skb_reserve
skb_reserve(skb, hw->extra_tx_headroom);
skb_reserve(skb, hw->extra_tx_headroom);
skb_reserve(skb, hw->extra_tx_headroom);
skb_reserve(skb, hw->extra_tx_headroom);
skb_reserve(skb, NET_SKB_PAD);
skb_reserve(new, headroom); /* data and tail move headroom forward. */