Re: [PATCH] net: ethernet: mtk_eth_soc: allocate dummy netdev before registering netdevs

[Lorenzo Bianconi <lorenzo.bianconi@oss.qualcomm.com>]

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> From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
> From: Sandeep Haemoon <sandeeph@loljews.com>
> Date: Tue, 15 Sep 2026 22:00:00 +0300
> Subject: [PATCH] net: ethernet: mtk_eth_soc: allocate dummy netdev before registering netdevs
> 
> The DMA ring and NAPI are shared between the multiple MAC netdevs and are
> carried by an internal "dummy" netdev (eth->dummy_dev). mtk_probe() creates
> that dummy device only after the MAC netdevs have been registered, so for a
> brief window the netdevs are visible to the network core while
> eth->dummy_dev is still NULL.
> 
> If anything brings the first netdev up during that window (e.g. netifd
> opening the WAN link the instant the interface is registered), mtk_open()
> takes the first-open path and runs mtk_start_dma() -> mtk_dma_init() ->
> mtk_rx_alloc(), which calls __xdp_rxq_info_reg() with eth->dummy_dev ==
> NULL. That triggers
> 
>   WARNING: CPU: ... Missing net_device from driver
> 
> in net/core/xdp.c and returns -ENODEV, so mtk_open() fails and the
> interface (e.g. the WAN port on MediaTek MT7988) stays down until it is
> manually brought up after probe has completed.
> 
> Allocate the dummy netdev and add the shared tx/rx NAPI to it before
> registering the MAC netdevs, and free it from the probe error path instead
> of the now-unneeded err_unreg_netdev cleanup.

I think the Fix is right, but there is a preliminary bug to fix I guess.
Can you please rebase your patch on top of the following one? Thanks.

https://lore.kernel.org/netdev/20260916-mtk_eth_soc-netdev-fix-v1-1-5dac50eb65b1@oss.qualcomm.com/T/#u

Regards,
Lorenzo

> 
> Signed-off-by: Sandeep Haemoon <sandeeph@loljews.com>
> ---
>  drivers/net/ethernet/mediatek/mtk_eth_soc.c | 31 ++++++++++++++--------
>  1 file changed, 18 insertions(+), 13 deletions(-)
> 
> diff --git a/drivers/net/ethernet/mediatek/mtk_eth_soc.c b/drivers/net/ethernet/mediatek/mtk_eth_soc.c
> index fd7a49a..ae6a19b 100644
> --- a/drivers/net/ethernet/mediatek/mtk_eth_soc.c
> +++ b/drivers/net/ethernet/mediatek/mtk_eth_soc.c
> @@ -5337,6 +5337,22 @@ static int mtk_probe(struct platform_device *pdev)
>  		}
>  	}
>  
> +	/* we run 2 devices on the same DMA ring so we need a dummy device
> +	 * for NAPI to work. Allocate it before registering the netdevs so a
> +	 * concurrent ndo_open (e.g. netifd bringing the first netdev up) never
> +	 * observes eth->dummy_dev == NULL, which would make mtk_rx_alloc() ->
> +	 * __xdp_rxq_info_reg() warn ("Missing net_device from driver",
> +	 * net/core/xdp.c) and leave the interface down on boot.
> +	 */
> +	eth->dummy_dev = alloc_netdev_dummy(0);
> +	if (!eth->dummy_dev) {
> +		err = -ENOMEM;
> +		dev_err(eth->dev, "failed to allocated dummy device\n");
> +		goto err_deinit_ppe;
> +	}
> +	netif_napi_add(eth->dummy_dev, &eth->tx_napi, mtk_napi_tx);
> +	netif_napi_add(eth->dummy_dev, &eth->rx_napi, mtk_napi_rx);
> +
>  	for (i = 0; i < MTK_MAX_DEVS; i++) {
>  		if (!eth->netdev[i])
>  			continue;
> @@ -5351,17 +5367,6 @@ static int mtk_probe(struct platform_device *pdev)
>  				   eth->netdev[i]->base_addr, eth->irq[MTK_FE_IRQ_SHARED]);
>  	}
>  
> -	/* we run 2 devices on the same DMA ring so we need a dummy device
> -	 * for NAPI to work
> -	 */
> -	eth->dummy_dev = alloc_netdev_dummy(0);
> -	if (!eth->dummy_dev) {
> -		err = -ENOMEM;
> -		dev_err(eth->dev, "failed to allocated dummy device\n");
> -		goto err_unreg_netdev;
> -	}
> -	netif_napi_add(eth->dummy_dev, &eth->tx_napi, mtk_napi_tx);
> -	netif_napi_add(eth->dummy_dev, &eth->rx_napi, mtk_napi_rx);
>  
>  	platform_set_drvdata(pdev, eth);
>  	schedule_delayed_work(&eth->reset.monitor_work,
> @@ -5369,9 +5374,9 @@ static int mtk_probe(struct platform_device *pdev)
>  
>  	return 0;
>  
> -err_unreg_netdev:
> -	mtk_unreg_dev(eth);
>  err_deinit_ppe:
> +	if (eth->dummy_dev)
> +		free_netdev(eth->dummy_dev);
>  	mtk_ppe_deinit(eth);
>  	mtk_mdio_cleanup(eth);
>  err_free_dev:
> -- 
> 2.39.2
> 

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