waldi added s390 support to initramfs-tools as of version 0.54. 0.55 is in incoming.debian.org and should be available in unstable soon. klibc-1.2.4-1 has an important s390 fix. so testing those releases are highly welcome. you can rebuild your initramfs after installation of aboves packages with update-initramfs -u -k <kernel-version> -- maks