PTR_DIFF
hdr_off = PTR_DIFF(ptr, hdr);
hdr_off = PTR_DIFF(ptr, hdr);
writel(PTR_DIFF(previous, card->tsq.base),
writel(PTR_DIFF(previous, card->rsq.base), card->membase + RSQH);
EFX_WARN_ON_ONCE_PARANOID((PTR_DIFF(tcp_hdr(skb), skb->data) +