Re: [PATCH net] net: stmmac: leave the datapath running for normal-size MTU changes

[Nicolai Buchwitz <nb@tipi-net.de>]

Hi James

On 1.10.2026 06:39, James Hilliard wrote:
> When both MTUs are at most ETH_DATA_LEN, the receive buffer size and 
> MAC
> receive limit do not change. Update the MTU without restarting the
> datapath.
> 
> This also avoids rebinding a live AF_XDP pool to a temporary RXQ. XDP
> rejects jumbo MTUs, so all supported XDP MTU changes take this path.
> Jumbo transitions remain on the reopen path until the rollback change.
> 
> Fixes: 3470079687448 ("net: ethernet: stmicro: stmmac: permit MTU 
> change with interface up")
> Signed-off-by: James Hilliard <james.hilliard1@gmail.com>
> ---
>  drivers/net/ethernet/stmicro/stmmac/stmmac_main.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c 
> b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
> index 3f34d491c959..03cd14d10356 100644
> --- a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
> +++ b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
> @@ -6161,7 +6161,12 @@ static int stmmac_change_mtu(struct net_device 
> *dev, int new_mtu)
>  	if ((txfifosz < new_mtu) || (new_mtu > BUF_SIZE_16KiB))
>  		return -EINVAL;
> 
> -	if (netif_running(dev)) {
> +	/* Normal-size frames use the same buffers and MAC receive limits.
> +	 * In particular, do not disturb a live AF_XDP pool: XDP does not
> +	 * support jumbo frames, so it never needs the ring replacement 
> below.
> +	 */
> +	if (netif_running(dev) &&
> +	    (dev->mtu > ETH_DATA_LEN || mtu > ETH_DATA_LEN)) {
>  		netdev_dbg(priv->dev, "restarting interface to change its MTU\n");
>  		/* Try to allocate the new DMA conf with the new mtu */
>  		dma_conf = stmmac_setup_dma_desc(priv, mtu);
> 
> ---
> base-commit: a7bfaba4823e3c165bb2004c74eff7c096672bc7
> change-id: 20260930-stmmac-mtu-fastpath-68813ead3b90
> 
> Best regards,
> --
> James Hilliard <james.hilliard1@gmail.com>

Reviewed-by: Nicolai Buchwitz <nb@tipi-net.de>

Thanks,
Nicolai