Re: [PATCH net-next] net: macb: allow MTU changes while the interface is running

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

On 17.3.2026 20:47, Breno Leitao wrote:
> On Tue, Mar 17, 2026 at 12:42:04PM -0700, Breno Leitao wrote:
>> On Mon, Mar 16, 2026 at 10:27:20AM +0100, Nicolai Buchwitz wrote:
>> > macb_change_mtu() currently returns -EBUSY if the interface is running,
>> > requiring users to bring the interface down before changing the MTU. This
>> > is unnecessarily restrictive.
>> >
>> > Instead, close and reopen the interface around the MTU change so that RX
>> > DMA buffers are reallocated for the new MTU. This is the same approach
>> > used by many other network drivers (e.g. igb, tg3, stmmac).
>> >
>> > Signed-off-by: Nicolai Buchwitz <nb@tipi-net.de>
>> > ---
>> >  drivers/net/ethernet/cadence/macb_main.c | 9 +++++++--
>> >  1 file changed, 7 insertions(+), 2 deletions(-)
>> >
>> > diff --git a/drivers/net/ethernet/cadence/macb_main.c b/drivers/net/ethernet/cadence/macb_main.c
>> > index 5e27e0e87a55..8dd01031250d 100644
>> > --- a/drivers/net/ethernet/cadence/macb_main.c
>> > +++ b/drivers/net/ethernet/cadence/macb_main.c
>> > @@ -3262,11 +3262,16 @@ static int macb_close(struct net_device *dev)
>> >
>> >  static int macb_change_mtu(struct net_device *dev, int new_mtu)
>> >  {
>> > -	if (netif_running(dev))
>> > -		return -EBUSY;
>> > +	bool was_running = netif_running(dev);
>> > +
>> > +	if (was_running)
>> > +		macb_close(dev);
>> >
>> >  	WRITE_ONCE(dev->mtu, new_mtu);
>> >
>> > +	if (was_running)
>> > +		return macb_open(dev);
>> 
>> Would you like to keep the new_mtu set, even if macb_open() fails?
> 
> Looking a bit further, I found that macb_set_ringparam() doesn't return
> failure if macb_open() fails. Should it be fixed also?
> 
> Should we have something similar to your code above?
> 
> diff --git a/drivers/net/ethernet/cadence/macb_main.c 
> b/drivers/net/ethernet/cadence/macb_main.c
> index 5bc35f651ebd2..3c43fc85af79d 100644
> --- a/drivers/net/ethernet/cadence/macb_main.c
> +++ b/drivers/net/ethernet/cadence/macb_main.c
> @@ -3516,7 +3516,7 @@ static int macb_set_ringparam(struct net_device 
> *netdev,
>         bp->tx_ring_size = new_tx_size;
> 
>         if (reset)
> -               macb_open(bp->dev);
> +               return macb_open(bp->dev);
> 
>         return 0;
>  }

Makes sense. I will send a separate patch for this.

Cheers
Nicolai