Subject: Re: [PATCH net-next] net: macb: allow MTU change when the interface is up

From: Simon Horman <[email-masked]>
Date: Mon, 25 Mar 2024 20:54:01 +0000

On Mon, Mar 25, 2024 at 04:27:35PM +0100, [email-masked] wrote:
> From: Thomas Perrot <[email-masked]>
> 
> The macb_change_mtu callback doesn't allow MTU changes when the
> interface is up because the MTU is used to set the buffer size on
> GEM.
> 
> So, to be able to change the MTU from userspace it is necessary to
> explicitly down then to up the interface, which is problematic in
> some cases.
> 
> Then we suggest to reset the interface when the interface is already
> up to allow to change the MTU, regardless of if the interface is up
> or not.
> 
> Signed-off-by: Thomas Perrot <[email-masked]>

Hi Thomas,

I'm not sure that it is expected behaviour for an interface
to reset like this when a change of MTU is requested.
While conversely I think it is common (if not entirely desirable)
to prohibit changing the MTU when an interface is up.
What is the problem being addressed here?

> ---
>  drivers/net/ethernet/cadence/macb_main.c | 12 ++++++++++--
>  1 file changed, 10 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/net/ethernet/cadence/macb_main.c b/drivers/net/ethernet/cadence/macb_main.c
> index 898debfd4db3..5c20b162c5da 100644
> --- a/drivers/net/ethernet/cadence/macb_main.c
> +++ b/drivers/net/ethernet/cadence/macb_main.c
> @@ -3019,11 +3019,19 @@ 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;
> +	int reset = 0;

nit: reset could be bool and set to true and false.

> +
> +	/* On GEM the buffer size is based on the MTU */
> +	if (netif_running(dev)) {
> +		macb_close(dev);
> +		reset = 1;
> +	}
>  
>  	dev->mtu = new_mtu;
>  
> +	if (reset)
> +		macb_open(dev);
> +
>  	return 0;
>  }
>  
> -- 
> 2.44.0
> 
>