I’m trying to configure the HDMI mode as well. I am running core20 on a RPI4.
I made sure to include @ijohnson 's PR (snap refresh snapd --edge), and I’m now running snapd 2.49.2+git1051.ge408012.
What worked was using the dash hdmi-cvt and not the underscore hdmi_cvt
snap set system pi-config.hdmi-cvt="1024 600 60 6 0 0 0"
However, I haven’t had success in confirming if any of these HDMI changes are being applied (the screen looks the same with and without). I can confirm my values are being set by seeing them listed with snap get system pi-config.
The obvious visual confirmation if the modes are being applied is a 90 Deg rotation (pi-config.display-rotate=1), which I do not see.
Furthermore, I don’t know where to find the config.txt file, can someone let me know the full path to the file? By looking around I found /writable/system-data/var/lib/snapd/seed/config.txt, but from what I understand I should not make changes in this file in Core20 (as it gets re-created at boot?).