Re: [PATCH v3 net-next 4/5] net: fec: add change_mtu to support dynamic buffer allocation

[Andrew Lunn <andrew@lunn.ch>]

> +static int fec_change_mtu(struct net_device *ndev, int new_mtu)
> +{
> +	struct fec_enet_private *fep = netdev_priv(ndev);
> +	int order, done;
> +
> +	order = get_order(new_mtu + ETH_HLEN + ETH_FCS_LEN);
> +	if (fep->pagepool_order == order) {
> +		WRITE_ONCE(ndev->mtu, new_mtu);
> +		return 0;
> +	}
> +
> +	fep->pagepool_order = order;
> +	fep->rx_frame_size = (PAGE_SIZE << order) - FEC_ENET_XDP_HEADROOM
> +			     - SKB_DATA_ALIGN(sizeof(struct skb_shared_info));
> +
> +	if (!netif_running(ndev)) {
> +		WRITE_ONCE(ndev->mtu, new_mtu);
> +		return 0;
> +	}
> +
> +	/* Stop TX/RX and free the buffers */
> +	napi_disable(&fep->napi);
> +	netif_tx_disable(ndev);
> +	read_poll_timeout(fec_enet_rx_napi, done, (done == 0),
> +			  10, 1000, false, &fep->napi, 10);
> +	fec_stop(ndev);
> +	fec_enet_free_buffers(ndev);
> +
> +	WRITE_ONCE(ndev->mtu, new_mtu);
> +
> +	/* Create the pagepool according the new mtu */
> +	if (fec_enet_alloc_buffers(ndev) < 0)
> +		return -ENOMEM;
> +

This is the wrong order. You need to first allocate the new buffers
and then free the old ones. If the allocation fails, you still have a
working interface using the smaller buffers, and the MTU change just
returns -ENOMEM. If you free the buffers and the allocation of the new
buffers fail, you interface is dead, because it has no buffers.

	Andrew