From: Krzysztof Halasa <khc@pm.waw.pl>
Date: Tue, 01 Aug 2006 03:04:28 +0200

hdlc_fr: logical PVC devices have no headers (plain IPv4 etc. as seen
by tcpdump), but they append FR headers (4 or 10 bytes long) just
before passing the skb to physical device.

If you hooked up fr_hard_header into dev->hard_header instead of
invoking it via pvc_xmit(), everything would be fine.

The complexity of this function arises from the fact that it prepends
headers of differing lengths depending upon the protocol type
being encapsulated, and this is the problem you should aim to
solve.

Alexey, any suggestions on how to handle this kind of thing?