-i, -u

If the -i suffix of the primary inflection is the locative -i, as
some claim, and the whole word therefore is a locativic construction
in the style of English progressive 'be V-ing', then the rest of the
word must be some kind of gerund, or verbal noun, or rather a
personal such (because of the personal endings -m, -s, -t),
thus: "my V-ing", "thy V-ing", "his V-ing".

But the secondary inflection has no -i. That makes no sense.
Contrasting a locativic -i with nothing (with no "case sense") is
meaningless (unless it's an endingless locative?).

Some say there was another locative suffix -u (based on the loc.pl.
-si, -su). That makes two of them, -i, -u, meaning "here", "there"?
Perhaps the -u was used in the secondary inflection and later lost?
The OCS I left its mark in Russian by palatalisation, the U left
nothing; something similar is the case in Japanese. Would an -u
explain the -U of 3rd sg. in OCS?

Torsten