You may be interested in this : https://wiki.postmarketos.org/wiki/Category:Interface

Those interfaces exists and are pretty mature compared to their current marketshare which is near zero.

What is missing though is a real developer/hacker community around Linux on mobile because even installing Linux on a smartphone is a huge PITA starting by being lucky enough to own or find a compatible terminal. Something from the Raspberry PI foundation with official support, clear and easy to buy would be absolutely groundbreaking and could became THE platform to develop Linux on smartphones.

Linux is really not that far behind, it just lacks a real community.