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From:   Xie He <xie.he.0141@gmail.com>
Date:   Mon, 23 Nov 2020 02:08:03 -0800
Message-ID: <CAJht_EO+enBOFMkVVB5y6aRnyMEsOZtUBJcAvOFBS91y7CauyQ@mail.gmail.com>
Subject: Re: [PATCH net-next v4 2/5] net/lapb: support netdev events
To:     Martin Schiller <ms@dev.tdt.de>
Cc:     Andrew Hendry <andrew.hendry@gmail.com>,
        "David S. Miller" <davem@davemloft.net>,
        Jakub Kicinski <kuba@kernel.org>,
        Linux X25 <linux-x25@vger.kernel.org>,
        Linux Kernel Network Developers <netdev@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>
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On Mon, Nov 23, 2020 at 1:36 AM Xie He <xie.he.0141@gmail.com> wrote:
>
> Some drivers don't support carrier status and will never change it.
> Their carrier status will always be UP. There will not be a
> NETDEV_CHANGE event.
>
> lapbether doesn't change carrier status. I also have my own virtual
> HDLC WAN driver (for testing) which also doesn't change carrier
> status.
>
> I just tested with lapbether. When I bring up the interface, there
> will only be NETDEV_PRE_UP and then NETDEV_UP. There will not be
> NETDEV_CHANGE. The carrier status is alway UP.
>
> I haven't tested whether a device can receive NETDEV_CHANGE when it is
> down. It's possible for a device driver to call netif_carrier_on when
> the interface is down. Do you know what will happen if a device driver
> calls netif_carrier_on when the interface is down?

I just did a test on lapbether and saw there would be no NETDEV_CHANGE
event when the netif is down, even if netif_carrier_on/off is called.
So we can rest assured of this part.