Function iflib_ether_pad

freebsd/net/iflib.c:3348–3386

* Pad an mbuf to ensure a minimum ethernet frame size. * min_frame_size is the frame size (less CRC) to pad the mbuf to */

 * min_frame_size is the frame size (less CRC) to pad the mbuf to
 */
static __noinline int
iflib_ether_pad(device_t dev, struct mbuf **m_head, uint16_t min_frame_size)
{
	/*
	 * 18 is enough bytes to pad an ARP packet to 46 bytes, and
	 * and ARP message is the smallest common payload I can think of
	 */
	static char pad[18];	/* just zeros */
	int n;
	struct mbuf *new_head;

	if (!M_WRITABLE(*m_head)) {
		new_head = m_dup(*m_head, M_NOWAIT);
		if (new_head == NULL) {
			m_freem(*m_head);
			device_printf(dev, "cannot pad short frame, m_dup() failed");
			DBG_COUNTER_INC(encap_pad_mbuf_fail);
			DBG_COUNTER_INC(tx_frees);
			return ENOMEM;
		}
		m_freem(*m_head);
		*m_head = new_head;
	}

	for (n = min_frame_size - (*m_head)->m_pkthdr.len;
	     n > 0; n -= sizeof(pad))
		if (!m_append(*m_head, min(n, sizeof(pad)), pad))
			break;

	if (n > 0) {
		m_freem(*m_head);
		device_printf(dev, "cannot pad short frame\n");
		DBG_COUNTER_INC(encap_pad_mbuf_fail);
		DBG_COUNTER_INC(tx_frees);
		return (ENOBUFS);
	}

	return 0;
}

static int
iflib_encap(iflib_txq_t txq, struct mbuf **m_headp)