netdevsim: set the carrier when the device goes up

Bringing a linked netdevsim device down and then up causes communication
failure because both interfaces lack carrier. Basically a ifdown/ifup on
the interface make the link broken.

Commit 3762ec05a9fbda ("netdevsim: add NAPI support") added supported
for NAPI, calling netif_carrier_off() in nsim_stop(). This patch
re-enables the carrier symmetrically on nsim_open(), in case the device
is linked and the peer is up.

Signed-off-by: Breno Leitao <[e-mail]>
Fixes: 3762ec05a9fbda ("netdevsim: add NAPI support")
Reviewed-by: Andrew Lunn <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

drivers/net/netdevsim/netdev.c

···
 static int nsim_open(struct net_device *dev)
 {
 	struct netdevsim *ns = netdev_priv(dev);
+	struct netdevsim *peer;
 	int err;
 
 	netdev_assert_locked(dev);
···
 		return err;
 
 	nsim_enable_napi(ns);
+
+	peer = rtnl_dereference(ns->peer);
+	if (peer && netif_running(peer->netdev)) {
+		netif_carrier_on(dev);
+		netif_carrier_on(peer->netdev);
+	}
 
 	return 0;
 }