The relevant developers are of the opinion (one that I agree with) that it does not require a clean room process. A clean room process is a gold standard for preventing accidental copyright infringment, but is not necessary and is enough work they don't consider it worthwhile. Reading (and even to the extent that it's compatible with the license modifying) the original source code to learn how the underlying hardware works is not copyright infringment and does not make future works you produce derivative works of the original.

Source/explanation in their words instead of mine: https://asahilinux.org/copyright/ (See: "Reverse engineering policy")