[arghwhat]

Note that this requires a clean-room process - reading the original source code as inspiration usually taints any work derived from the observations.

This is why open source developers actively avoid reading leaked sources from proprietary projects for example. "open source" code with too restrictive licenses is no different from closed source code.

[gpm]

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")

[arghwhat]

The policy of those developers does not mean much, and while the policy of upstream kernel developers that need to accept the code can be interesting, but ultimately only a lawyer's opinion have any meaning here.

They might coincidentally be right of course, but their advice, policy and opinion in this legal matters mean as little as yours and mine.