Help me setup HDMI Dummy Plug

[Weraspowa]

Hello, I just got my steamdeck and dock with HDMI dummy plug.

How do I set up this?

I want it to output 1440p 120hz to the dummy plug, so my sunshine streaming server can send a stream of Deck's desktop to my main PC.

I found a guide online telling me to use Xrandr to add mode and then set it to hdmi-1 output. I tried to follow the guide, but it was unsuccessful. My deck defaults to its 1080p 60hz default resolution, dictated by Edid of that dummy plug. If I set 1440p ress via KDE's settings, I get an error telling me it was not successful, and it immediately reverts to 1080p.

I will try to follow up below with the logs and xrandr output when I figure out how to post it here.

[Weraspowa]

 (deck@steamdeck ~)$ #now deck is in 1080p mode, in dock, with HDMI dummy plug on default settings.
(deck@steamdeck ~)$ #Following along the archwiki guide on xrandr and How to set custom ressolution section
(deck@steamdeck ~)$ # url : https://wiki.archlinux.org/title/Xrandr
(deck@steamdeck ~)$ #get modeline
(deck@steamdeck ~)$ cvt 2560 1440 120
# 2560x1440 119.95 Hz (CVT) hsync: 185.33 kHz; pclk: 661.25 MHz
Modeline "2560x1440_120.00" 661.25 2560 2784 3064 3568 1440 1443 1448 1545 -hsync +vsync
(deck@steamdeck ~)$ ^Candr --newmode 1440p120 661.25 2560 2784 3064 3568 1440 1443 1448 1545 -hsync +vsync
(130)(deck@steamdeck ~)$ # add mode to Xrandr
(130)(deck@steamdeck ~)$ xrandr --newmode 1440p120 661.25 2560 2784 3064 3568 1440 1443 1448 1545 -hsync +vsync
(deck@steamdeck ~)$ #admode and activate 1440p120 mode on displayport 0
(1)(deck@steamdeck ~)$ xrandr
Screen 0: minimum 320 x 200, current 1920 x 1080, maximum 16384 x 16384
eDP connected (normal left inverted right x axis y axis)
800x1280 60.00 +
256x160 58.79
DisplayPort-0 connected primary 1920x1080+0+0 (normal left inverted right x axis y axis) 1872mm x 1053mm
1920x1080 60.00*+
1680x1050 59.95
1600x900 60.00
1280x1024 75.02 60.02
1440x900 59.89
1280x800 59.81
1152x864 75.00
1280x720 60.00
1024x768 75.03 70.07 60.00
832x624 74.55
800x600 72.19 75.00 60.32 56.25
640x480 75.00 72.81 66.67 59.94
720x400 70.08
2560x1440_144.00 143.91
1440p120 (0x751) 661.250MHz -HSync +VSync
h: width 2560 start 2784 end 3064 total 3568 skew 0 clock 185.33KHz
v: height 1440 start 1443 end 1448 total 1545 clock 119.95Hz
(1)(deck@steamdeck ~)$ xrandr --addmode DisplayPort-0 1440p120
(deck@steamdeck ~)$ xrandr --output DisplayPort-0 --mode 1440p120
(deck@steamdeck ~)$ #now it sets itself to 1440p120hz and imidietly revers to 1080p 60hz
(deck@steamdeck ~)$ #in settings there is 2560x1440 ress above the 1080p
(deck@steamdeck ~)$ #setting it screen blinks and reversts to 1080p.
(deck@steamdeck ~)$ #doing exactly this works on my 5900hx,6800m,EndeavorOS,Asus ROG Strix g15 amd advantage edition and it just werks.
(deck@steamdeck ~)$ # HOW? What's brouken? Am I crazy?

[RSebire]

If you’re using the steam deck dock, make sure it’s updated.
And make sure you HDMI cable is up to the specifications your after too.

[Prezidentas]

you can setup a custom edid profile on an output with a Linux boot option on grub or whatever does the deck use, however with the readonly nature of steamOS it might be easier to get a better dummy plug. also, you could install a different Linux distro if you won't be using the deck portably anymore.

[ReBoot]

Why not getting a dummy plug with the right EDID profile?