I tried to do a partial upgrade from Debian 9 to Debian 10 so that I could then resolve any kind of package conflicts to do a full upgrade. The partial upgrade was successful but I got the same problem as others have when they tried to upgrade to Debian 10. I figured out what had happened. I remotely control my Gemini PDA via ssh and edited the xorg.conf file. It looks like something surrounding the 'hwcomposer' driver is failing with Xorg as using 'fbdev' gets me a screen. I had to also use 'xdm' instead of 'sddm' as 'sddm' failed to load. I'm going to look into this further, but it looks like the Xorg fork being used to get 'glamor' rendering with the 'hwcomposer' method is failing severely and it seems to be very outdated. Maybe a Wayland desktop environment that supports 'hwcomposer' might help with this.
I'm unsure if Gemian uses a heavily modified version of Halium or just the Halium initramfs.
As far as the kernel goes, I tried to bring the vendor code over to AOSP 4.4 and it's quite a lot to say the least. I postponed it for now though. It may not even work at the end of it all, who knows though!
April 2025 EDIT: I was able to confirm that Gemian on the Gemini PDA is using LineageOS 14.1 (Android 7.1) sources for Halium 7.1; the closest I got Debian Buster running was a desktop but the device would immediately restart for some reason. Using or at least referencing the work done for UBports for Gemini PDA, it's possible to get Halium 9 working with a Project Treble implementation; requiring the Android 8.1 firmware to be present on the Gemini PDA. An advantage of a Halium 9-based port would be hwcomposer2 support; allowing for current KWin (for KDE) to work on Wayland (as current efforts only support hwcomposer2 for KWin Wayland; something that Halium Android 7.1 does not have).
