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

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

On 17.3.2026 20:42, 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?

I think keeping the new MTU even if macb_open() fails is fine. The 
interface is down at that point anyway?
igb does the same and even discards the igb_up() return value entirely. 
At least we propagate the error.

> 
> Other than that, it looks good to me.
> --breno

Thanks
Nicolai