[Bug 1763202] Re: Lenovo X1 Carbon 6th Gen. HDMI not working for external monitor

I'm having same issue running kernel 4.18.0-15 on an X1 extreme using
thunderbolt 3 dock gen 2 connected from dock to monitor via HDMI cable.

Display settings detects monitor but monitor shows no signal.

System is dual boot.

Works on Windows, not on Ubuntu

I am waiting to purchase a DisplayPort cable to see if that makes a
difference but otherwise it doesn't work.

Plugging HDMI directly into laptop (not dock) works.

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Title:
Lenovo X1 Carbon 6th Gen. HDMI not working for external monitor

Status in linux package in Ubuntu:
Fix Released

Bug description:
I have an X1 Carbon 6th Gen. (Intel Core i7 8th Gen, 16 GB RAM, 512 GB
SSD).

When I try to attach an external monitor via an HDMI cable the monitor
receives no signal. Similarly, when I use the Thunderbold dock and
attach two monitors (one via HDMI, one with DisplayPort) only the
monitor attached with DisplayPort works; the monitor attached with
HDMI stays black.

This made me think if this is really a bug in the software. I saw people describing this issue with Windows installed; the solution was to hit a special key combination. Unfortunately, the multimedia keys all not all functional, specifically the F7 key, which seems to meant for "activate external monitor", has no function.
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ApportVersion: 2.20.9-0ubuntu4
Architecture: amd64
AudioDevicesInUse:
USER PID ACCESS COMMAND
/dev/snd/controlC1: peter 3510 F.... pulseaudio
/dev/snd/controlC0: peter 3510 F.... pulseaudio
CurrentDesktop: ubuntu:GNOME
DistroRelease: Ubuntu 18.04
HibernationDevice: RESUME=UUID=8aee1825-7879-4889-8de3-e57900548385
InstallationDate: Installed on 2018-04-06 (5 days ago)
InstallationMedia: Ubuntu 18.04 LTS "Bionic Beaver" - Alpha amd64 (20180403)
MachineType: LENOVO 20KHCTO1WW
Package: linux (not installed)
ProcFB:
0 EFI VGA
1 inteldrmfb
ProcKernelCmdLine: BOOT_IMAGE=/vmlinuz-4.15.0-13-generic.efi.signed root=/dev/mapper/ubuntu--vg-root ro quiet splash vt.handoff=1
ProcVersionSignature: Ubuntu 4.15.0-13.14-generic 4.15.10
RelatedPackageVersions:
linux-restricted-modules-4.15.0-13-generic N/A
linux-backports-modules-4.15.0-13-generic N/A
linux-firmware 1.173
Tags: bionic
Uname: Linux 4.15.0-13-generic x86_64
UpgradeStatus: No upgrade log present (probably fresh install)
UserGroups: adm cdrom dip docker lpadmin plugdev sambashare sudo
_MarkForUpload: True
dmi.bios.date: 02/27/2018
dmi.bios.vendor: LENOVO
dmi.bios.version: N23ET37W (1.12 )
dmi.board.asset.tag: Not Available
dmi.board.name: 20KHCTO1WW
dmi.board.vendor: LENOVO
dmi.board.version: SDK0J40709 WIN
dmi.chassis.asset.tag: No Asset Information
dmi.chassis.type: 10
dmi.chassis.vendor: LENOVO
dmi.chassis.version: None
dmi.modalias: dmi:bvnLENOVO:bvrN23ET37W(1.12):bd02/27/2018:svnLENOVO:pn20KHCTO1WW:pvrThinkPadX1Carbon6th:rvnLENOVO:rn20KHCTO1WW:rvrSDK0J40709WIN:cvnLENOVO:ct10:cvrNone:
dmi.product.family: ThinkPad X1 Carbon 6th
dmi.product.name: 20KHCTO1WW
dmi.product.version: ThinkPad X1 Carbon 6th
dmi.sys.vendor: LENOVO

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