:pnLatitude5501:pvr:rvnDellInc.:rn0Y8H01:rvrA00:cvnDellInc.:ct10:cvr:
dmi.product.family: Latitude
dmi.product.name: Latitude 5501
dmi.sys.vendor: Dell Inc.

[Gregor Vindry (gv5470)]

I updated to 4.15.0-1079-oem, and the problem is gone for me now.