[dpdk-dev] [PATCH v6 1/6] ethdev: fix max Rx packet length

[Xu, Rosen]

Hi,

> -----Original Message-----
> From: Yigit, Ferruh <ferruh.yigit at intel.com>
> Sent: Tuesday, October 12, 2021 7:54
> To: Jerin Jacob <jerinj at marvell.com>; Li, Xiaoyun <xiaoyun.li at intel.com>;
> Chas Williams <chas3 at att.com>; Min Hu (Connor) <humin29 at huawei.com>;
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> Cc: Yigit, Ferruh <ferruh.yigit at intel.com>; dev at dpdk.org; Huisong Li
> <lihuisong at huawei.com>
> Subject: [PATCH v6 1/6] ethdev: fix max Rx packet length
> 
> 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 at intel.com>
> Acked-by: Ajit Khaparde <ajit.khaparde at broadcom.com>
> Acked-by: Somnath Kotur <somnath.kotur at broadcom.com>
> Acked-by: Huisong Li <lihuisong at huawei.com>
> ---
> Cc: Min Hu (Connor) <humin29 at 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: Rosen Xu <rosen.xu at intel.com>