Hello!

Changing or expanding the link level headers should only mess
up the hw checksum if you are using CHECKSUM_HW, is that what
your skge driver is using?

Well, even if this driver uses CHECKSUM_UNNECESSARY, the bug is still
apparently present for another devices which use CHECKSUM_HW.

Actually, the bug is in ppp_input(). It is responsible for setting correct
ip_summed before doing netif_rx(). It could even optimize subtracting
checksum of stripped ppp header from ip_summed (f.e. ipgre_rcv() do this)

But suggested fix is still correct. Unless there are another users of
ppp_input() with the same problem.

Alexey