Thinkpad T460 no longer gets external DisplayPort monitor info

Ask Fedora
1

I have an external Acer Technologies B243PWL monitor plugged into my Thinkpad T460 running Fedora KDE spin. xrandr and System Settings > Display Configuration have both always detected the monitor brand and its 1920x1200 resolution as device DP-1, and KDE Sound uses its speakers as “Digital Stereo (HDMI) Output” profile. The monitor’s on-screen control shows “DDC/CI on”.

Until… it stopped working last week. Display Configuration just shows an unnamed device DP1 with resolutions only up to 1024x768, and the speakers aren’t available as a profile.

This happened just after an upgrade to 6.11.7 kernel, but I rebooted back to 6.11.6 and had the same problem. I plugged in a TV over the laptop’s HDMI port and that was detected as a high-res display and sound profile. It seems the problem could be the monitor, the DisplayPort cable, or the laptop (or software); unfortunately I only have one of each.

I’m trying to figure out the low-level software that reads this stuff, so I can turn on verbose debugging. I found nothing relevant in journalctl. I found ddcutil most of whose commands print “Display not found” without context and don’t display any DP-1 info (just eDP-1, the Laptop’s built-in display). There is a /sys/devices/pci0000:00/0000:00:02.0/drm/card1/card1-DP-1/modes that has the minimal set of resolutions, also in the same directory there’s a status file that contains “connected”, an empty edid file, etc.

Meanwhile, is there a way to tell software that I have a 1920x1200 monitor plugged in even though Linux+KDE doesn’t detect it? I notice an empty /sys/kernel/debug/dri/0000:00:02.0/DP-1/edid_override file…

Thanks for any insight!

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Added multiple-displays

3

I noticed that dnf update on 2024-11-17 also upgraded intel-gpu-firmware and intel-audio-firmware from -20241017 to -2024-1110. I downgraded both and that didn’t help (does this actually cause the older firmware to be loaded?). I then attempted a sudo dnf history rollback to an earlier known-good configuration, but that complained about dozens of unavailable packages and “none of the providers can be installed”, so I did sudo dnf history rollback --skip-unavailable 4 back to before 2024-11-13. Didn’t help.

So it seems more likely hardware than software. I instead connected the Thinkpad and Acer monitor with an HDMI → DVI cable and KDE Plasma immediately recognized it as a 1920x1200 display, but I don’t think DVI does audio. I’ve ordered a new mini DisplayPort → DisplayPort cable from Monoprice.

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I plugged in the new cable, and immediately KDE Plasma detected my external 1920x1200 Acer monitor and its HDMI / DisplayPort audio output.