If you keep the previous initialization to zero, I argue that this line should use '|='.

True, I'll update the diff.

I suspect the option (and command line arg etc.) should be "softupdates" - see for example the description in the tunefs man page for -n Turn on/off soft updates

isn't this always setting the flag, not just when requested?

Agreed, I'll change that

That was my quick hack ... will update the diff without it

Note that the option framework has OPT_BOOL see e.g. iso9660 allow-deep-trees. Based on that example I think this could be -soft-updates, which does not take a parameter and just enables SU if specified?

comment ought to be "soft updates' too :-)

OPT_BOOL just define a INT32 with values from 0 to 1.
-o softupdates works and is equivalent to -o softupdates=1

Unrelated to the review at hands, would be nice to switch ffs_options to use designated initializers.