Hi,
Archlinuxarm ARMv8 (AArch64) uses uboot to boot the system
You have to modify /boot/boot.txt with your rootflags
mine look like this:
setenv bootargs console=ttyS1,115200 console=tty0 root=PARTUUID=${uuid} rootflags=subvol=root,compress rw rootwait smsc95xx.macaddr="${usbethaddr}"
So you have to insert “rootflags=subvol=root,compress” to boot from your btrfs subvolume!
After editing the text file you have to run
cd /boot/
./mkscr
This will recreate your boot.scr
Have fun!