net: stmmac: allow changing the MTU while the interface is running

From: Niklas Cassel <[email protected]>

Setting ethtool ops for stmmac is only allowed when the interface is up. Setting MTU (a netdev op) for stmmac is only allowed when the interface is down.

It seems that the only reason why MTU cannot be changed when running is that we have not bothered to implement a nice way to dealloc/alloc the descriptor rings.

To make it less confusing for the user, call ndo_stop() and ndo_open() from ndo_change_mtu(). This is not a nice way to dealloc/alloc the descriptor rings, since it will announce that the interface is being brought down/up to user space, but there are several other drivers doing it this way, and it is arguably better than just returning -EBUSY.

---
 drivers/net/ethernet/stmicro/stmmac/stmmac_main.c | 12 +++++-------
 1 file changed, 5 insertions(+), 7 deletions(-)

index c1c63197ff73..fd268dc0df02 100644
@@ -3109,17 +3109,15 @@ static void stmmac_set_rx_mode(struct net_device *dev)
  */
 static int stmmac_change_mtu(struct net_device *dev, int new_mtu)
 {
-	struct stmmac_priv *priv = netdev_priv(dev);
-
-	if (netif_running(dev)) {
-		netdev_err(priv->dev, "must be stopped to change its MTU\n");
-		return -EBUSY;
-	}
-
 	dev->mtu = new_mtu;
 
 	netdev_update_features(dev);
 
+	if (netif_running(dev)) {
+		stmmac_release(dev);
+		stmmac_open(dev);
+	}
+
 	return 0;
 }
 

2.11.0

From: Niklas Cassel <[email protected]>
Date: Tue, 4 Apr 2017 14:18:54 +0200

You can't do this with no error handling.

Instead, you must do this using a "prepare", "commit" sequence. First making sure you can reallocate all necessary resources, and make the config change, before actually doing so.

You're not even checking if the re-open fails, meaning that an MTU change can cause the interface to shut down. That is simply not acceptable.