netdev - Re: [PATCH] Make virtio_net support carrier detection

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Date:	Thu, 12 Mar 2009 13:05:10 +0200
From:	Pantelis Koukousoulas <pktoss@...il.com>
To:	Rusty Russell <rusty@...tcorp.com.au>
Cc:	netdev@...r.kernel.org
Subject: Re: [PATCH] Make virtio_net support carrier detection

On Thu, Mar 12, 2009 at 12:23 PM, Pantelis Koukousoulas
<pktoss@...il.com> wrote:
>> No, that's absolutely sane behavior, the previous was buggy.  If a network
>> doesn't support carrier, you shouldn't look for it.
>>
>> Since they've fixed this in Network Manager, I'm not tempted to lie about it
>> in the driver (though the distributions might choose to).
>>
>> Rusty.
>>
>
> Ok, if "lying" is reporting that the driver supports carrier
> detection, this looks
> like an innocent lie :). Because the carrier *is* on, this is the truth and you
> could just as well claim that the virtual hardware supports detection
> as that it doesn't :)

After reading this one more time, it doesn't make much sense, so let me try
to explain my point of view more clearly:
For the wired case, carrier status == "is cable connected?"
(NetworkManager is asking that)

So,

* virtio saying "Hey, I have no idea if cable is connected or not. I
have no way to know that".
  (aka carrier detect not supported)

I think this is closer to lying than:

* "Hey, I know that the cable is connected". == carrier detection
supported. carrier is on

Because unlike prehistoric hardware / incomplete drivers, virtio *does* know :)

Even if someone comes up with a patch at the hypervisor / emulator
level that allows
the user to set carrier status to on or off, then pressure should be
applied on them
to implement this change in a way that makes this known to the virtio_net driver
for the same reasons that actual hardware does it (i.e., users like / want this
functionality :)

Hope this is clearer,
Pantelis
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