External monitor is not recognized as a monitor/no video output when connected directly via USB-C <-> USB-C

[Mugi4ok]

Your system information

• Steam client version: 1721173382
• SteamOS version: 3.5.19
• Opted into Steam client beta?: No
• Opted into SteamOS beta?: No
• Have you checked for updates in Settings > System?: Yes

Please describe your issue in as much detail as possible:

When connecting an external monitor (LG 34WP75C-B) directly to Steam Deck using USB-C <-> USB-C cable there is no video output, Steam Deck does not recognize it as an external monitor whatsoever but rather as a USB hub and a charging device. Monitor does not detect any output, displaying the corresponding message. Both devices do support AltDP mode per specs, monitor works as intended with other devices via USB-C, but not with Deck/SteamOS.

I suspect this has something to do with AMD drivers and handshake, especially after reading through this issue which is partly similar to my problem: https://github.com/ValveSoftware/SteamOS/issues/1399 Unfortunately, I am not Linux-savvy enough to patch the kernel to see if the suggested fix helps with my problem as well. Additionally, this thread on Steam forum suggests that the issue majorly affects LG users: https://steamcommunity.com/app/1675200/discussions/1/3416556480597288512/?ctp=6 Still, given that the monitor works fine with the rest of devices, I believe it is fair to consider this a SteamOS issue.

It is also worth to mention that I have recently attempted to opt for several beta builds, including the master branch, but the issue is still present.

Steps for reproducing this issue:

1. Connect external LG 34WP75C-B monitor to Steam Deck directly via USB-C <-> USB-C cable.
2. Observe no output to the external display.

Log snippet from dmesg when connecting the monitor

[16406.593745] usb 1-1: new high-speed USB device number 10 using xhci_hcd
[16406.744686] usb 1-1: New USB device found, idVendor=0bda, idProduct=5411, bcdDevice= 2.03
[16406.744696] usb 1-1: New USB device strings: Mfr=1, Product=2, SerialNumber=0
[16406.744698] usb 1-1: Product: USB2.1 Hub
[16406.744700] usb 1-1: Manufacturer: Generic
[16406.786675] hub 1-1:1.0: USB hub found
[16406.787936] hub 1-1:1.0: 5 ports detected
[16407.130410] usb 1-1.3: new full-speed USB device number 11 using xhci_hcd
[16407.260088] usb 1-1.3: New USB device found, idVendor=043e, idProduct=9a39, bcdDevice= 4.17
[16407.260098] usb 1-1.3: New USB device strings: Mfr=1, Product=3, SerialNumber=4
[16407.260101] usb 1-1.3: Product: LG Monitor Controls
[16407.260104] usb 1-1.3: Manufacturer: LG Electronics Inc.
[16407.260106] usb 1-1.3: SerialNumber: 312NTUWCA374
[16407.342441] hid-generic 0003:043E:9A39.000B: hiddev97,hidraw6: USB HID v1.11 Device [LG Electronics Inc. LG Monitor Controls] on usb-0000:04:00.3-1.3/input0

If I can provide any additional information/logs to help better identify the issue -- I'll be glad to do so.

[deftdawg]

SteamOS 3.7 w/ Linux 6.11 has been released - might resolve this issue

https://store.steampowered.com/news/app/1675200/view/820329049655084700

[Mugi4ok]

Thank you for the heads-up, @deftdawg, much appreciated! Also, thank you for your research around the issue you faced.

Unfortunately, it seems that my issue is not fixed by this update, and nor did it get any attention from the staff team.

The behavior I get from the monitor after software update remains the same, i.e. "No output detected". Along the way I also tried numerous advice I found across the Internet, like resetting PD controller, deleting ~/.config/gamescope/modes.cfg file (even though it is empty), multiple "pseudo-magical" combinations of rebooting and reconnecting the input cable, trying at least 5 different cables, etc.

What is even more disheartening is that, reportedly, OLED does not have this issue. Still not losing hope for this issue to get addressed one day, but since the issue does not seem to be too widespread and does not affect OLED users, realistically it is unlikely to get fixed.