doc: announce deprecation of maximum Rx length field

The configuration related to the MTU is complex
and have some design issues.
This area should be redesigned a bit.

Signed-off-by: Ferruh Yigit <ferruh.yigit@intel.com>
Acked-by: Konstantin Ananyev <konstantin.ananyev@intel.com>
Acked-by: Ajit Khaparde <ajit.khaparde@broadcom.com>
Acked-by: Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>
Acked-by: Matan Azrad <matan@nvidia.com>
Acked-by: Thomas Monjalon <thomas@monjalon.net>

diff --git a/doc/guides/rel_notes/deprecation.rst b/doc/guides/rel_notes/deprecation.rst
index da47718..2f498a0 100644 (file)
--- a/doc/guides/rel_notes/deprecation.rst
+++ b/doc/guides/rel_notes/deprecation.rst
@@ -64,6 +64,31 @@ Deprecation Notices
   In 19.11 PMDs will still update the field even when the offload is not
   enabled.
 
+* ethdev: ``uint32_t max_rx_pkt_len`` field of ``struct rte_eth_rxmode``, will be
+  replaced by a new ``uint32_t mtu`` field of ``struct rte_eth_conf`` in v21.11.
+  The new ``mtu`` field will be used to configure the initial device MTU via
+  ``rte_eth_dev_configure()`` API.
+  Later MTU can be changed by ``rte_eth_dev_set_mtu()`` API as done now.
+  The existing ``(struct rte_eth_dev)->data->mtu`` variable will be used to store
+  the configured ``mtu`` value,
+  and this new ``(struct rte_eth_dev)->data->dev_conf.mtu`` variable will
+  be used to store the user configuration request.
+  Unlike ``max_rx_pkt_len``, which was valid only when ``JUMBO_FRAME`` enabled,
+  ``mtu`` field will be always valid.
+  When ``mtu`` config is not provided by the application, default ``RTE_ETHER_MTU``
+  value will be used.
+  ``(struct rte_eth_dev)->data->mtu`` should be updated after MTU set successfully,
+  either by ``rte_eth_dev_configure()`` or ``rte_eth_dev_set_mtu()``.
+
+  An application may need to configure device for a specific Rx packet size, like for
+  cases ``DEV_RX_OFFLOAD_SCATTER`` is not supported and device received packet size
+  can't be bigger than Rx buffer size.
+  To cover these cases an application needs to know the device packet overhead to be
+  able to calculate the ``mtu`` corresponding to a Rx buffer size, for this
+  ``(struct rte_eth_dev_info).max_rx_pktlen`` will be kept,
+  the device packet overhead can be calculated as:
+  ``(struct rte_eth_dev_info).max_rx_pktlen - (struct rte_eth_dev_info).max_mtu``
+
 * ethdev: ``rx_descriptor_done`` dev_ops and ``rte_eth_rx_descriptor_done``
   will be removed in 21.11.
   Existing ``rte_eth_rx_descriptor_status`` and ``rte_eth_tx_descriptor_status``