>
> There is a confusion on setting max Rx packet length, this patch aims to
> clarify it.
>
> 'rte_eth_dev_configure()' API accepts max Rx packet size via
> 'uint32_t max_rx_pkt_len' field of the config struct 'struct
> rte_eth_conf'.
>
> Also 'rte_eth_dev_set_mtu()' API can be used to set the MTU, and result
> stored into '(struct rte_eth_dev)->data->mtu'.
>
> These two APIs are related but they work in a disconnected way, they
> store the set values in different variables which makes hard to figure
> out which one to use, also having two different method for a related
> functionality is confusing for the users.
>
> Other issues causing confusion is:
> * maximum transmission unit (MTU) is payload of the Ethernet frame. And
> 'max_rx_pkt_len' is the size of the Ethernet frame. Difference is
> Ethernet frame overhead, and this overhead may be different from
> device to device based on what device supports, like VLAN and QinQ.
> * 'max_rx_pkt_len' is only valid when application requested jumbo frame,
> which adds additional confusion and some APIs and PMDs already
> discards this documented behavior.
> * For the jumbo frame enabled case, 'max_rx_pkt_len' is an mandatory
> field, this adds configuration complexity for application.
>
> As solution, both APIs gets MTU as parameter, and both saves the result
> in same variable '(struct rte_eth_dev)->data->mtu'. For this
> 'max_rx_pkt_len' updated as 'mtu', and it is always valid independent
> from jumbo frame.
>
> For 'rte_eth_dev_configure()', 'dev->data->dev_conf.rxmode.mtu' is user
> request and it should be used only within configure function and result
> should be stored to '(struct rte_eth_dev)->data->mtu'. After that point
> both application and PMD uses MTU from this variable.
>
> When application doesn't provide an MTU during 'rte_eth_dev_configure()'
> default 'RTE_ETHER_MTU' value is used.
>
> Additional clarification done on scattered Rx configuration, in
> relation to MTU and Rx buffer size.
> MTU is used to configure the device for physical Rx/Tx size limitation,
> Rx buffer is where to store Rx packets, many PMDs use mbuf data buffer
> size as Rx buffer size.
> PMDs compare MTU against Rx buffer size to decide enabling scattered Rx
> or not. If scattered Rx is not supported by device, MTU bigger than Rx
> buffer size should fail.
>
> Signed-off-by: Ferruh Yigit <ferruh.yi...@intel.com>
> Acked-by: Ajit Khaparde <ajit.khapa...@broadcom.com>
> Acked-by: Somnath Kotur <somnath.ko...@broadcom.com>
> Acked-by: Huisong Li <lihuis...@huawei.com>
> ---
> Cc: Min Hu (Connor) <humi...@huawei.com>
>
> v2:
> * Converted to explicit checks for zero/non-zero
> * fixed hns3 checks
> * fixed some sample app rxmode.mtu value
> * fixed some sample app max-pkt-len argument and updated doc for it
>
> v3:
> * rebased
>
> v4:
> * fix typos in commit logs
>
> v5:
> * fix testpmd '--max-pkt-len=###' parameter for DTS jumbo frame test
>
> v6:
> * uint32_t type used in 'eth_dev_get_overhead_len()' helper function
> ---
Acked-by: Konstantin Ananyev <konstantin.anan...@intel.com>
> --
> 2.31.1
