chans/tmp file is written to by the installer
Restricting write access on chans/tmp, the runtime directory for foton filterfiles, causes model install to fail, but the installer should not be touching this directory except maybe to ensure it's created.
The folks at AS and NCU in Taiwan want to forbid write access to everyone except advligorts user, to discourage a habit that's developed there of editing the chans/tmp files directly with foton. I think that would generally be a good idea everywhere.