I am connecting a Dell Latitude 5285 to a Samsung S27H850 by a USB-C cable (Hama USB 3.1, "full featured") and sometimes Xorg detects a signal, but loses it quickly again. Connecting to the same monitor using an USB-C to display port adapter works. When plugging the monitor in, in my recent kernel the following message is printed: [217634.724851] usb 1-1: USB disconnect, device number 11 [217636.049722] acpi INT3400:00: Unsupported event [0x86] [217674.130258] usb 1-3: new high-speed USB device number 12 using xhci_hcd [217674.287390] hub 1-3:1.0: USB hub found [217674.288923] hub 1-3:1.0: 4 ports detected [217674.570263] usb 1-3.4: new full-speed USB device number 13 using xhci_hcd [217674.685149] usb 1-3.4: not running at top speed; connect to a high speed hub [217675.303530] acpi INT3400:00: Unsupported event [0x86] [21:56] manager:~% Because of the ACPI message, I am reporting this against ACPI - if it belongs to the USB section, please let me know. I tested the same cable & monitor with a Macbook, so the physical connection actually works fine. My installed software versions are: [21:56] manager:~% uname -r; pacman -Q | grep xorg 4.14.12-1-ARCH xorg-appres 1.0.4-1 xorg-bdftopcf 1.1-1 xorg-font-util 1.3.1-1 xorg-font-utils 7.6-4 xorg-fonts-alias 1.0.3-1 xorg-fonts-encodings 1.0.4-4 xorg-fonts-misc 1.0.3-5 xorg-luit 1.1.1-2 xorg-mkfontdir 1.0.7-8 xorg-mkfontscale 1.1.2-1 xorg-server 1.19.6-2 xorg-server-common 1.19.6-2 xorg-server-xvfb 1.19.6-2 xorg-setxkbmap 1.3.1-1 xorg-xauth 1.0.10-1 xorg-xbacklight 1.2.1-1 xorg-xdpyinfo 1.3.2-1 xorg-xev 1.2.2-1 xorg-xinit 1.3.4-4 xorg-xinput 1.6.2-1 xorg-xkbcomp 1.4.0-1 xorg-xkill 1.0.4-1 xorg-xmodmap 1.0.9-1 xorg-xrandr 1.5.0-1 xorg-xrdb 1.1.0-2 xorg-xset 1.2.3-1 xorg-xwd 1.0.6-1 xorg-xwininfo 1.1.3-1 [21:56] manager:~%
After upgrading to 4.14.13-1-ARCH the screen turns on and stays on, until I suspend and turn the notebook on again. After suspend, Linux shortly sees a signal, the image is seen on the external monitor and it turns off after a few seconds. Funnily, leaving the screen connected, it turns off and on repeatedly (after about a minute it turns on and turns off after a few seconds. While writing this bug report, characters I type are multipled (i.e. 12x n), when the screen is turning off or on.
[217636.049722] acpi INT3400:00: Unsupported event [0x86] this ACPI message is unrelated, plus it will be fixed after next merge window. For the display issue, please file a bug at freedesktop.org instead.