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

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

On 17.3.2026 23:23, Jakub Kicinski wrote:
> On Tue, 17 Mar 2026 18:00:02 +0100 Théo Lebrun wrote:
>> Reviewed-by: Théo Lebrun <theo.lebrun@bootlin.com>
> 
> I told you recently that open / close is not allowed.
> So why are you putting a rb tag on a patch which does exactly that?
> 
> https://lore.kernel.org/all/20260306190948.44d23f8f@kernel.org/

Just checking if I got the idea before submitting another approach.
Something like:


static int macb_change_mtu(struct net_device *dev, int new_mtu)
{
	// declarations
	// ...

	if (!netif_running(dev)) {
		WRITE_ONCE(dev->mtu, new_mtu);
		return 0;
	}

	// stop TX + NAPI
	// ...

	// stop MAC (macb_reset_hw), no phylink/PHY/PM touch
	// ...

	macb_free_consistent(bp);

	WRITE_ONCE(dev->mtu, new_mtu);
	macb_init_rx_buffer_size(bp, bufsz);

	err = macb_alloc_consistent(bp);
	if (err) {
		// device is down, no DMA buffers - same as igb
		return err;
	}

	// init rings, init HW, re-enable NAPI + TX
	// ...

	return 0;
}


On alloc failure the device would be left stopped, same as 
igb_change_mtu() does it. Or would you prefer stmmac-style 
pre-allocation?
That would mean refactoring macb_alloc_consistent() to return a separate 
resource set.

Thanks
Nicolai