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

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

Hi Théo

Thanks for the review and testing!

On 17.3.2026 18:00, Théo Lebrun wrote:
> Hello Nicolai,
> 
> On Mon Mar 16, 2026 at 10:27 AM CET, 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.
> 
> One valid reasoning for making the operation return -EBUSY is that
> macb_close() will lead to packet loss for a few seconds. Users might
> not expect an MTU change to trigger that. -EBUSY means user (be it
> human or software) becomes aware and has time for a second thought.
> 
> But clearly, it becomes annoying as the user in the long run, when you
> own your platform and know the implications.

Since setting the MTU already requires elevated rights and usually 
involves reconfiguring surrounding infrastructure that may also drop 
packets, I think the better UX is worth the brief interruption.

> 
>> 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);
>> +
>>  	return 0;
>>  }
> 
> Reviewed-by: Théo Lebrun <theo.lebrun@bootlin.com>
> Tested-by: Théo Lebrun <theo.lebrun@bootlin.com> # on eyeq5-epm
> 
> Thanks,
> 
> --
> Théo Lebrun, Bootlin
> Embedded Linux and Kernel engineering
> https://bootlin.com

Cheers
Nicolai