Re: [PATCH net-next v4 2/5] net/lapb: support netdev events

[Xie He]

On Mon, Nov 23, 2020 at 1:00 AM Martin Schiller <ms@xxxxxxxxxx> 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?