doc: announce ABI change for Tx preparation

doc: announce ABI change for Tx preparation

This is an ABI deprecation notice for DPDK 16.11 in librte_ether about
changes in rte_eth_dev and rte_eth_desc_lim structures.

As discussed in that thread:

http://dpdk.org/ml/archives/dev/2015-September/023603.html

Different NIC models depending on HW offload requested might impose
different requirements on packets to be TX-ed in terms of:

 - Max number of fragments per packet allowed
 - Max number of fragments per TSO segments
 - The way pseudo-header checksum should be pre-calculated
 - L3/L4 header fields filling
 - etc.

MOTIVATION:
-----------

1) Some work cannot (and didn't should) be done in rte_eth_tx_burst.
   However, this work is sometimes required, and now, it's an
   application issue.

2) Different hardware may have different requirements for TX offloads,
   other subset can be supported and so on.

3) Some parameters (eg. number of segments in ixgbe driver) may hung
   device. These parameters may be vary for different devices.

   For example i40e HW allows 8 fragments per packet, but that is after
   TSO segmentation. While ixgbe has a 38-fragment pre-TSO limit.

4) Fields in packet may require different initialization (like eg. will
   require pseudo-header checksum precalculation, sometimes in a
   different way depending on packet type, and so on). Now application
   needs to care about it.

5) Using additional API (rte_eth_tx_prep) before rte_eth_tx_burst let to
   prepare packet burst in acceptable form for specific device.

6) Some additional checks may be done in debug mode keeping tx_burst
   implementation clean.

PROPOSAL:
---------

To help user to deal with all these varieties we propose to:

1. Introduce rte_eth_tx_prep() function to do necessary preparations of
   packet burst to be safely transmitted on device for desired HW
   offloads (set/reset checksum field according to the hardware
   requirements) and check HW constraints (number of segments per
   packet, etc).

   While the limitations and requirements may differ for devices, it
   requires to extend rte_eth_dev structure with new function pointer
   "tx_pkt_prep" which can be implemented in the driver to prepare and
   verify packets, in devices specific way, before burst, what should to
   prevent application to send malformed packets.

2. Also new fields will be introduced in rte_eth_desc_lim:
   nb_seg_max and nb_mtu_seg_max, providing an information about max
   segments in TSO and non-TSO packets acceptable by device.

   This information is useful for application to not create/limit
   malicious packet.

APPLICATION (CASE OF USE):
--------------------------

1) Application should to initialize burst of packets to send, set
   required tx offload flags and required fields, like l2_len, l3_len,
   l4_len, and tso_segsz

2) Application passes burst to the rte_eth_tx_prep to check conditions
   required to send packets through the NIC.

3) The result of rte_eth_tx_prep can be used to send valid packets
   and/or restore invalid if function fails.

eg.

	for (i = 0; i < nb_pkts; i++) {

		/* initialize or process packet */

		bufs[i]->tso_segsz = 800;
		bufs[i]->ol_flags = PKT_TX_TCP_SEG | PKT_TX_IPV4
				| PKT_TX_IP_CKSUM;
		bufs[i]->l2_len = sizeof(struct ether_hdr);
		bufs[i]->l3_len = sizeof(struct ipv4_hdr);
		bufs[i]->l4_len = sizeof(struct tcp_hdr);
	}

	/* Prepare burst of TX packets */
	nb_prep = rte_eth_tx_prep(port, 0, bufs, nb_pkts);

	if (nb_prep < nb_pkts) {
		printf("tx_prep failed\n");

		/* drop or restore invalid packets */

	}

	/* Send burst of TX packets */
	nb_tx = rte_eth_tx_burst(port, 0, bufs, nb_prep);

	/* Free any unsent packets. */

Signed-off-by: Tomasz Kulasek <tomaszx.kulasek@intel.com>
Acked-by: Konstantin Ananyev <konstantin.ananyev@intel.com>
Acked-by: Thomas Monjalon <thomas.monjalon@6wind.com>
Acked-by: Olivier Matz <olivier.matz@6wind.com>