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From:   Xie He <xie.he.0141@gmail.com>
Date:   Mon, 23 Nov 2020 01:36:45 -0800
Message-ID: <CAJht_EMjO_Tkm93QmAeK_2jg2KbLdv2744kCSHiZLy48aXiHnw@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 1:00 AM Martin Schiller <ms@dev.tdt.de> wrote:
>
> AFAIK the carrier can't be up before the device is up. Therefore, there
> will be a NETDEV_CHANGE event after the NETDEV_UP event.
>
> This is what I can see in my tests (with the HDLC interface).
>
> Is the behaviour different for e.g. lapbether?

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?