awesomebytes

Hello community,
 
I have just been successful building my own image of Ubuntu focal + preempt_rt (kernel 5.15.76-rt53) for my Rock64 board for the first time (a bit of additional background at the end of my post). Exciting! I had a few hiccups along the way, and I need to figure out a few more things. So far I'm happy and impressed with the project.
 
However, given I want to share my work (and the resulting image(s)) I would like to ask for some pointers on what is the best way to go in regards of forking armbian/build and the changes I found myself doing.
 
I have added my kernel compilation flags to config/kernel/linux-rockchip64-current.config but I feel like maybe I should create a new file, something like 'linux-rockchip64-current-rt.config' but I do not know if the compile.sh will find it (or how to make it find it) if I do so. What would be the best way to go about it? I added the PREEMPT_RT patches to patch/kernel/rockchip64-current , but again, there should probably be a folder for RT maybe? (I wonder how to make compile.sh find it). I created userpatches/lib.config with the content 'KERNELBRANCH='tag:v5.15.76'' to use the latest PREEMPT_RT compatible kernel, ideally this could be scripted to use the latest closest to 'current', which also would avoid needing to modify the patch/kernel/BOARD_NAME folder. I also needed to remove the box64 package cause it would stop building my image (GPG key outdated?) is this package necessary for a minimal installation? I don't think I found I needed it so far. I left a comment about it in this github issue: https://github.com/armbian/build/issues/3968#issuecomment-1310390875 I needed to add a couple of additional cmdline flags, I edited by hand (after booting the board) the resulting /boot/armbianEnv.txt with 'extraargs=nohz_full=3 isolcpus=3' and that worked, but I'd like to create the image with that already done. What would be the best place to do that? userpatches/customize-image.sh ? If I place anything in userpatches/customize-image.sh am I able to run the build just from there?  
Thank you very much for any pointers you can give me about best practices to go about it, anything I did wrong, or templates to follow.
 
My first goal is to provide a reproducible build of ubuntu focal + preempt_rt for the rock64 board (which I'm very close!), but I'd be happy to extend the work to be able to apply the PREEMPT_RT patches to any board saving people a bit of the guess work I needed to do (at least for as long the patches are not part of the kernel by default!).
 
Additional background: I need to use Ubuntu as my plan is to use ROS 2 on it, and the officially supported distro is Ubuntu (and focal happens to be the most useful for my use-case). I am aiming, in conjunction with some members of the ROS real-time working group, to ease the entry barrier of learning to make real-time apps for robotics. I am using a Rock64 board because it's similar to a Raspberry Pi 4, as I am unable to source a Raspi4 and I believe it is the same for everyone.
 
Bonus: a quick test (running 10min) of cyclictest with stress on the system provided the following plot on latency (74us max, 15us min, about 20ish us average):