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.

When a device is brought up, if it has a peer and this peer device is
UP, set both carriers to on.

Signed-off-by: Breno Leitao <leitao@debian.org>
Fixes: 3762ec05a9fbda ("netdevsim: add NAPI support")
---
 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@debian.org>

On Mon, Oct 13, 2025 at 10:09:22AM -0700, Breno Leitao wrote:
> 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.
> 
> When a device is brought up, if it has a peer and this peer device is
> UP, set both carriers to on.
> 
> Signed-off-by: Breno Leitao <leitao@debian.org>
> Fixes: 3762ec05a9fbda ("netdevsim: add NAPI support")

It was not obvious what adding NAPI has to do with carrier status.  I
had to go look at 3762ec05a9fbda to see that that was when nsim_stop()
started to change the carrier on stop. This patch makes nsim_open()
somewhat symmetrical. If you need a respin, maybe expand the commit
message to explain this.

Otherwise this looks O.K. to me.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>

    Andrew

Hello Andrew,

On Mon, Oct 13, 2025 at 07:25:27PM +0200, Andrew Lunn wrote:
> On Mon, Oct 13, 2025 at 10:09:22AM -0700, Breno Leitao wrote:
> > 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.
> > 
> > When a device is brought up, if it has a peer and this peer device is
> > UP, set both carriers to on.
> > 
> > Signed-off-by: Breno Leitao <leitao@debian.org>
> > Fixes: 3762ec05a9fbda ("netdevsim: add NAPI support")
> 
> It was not obvious what adding NAPI has to do with carrier status.  I
> had to go look at 3762ec05a9fbda to see that that was when nsim_stop()
> started to change the carrier on stop. This patch makes nsim_open()
> somewhat symmetrical. If you need a respin, maybe expand the commit
> message to explain this.

Thanks for the review. I've sent a v2 with the updated message, given it
adds clarity to the change.

--breno