Re: [PATCH net-next v4 15/15] net: macb: use context swapping in .ndo_change_mtu()

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

Hi Théo

On 17.7.2026 21:48, Théo Lebrun wrote:
> Use newly introduced context buffer management to implement
> .ndo_change_mtu() as a context swap: allocate new context ->
> reconfigure HW -> free old context.
> 
> This resists memory pressure well by failing without closing the
> interface and it is much faster by avoiding PHY reinit.
> 
> AT91 EMAC is handled differently as their buffer management is separate
> and they don't do NAPI. We refuse them (-EBUSY) to avoid implementing
> context swapping for them.
> 
> Signed-off-by: Théo Lebrun <theo.lebrun@bootlin.com>
> ---
>  drivers/net/ethernet/cadence/macb_main.c | 22 ++++++++++++++++++++--
>  1 file changed, 20 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/net/ethernet/cadence/macb_main.c 
> b/drivers/net/ethernet/cadence/macb_main.c
> index 5792647eb0a6..1de66f442c59 100644
> --- a/drivers/net/ethernet/cadence/macb_main.c
> +++ b/drivers/net/ethernet/cadence/macb_main.c
> @@ -3493,11 +3493,29 @@ static int macb_close(struct net_device 
> *netdev)
> 
>  static int macb_change_mtu(struct net_device *netdev, int new_mtu)
>  {
> -	if (netif_running(netdev))
> -		return -EBUSY;
> +	struct macb *bp = netdev_priv(netdev);
> +	bool running = netif_running(netdev);
> +	struct macb_context *new_ctx;
> +
> +	if (running) {
> +		/* Context swapping is not supported for AT91. */
> +		if (bp->caps & MACB_CAPS_MACB_IS_EMAC)
> +			return -EBUSY;
> +
> +		new_ctx = macb_context_alloc(bp, new_mtu,
> +					     bp->configured_rx_ring_size,
> +					     bp->configured_tx_ring_size);
> +		if (IS_ERR(new_ctx))
> +			return PTR_ERR(new_ctx);
> +
> +		macb_context_swap_start(bp);
> +	}
> 
>  	WRITE_ONCE(netdev->mtu, new_mtu);
> 
> +	if (running)
> +		macb_context_swap_end(bp, new_ctx);
> +
>  	return 0;
>  }

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

Thanks
Nicolai