Re: [dpdk-dev] [PATCH v4 1/2] ethdev: fix MTU doesn't update when jumbo frame disabled

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

On 1/25/2021 8:32 AM, Steve Yang wrote:
> The MTU value should be updated to 'max_rx_pkt_len - overhead'
> no matter if the JUMBO FRAME offload enabled. If not update this MTU,
> use will get the wrong MTU info via some command.
> E.g.: 'show port info all' in testpmd tool.
> 
> Actually, the 'max_rx_pkt_len' has been used for other purposes in many
> places now, even though the 'max_rx_pkt_len' is expected 'Only used if
> JUMBO_FRAME enabled'.
> 
> For examples,
> 'max_rx_pkt_len' perhaps can be used as the 'rx_ctx.rxmax' in i40e.
> 
> Fixes: bf0f90d92d30 ("ethdev: fix max Rx packet length check")
> 
> Signed-off-by: Steve Yang <stevex.yang@intel.com>
> ---
>   lib/librte_ethdev/rte_ethdev.c | 8 ++++----
>   1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/lib/librte_ethdev/rte_ethdev.c b/lib/librte_ethdev/rte_ethdev.c
> index daf5f24f7e..42857e3b67 100644
> --- a/lib/librte_ethdev/rte_ethdev.c
> +++ b/lib/librte_ethdev/rte_ethdev.c
> @@ -1421,10 +1421,6 @@ rte_eth_dev_configure(uint16_t port_id, uint16_t nb_rx_q, uint16_t nb_tx_q,
>   			ret = -EINVAL;
>   			goto rollback;
>   		}
> -
> -		/* Scale the MTU size to adapt max_rx_pkt_len */
> -		dev->data->mtu = dev->data->dev_conf.rxmode.max_rx_pkt_len -
> -				overhead_len;
>   	} else {
>   		uint16_t pktlen = dev_conf->rxmode.max_rx_pkt_len;
>   		if (pktlen < RTE_ETHER_MIN_MTU + overhead_len ||
> @@ -1434,6 +1430,10 @@ rte_eth_dev_configure(uint16_t port_id, uint16_t nb_rx_q, uint16_t nb_tx_q,
>   						RTE_ETHER_MTU + overhead_len;
>   	}
>   
> +	/* Scale the MTU size to adapt max_rx_pkt_len */
> +	dev->data->mtu = dev->data->dev_conf.rxmode.max_rx_pkt_len -
> +				overhead_len;
> +
>   	/*
>   	 * If LRO is enabled, check that the maximum aggregated packet
>   	 * size is supported by the configured device.
> 

This will cause 'max_rx_pkt_len' taken into account even 'JUMBO_FRAME' is not 
set, this is against the API documentation.

PMD can ignore the 'max_rx_pkt_len' when 'JUMBO_FRAME' is not set, so updating 
the 'max_rx_pkt_len' is relatively harmless, but updating 'MTU' can have affect.

Instead what do you think explicitly call 'rte_eth_dev_set_mtu()' in the testpmd 
when the MTU of the port needs to be changed because of "port config all 
max-pkt-len" command?