linux-kernel - [PATCH net v2] netdevsim: set the carrier when the device goes up

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Message-Id: <20251014-netdevsim_fix-v2-1-53b40590dae1@debian.org>
Date: Tue, 14 Oct 2025 02:17:25 -0700
From: Breno Leitao <leitao@...ian.org>
To: Jakub Kicinski <kuba@...nel.org>, Andrew Lunn <andrew+netdev@...n.ch>, 
 "David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, 
 Paolo Abeni <pabeni@...hat.com>, David Wei <dw@...idwei.uk>
Cc: netdev@...r.kernel.org, linux-kernel@...r.kernel.org, 
 kernel-team@...a.com, Breno Leitao <leitao@...ian.org>, 
 Andrew Lunn <andrew@...n.ch>
Subject: [PATCH net v2] 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 <leitao@...ian.org>
Fixes: 3762ec05a9fbda ("netdevsim: add NAPI support")
Reviewed-by: Andrew Lunn <andrew@...n.ch>
---
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/20251013-netdevsim_fix-v1-1-357b265dd9d0@debian.org
---
 drivers/net/netdevsim/netdev.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/drivers/net/netdevsim/netdev.c b/drivers/net/netdevsim/netdev.c
index ebc3833e95b44..fa1d97885caaf 100644
--- a/drivers/net/netdevsim/netdev.c
+++ b/drivers/net/netdevsim/netdev.c
@@ -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 <leitao@...ian.org>