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.

---
Changes in v2:
- Reword the commit message to reference the symmetry between
nsim_open() and nsim_stop() in regarding to carrier. (Andrew Lunn)
- Link to v1: https://lore.kernel.org/r/[email protected]

---
 drivers/net/netdevsim/netdev.c | 7 +++++++
 1 file changed, 7 insertions(+)

index ebc3833e95b44..fa1d97885caaf 100644
@@ -545,6 +545,7 @@ static void nsim_enable_napi(struct netdevsim *ns)
 static int nsim_open(struct net_device *dev)
 {
 	struct netdevsim *ns = netdev_priv(dev);
+	struct netdevsim *peer;
 	int err;
 
 	netdev_assert_locked(dev);

@@ -555,6 +556,12 @@ static int nsim_open(struct net_device *dev)
 
 	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;
 }
 

---
base-commit: 0b4b77eff5f8cd9be062783a1c1e198d46d0a753
change-id: 20251013-netdevsim_fix-b9a8a5064f8f

Best regards,

Breno Leitao <[email protected]>