Re: [dpdk-dev] [PATCH v6 2/2] librte_ethdev: fix MTU size exceeds max rx packet length

[Ferruh Yigit]

On 10/22/2020 9:48 AM, SteveX Yang wrote:
If max rx packet length is smaller then MTU + Ether overhead, that will
drop all MTU size packets.
Update the MTU size according to the max rx packet and Ether overhead.

Fixes: 59d0ecdbf0e1 ("ethdev: MTU accessors")

Signed-off-by: SteveX Yang <stevex.y...@intel.com>
---
  lib/librte_ethdev/rte_ethdev.c | 14 ++++++++++++++
  1 file changed, 14 insertions(+)

diff --git a/lib/librte_ethdev/rte_ethdev.c b/lib/librte_ethdev/rte_ethdev.c
index b12bb3854..17f1c33ac 100644
--- a/lib/librte_ethdev/rte_ethdev.c
+++ b/lib/librte_ethdev/rte_ethdev.c
@@ -1290,6 +1290,8 @@ rte_eth_dev_configure(uint16_t port_id, uint16_t nb_rx_q, 
uint16_t nb_tx_q,
        struct rte_eth_dev *dev;
        struct rte_eth_dev_info dev_info;
        struct rte_eth_conf orig_conf;
+       uint16_t overhead_len;
+       uint16_t max_rx_pktlen;
        int diag;
        int ret;
  
@@ -1415,6 +1417,18 @@ rte_eth_dev_configure(uint16_t port_id, uint16_t nb_rx_q, uint16_t nb_tx_q,
                                                        RTE_ETHER_MAX_LEN;
        }
  
+	/*
+        * Update MTU value if MTU + OVERHEAD exceeds the max_rx_pkt_len
+        */

I am not sure this conditional update is required, the target is keep 
'max_rx_pktlen' & 'mtu' in sync.

So why not just:
dev->data->mtu = max_rx_pktlen - overhead_len;


+       max_rx_pktlen = dev->data->dev_conf.rxmode.max_rx_pkt_len;
+       if (dev_info.max_rx_pktlen && dev_info.max_mtu)
+               overhead_len = dev_info.max_rx_pktlen - dev_info.max_mtu;
+       else
+               overhead_len = RTE_ETHER_HDR_LEN + RTE_ETHER_CRC_LEN;
+
+       if (max_rx_pktlen < dev->data->mtu + overhead_len)
+               dev->data->mtu = max_rx_pktlen - overhead_len;
+
        /*
         * If LRO is enabled, check that the maximum aggregated packet
         * size is supported by the configured device.