[dpdk-dev] [RFC v2] doc: announce max Rx packet len field deprecation

[Ferruh Yigit <ferruh.yigit@intel.com>]

Signed-off-by: Ferruh Yigit <ferruh.yigit@intel.com>
Acked-by: Konstantin Ananyev <konstantin.ananyev@intel.com>
---
Cc: Thomas Monjalon <thomas@monjalon.net>
Cc: Andrew Rybchenko <arybchenko@solarflare.com>
Cc: Konstantin Ananyev <konstantin.ananyev@intel.com>
Cc: Matan Azrad <matan@nvidia.com>
Cc: Olivier Matz <olivier.matz@6wind.com>
Cc: Jerin Jacob <jerinj@marvell.com>

v2:
* ``uint32_t mtu`` moved to ``struct rte_eth_conf``
* The "Driver is responsible from updating ``(struct
  rte_eth_dev)->data->mtu``" updated because ethdev layer also can do
  this. The intention there was both APIs should update the variable.

Another open question is from Andrew, if we can remove the ``uint32_t
max_rx_pkt_len`` completely from the ``rte_eth_dev_configure()``.
This may force applications to have one more additional
``rte_eth_dev_set_mtu()`` call for device initialization, but if
applications are OK with the default values most of times, agree that
removing is easier solution, please comment.
---
 doc/guides/rel_notes/deprecation.rst | 25 +++++++++++++++++++++++++
 1 file changed, 25 insertions(+)

diff --git a/doc/guides/rel_notes/deprecation.rst b/doc/guides/rel_notes/deprecation.rst
index 96986fabd598..73e43581d615 100644
--- a/doc/guides/rel_notes/deprecation.rst
+++ b/doc/guides/rel_notes/deprecation.rst
@@ -88,6 +88,31 @@ Deprecation Notices
   will be limited to maximum 256 queues.
   Also compile time flag ``RTE_ETHDEV_QUEUE_STAT_CNTRS`` will be removed.
 
+* 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``
+
 * sched: To allow more traffic classes, flexible mapping of pipe queues to
   traffic classes, and subport level configuration of pipes and queues
   changes will be made to macros, data structures and API functions defined
-- 
2.26.2