TX_OVERHEAD
ndev->needed_headroom = TX_OVERHEAD;
need_pages = (pkt_len + TX_OVERHEAD + 127) >> 7;
(headroom < (TX_OVERHEAD + spi_len)) ||
size_t h = max((TX_OVERHEAD + spi_len) - headroom, 0);
memcpy(skb_push(skb, TX_OVERHEAD), &info.tx_overhead, TX_OVERHEAD);
skb->data + 4, TX_OVERHEAD, 0);
skb->data + 4 + TX_OVERHEAD,
skb->len - TX_EOP_SIZE - 4 - TX_OVERHEAD, 0);