On Mon, Apr 25, 2016 at 07:35:36PM +0200, Michael Kerrisk (man-pages) wrote:

quoted

I'd rather update the man page - EOPNOTSUPP is a much more descriptive
error code for this case.  I'll send you a patch.

Unless I'm misunderstanding something here, you're proposing something
very inconsistent. The standard error for unknown flag bits is EINVAL.
This is so for dozens of systems calls (check the man pages; you might
find a rare exception, but that's the point, they are exceptions). It
seems to me here that it's really the implementation that needs
fixing, not the man page!

For new filesystem calls we try to use EOPNOTSUPP as much as possible,
e.g. fallocate.