Re: [dpdk-dev] [PATCH v7 1/6] ethdev: fix max Rx packet length

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

On 10/18/2021 2:48 PM, Ferruh Yigit wrote:
> 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.yigit@intel.com>
> Acked-by: Ajit Khaparde <ajit.khaparde@broadcom.com>
> Acked-by: Somnath Kotur <somnath.kotur@broadcom.com>
> Acked-by: Huisong Li <lihuisong@huawei.com>
> Acked-by: Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>
> Acked-by: Konstantin Ananyev <konstantin.ananyev@intel.com>
> Acked-by: Rosen Xu <rosen.xu@intel.com>
> Acked-by: Hyong Youb Kim <hyonkim@cisco.com>

Series applied to dpdk-next-net/main, thanks.