Linux alsa midi-in support
The linux alsa midi-in implementation is missing.
Do you have a MIDI-in device to test it?
Yes on linux there is working hardware and virtual midi setup.
Wrote an impl a year ago only never had success with openMSX setup+roms with correct software for and emulated midi interface to test the msx side of things. Raw untested alsa midi-in impl; https://github.com/openMSX/openMSX/compare/master...immetoo:alsa_midi_impl
I can help you setup openMSX to test it. I think MSX software ike MIDI2OPL4 (routes incoming MIDI to the OPL4) or Synthesix (routes incoming MIDI to the SCC) can be used to test. There's also MIDISlav and Tiny Sound Module, for instance.