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From: David Miller <davem@davemloft.net>
Subject: Re: [PATCH] Make virtio_net support carrier detection
Date: Sun, 15 Mar 2009 20:15:36 -0700 (PDT)
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From: Rusty Russell <rusty@rustcorp.com.au>
Date: Mon, 16 Mar 2009 13:43:13 +1030

> On Monday 16 March 2009 13:28:06 David Miller wrote:
> > > Or should all devices which can't detect carrier set it to on, so
> > > older NetworkManagers work?
> > 
> > It is my opinion that this is the only correct behavior.
> > 
> > If you don't know, it's on.  Because "on" means "usable".  And you
> > can't fault NetworkManager or any other tool for making that kind of
> > decision.
> 
> Yes, I agree that NM has to assume unknown == on.  But I still think the
> kernel is right as it is to return -EOPNOTSUPP and let userspace deal with it.
> 
> "Let's fix broken userspace in the kernel" arguments make me nervous ;)

This is right from one perspective.

But from another, -EOPNOTSUPP means "half-written driver, kick maintainer"
:-)