net: ethernet: stmicro: stmmac: permit MTU change with interface up

Remove the limitation where the interface needs to be down to change
MTU by releasing and opening the stmmac driver to set the new MTU.
Also call the set_filter function to correctly init the port.
This permits to remove the EBUSY error while the ethernet port is
running permitting a correct MTU change if for example a DSA request
a MTU change for a switch CPU port.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/stmicro/stmmac/stmmac_main.c

@@ -5551,18 +5551,15 @@ static int stmmac_change_mtu(struct net_device *dev, int new_mtu)
 {
 	struct stmmac_priv *priv = netdev_priv(dev);
 	int txfifosz = priv->plat->tx_fifo_size;
+	struct stmmac_dma_conf *dma_conf;
 	const int mtu = new_mtu;
+	int ret;
 
 	if (txfifosz == 0)
 		txfifosz = priv->dma_cap.tx_fifo_size;
 
 	txfifosz /= priv->plat->tx_queues_to_use;
 
-	if (netif_running(dev)) {
-		netdev_err(priv->dev, "must be stopped to change its MTU\n");
-		return -EBUSY;
-	}
-
 	if (stmmac_xdp_is_enabled(priv) && new_mtu > ETH_DATA_LEN) {
 		netdev_dbg(priv->dev, "Jumbo frames not supported for XDP\n");
 		return -EINVAL;
@@ -5574,8 +5571,29 @@ static int stmmac_change_mtu(struct net_device *dev, int new_mtu)
 	if ((txfifosz < new_mtu) || (new_mtu > BUF_SIZE_16KiB))
 		return -EINVAL;
 
-	dev->mtu = mtu;
+	if (netif_running(dev)) {
+		netdev_dbg(priv->dev, "restarting interface to change its MTU\n");
+		/* Try to allocate the new DMA conf with the new mtu */
+		dma_conf = stmmac_setup_dma_desc(priv, mtu);
+		if (IS_ERR(dma_conf)) {
+			netdev_err(priv->dev, "failed allocating new dma conf for new MTU %d\n",
+				   mtu);
+			return PTR_ERR(dma_conf);
+		}
 
+		stmmac_release(dev);
+
+		ret = __stmmac_open(dev, dma_conf);
+		kfree(dma_conf);
+		if (ret) {
+			netdev_err(priv->dev, "failed reopening the interface after MTU change\n");
+			return ret;
+		}
+
+		stmmac_set_rx_mode(dev);
+	}
+
+	dev->mtu = mtu;
 	netdev_update_features(dev);
 
 	return 0;