doc: announce ethdev API change for Rx split

The DPDK datapath in the transmit direction is very flexible.
The applications can build multi-segment packets and manages
almost all data aspects - the memory pools where segments
are allocated from, the segment lengths, the memory attributes
like external, registered, etc.

In the receiving direction, the datapath is much less flexible,
the applications can only specify the memory pool to configure
the receiving queue and nothing more. The packet being received
can only be pushed to the chain of the mbufs of the same data
buffer size and allocated from the same pool. In order to extend
the receiving datapath buffer description it is proposed to add
the new fields into rte_eth_rxconf structure:

struct rte_eth_rxconf {
    ...
    uint16_t rx_split_num; /* number of segments to split */
    uint16_t *rx_split_len; /* array of segment lengths */
    struct rte_mempool **mp; /* array of segment memory pools */
    ...
};

The non-zero value of rx_split_num field configures the receiving
queue to split ingress packets into multiple segments to the mbufs
allocated from various memory pools according to the specified
lengths. The zero value of rx_split_num field provides the
backward compatibility and queue should be configured in a regular
way (with single/multiple mbufs of the same data buffer length
allocated from the single memory pool).

The new approach would allow splitting the ingress packets into
multiple parts pushed to the memory with different attributes.
For example, the packet headers can be pushed to the embedded data
buffers within mbufs and the application data into the external
buffers attached to mbufs allocated from the different memory
pools. The memory attributes for the split parts may differ
either - for example the application data may be pushed into
the external memory located on the dedicated physical device,
say GPU or NVMe. This would improve the DPDK receiving datapath
flexibility preserving compatibility with existing API.

The proposed extended description of receiving buffers might be
considered by other vendors to be involved into similar features
support, it is the subject for the further discussion.

Signed-off-by: Viacheslav Ovsiienko <viacheslavo@mellanox.com>
Acked-by: Jerin Jacob <jerinj@marvell.com>
Acked-by: Andrew Rybchenko <arybchenko@solarflare.com>
Acked-by: Thomas Monjalon <thomas@monjalon.net>
Acked-by: Ferruh Yigit <ferruh.yigit@intel.com>

diff --git a/doc/guides/rel_notes/deprecation.rst b/doc/guides/rel_notes/deprecation.rst
index 30f1dd7..aaf1d68 100644 (file)
--- a/doc/guides/rel_notes/deprecation.rst
+++ b/doc/guides/rel_notes/deprecation.rst
@@ -163,6 +163,11 @@ Deprecation Notices
   In 19.11 PMDs will still update the field even when the offload is not
   enabled.
 
+* ethdev: Add new fields to ``rte_eth_rxconf`` to configure the receiving
+  queues to split ingress packets into multiple segments according to the
+  specified lengths into the buffers allocated from the specified
+  memory pools. The backward compatibility to existing API is preserved.
+
 * ethdev: ``rx_descriptor_done`` dev_ops and ``rte_eth_rx_descriptor_done``
   will be deprecated in 20.11 and will be removed in 21.11.
   Existing ``rte_eth_rx_descriptor_status`` and ``rte_eth_tx_descriptor_status``