I have a Debian rootfs as a tar file. I would like to specify it as the initramfs source in Kconfig (using PetaLinux), but the source must be a cpio archive.
How can I convert the .tar to .cpio, maintaining its structure, permissions, etc.?
Since there is no program that offers this kind of conversion, you need to unpack the tar archive using:
tar xpf archive.tar
as root and later to create a cpio archive from that extracted directory tree.