Some notes on the FiiO M3K Mini HiFi. As of July 2021, the M3k is available from amazon.com . A build of rockbox for the device exists, and works just great!
This is the native port, recommended nowadays.
This was the first “port”, rockbox more running in userspace as alternative to the native firmware. On each start, one has to select “rockbox”. The native port is recommended nowadays.