net/mlx5: get_mtu returns 1500 on startup instead of kernel driver MTU

https://bugs.dpdk.org/show_bug.cgi?id=1768

            Bug ID: 1768
           Summary: net/mlx5: get_mtu returns 1500 on startup instead of
                    kernel driver MTU
           Product: DPDK
           Version: 24.11
          Hardware: All
                OS: All
            Status: UNCONFIRMED
          Severity: normal
          Priority: Normal
         Component: ethdev
          Assignee: [email protected]
          Reporter: [email protected]

Target Milestone: ---

Hello,

I observe this issue with both DPDK 24.11 and 25.07 on Debian 12 with DOCA 3.0.0 and a ConnectX-5 NIC. My code calls rte_eth_dev_get_mtu() to determine the existing MTU value of the kernel driver before initializing DPDK. Then it sets rte_eth_conf.rxmode.mtu equal to the returned value and calls rte_eth_dev_configure(). This way DPDK uses the same MTU as the kernel driver.

However, when running this code I observe the initial rte_eth_dev_get_mtu() call always returns 1500B. I have tried setting kernel MTUs of 800, 3000, 4000, and 9000B. I added tracing to lib/ethdev and drivers/net/mlx5 and believe the issue is because while mlx5 determines and stores the current MTU, that value is not propagated back to the ethdev structure.

My attempt at a timeline:
* mlx5_dev_spawn() sets priv->mtu = RTE_ETHER_MTU (1500)
* mlx5_dev_spawn() sets eth_dev = rte_eth_dev_allocate()
* rte_eth_dev_allocate() sets eth_dev->data->mtu = RTE_ETHER_MTU (1500)
* mlx5_dev_spawn() gets actual MTU calling mlx5_get_mtu(&priv->mtu) #### eth_dev->data->mtu doesn't get updated ####
* I call rte_eth_dev_get_mtu() which returns 1500

For testing, I tried this patch to set the rte_eth_dev_data.mtu field equal to the value returned from mlx5_get_mtu(). With this change, my code works as expected. I hope something like this can be merged to allow cooperation between the kernel and DPDK drivers. Thank you for your time and your work on mlx5!

@@ -1594,6 +1594,7 @@ mlx5_dev_spawn(struct rte_device *dpdk_dev,
        }
        DRV_LOG(DEBUG, "port %u MTU is %u", eth_dev->data->port_id,
                priv->mtu);
+       ethdev->data->mtu = priv->mtu;
        /* Initialize burst functions to prevent crashes before link-up. */
        eth_dev->rx_pkt_burst = rte_eth_pkt_burst_dummy;
        eth_dev->tx_pkt_burst = rte_eth_pkt_burst_dummy;

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