understanding how nf_bridge_maybe_copy_header works

Sorry for the crosspost, but I was not sure which list this question
would apply to.

I am having some difficulty understanding something and I was hoping
that someone can shed some light on the subject for me.

In the br_dev_queue_push_xmit() routine a call is made to
nf_bridge_maybe_copy_header(), which seems to copy information from
skb->nf_bridge->data to the skb->data.  This will happen when in being
used by the bridge.  But, I can't seem to figure out how this data is
copied into the skb->nf_bridge->data in the first place.  If you follow
the code in the br_netfilter.c file for VLAN frames, I never see how
skb->nf_bridge->data is set, yet it seems to copy the data from there. 
So, I am missing something.  But, I can't seem to figure out what it
is?  Can anyone help me?  Thanks in advance.

--joubert