Date:	Thu, 12 Mar 2009 09:55:34 -0400
From:	Dan Williams <dcbw@...hat.com>
To:	David Miller <davem@...emloft.net>
Cc:	pktoss@...il.com, rusty@...tcorp.com.au, netdev@...r.kernel.org
Subject: Re: [PATCH] Make virtio_net support carrier detection

On Thu, 2009-03-12 at 09:41 -0400, Dan Williams wrote:
> On Thu, 2009-03-12 at 05:52 -0700, David Miller wrote:
> > From: Pantelis Koukousoulas <pktoss@...il.com>
> > Date: Thu, 12 Mar 2009 14:47:09 +0200
> > 
> > > > The problem is that there's no "my carrier detection is accurate" flag
> > > > for drivers.  Drivers that don't support carrier detection (say, my
> > > > Belkin PCMCIA NE2k card) always report "carrier on", but of course don't
> > > > support carrier detection.  So when NetworkManager looks at the device
> > > > and sees "hey, there's a carrier!" it will activate it, because a
> > > > carrier means the cable is plugged in or the PHY *thinks* a cable is
> > > > plugged in.
> > > >
> > > > So NetworkManager checks whether the device supports ethtool get_link or
> > > > MII register carrier status in lieu of a general kernel driver flag for
> > > > "I support carrier detection".
> > > >
> > > > Carrier is not on/off, it needs to be tristate, like on/off/unknown.  If
> > > > it was unknown, NM could make an intelligent decision about this without
> > > > resorting to ethtool/MII checks.  But we don't have that.
> > > 
> > > This looks like an independent problem imho, one of kernel<->userspace
> > > API. The current 'defacto' way of finding out if detection is supported or
> > > not (ethtool) seems to give the needed information even if somewhat ugly.
> > > 
> > > The issue discussed here as I understand it is whether virtio should or
> > > should not support carrier status reporting. IMHO it should, since
> > > it is useful functionality and doesn't cost much.
> > > 
> > > Dan, What is your opinion on that?
> > 
> > I think Dan is right.
> > 
> > If the driver doesn't provide an explicit carrier, there is no way to
> > know for sure that it is on of off.  If NetworkManager decides that
> > this means on, and it's really off, that isn't so nice.
> > 
> > And lots of very old drivers in the tree don't have a link indication
> > handler, so this is a very real issue.
> > 
> > Not adding a link state handler to virtio_net for pompous reasons like
> > some theoretical "clean design" claim is idiotic, and in the end bad
> > for users who are using existing versions of NetworkManager.
> > 
> > I also think the NetworkManager change is wrong, lack of link
> > indication support does not mean the link is always on, not by a
> > country mile.  Tell that to all the ancient ethernet drivers in
> > our tree.
> 
> Checking up more this morning in NetworkManager, here's the current
> behavior with 0.7.x.
> 
> If the device doesn't support carrier detection (determined by checking
> for ethtool or MII ioctl support), NetworkManager will *not* listen for
> netlink carrier events.  These are triggered by
> netif_carrier_on()/netif_carrier_off(), which even drivers that don't
> support carrier detection will still sometimes send.
> 
> However, NetworkManager will assume carrier=on, because NM is designed
> such that if the carrier is not on, the nothing can be done with the
> device because there's no cable connected.
> 
> It will then try to automatically bring up any valid 'autoconnect'
> ethernet connection.  This will obviously fail, and when NM runs out of
> valid 'autoconnect' ethernet configs, the device will be dormant until
> the user explicitly chooses to activate it from the UI.
> 
> This shouldn't be an issue, because NM supports multiple active devices,
> and thus if you have a wifi connection as well, that will successfully
> activate and wifi will get the default route while the ethernet device
> is still trying its hardest to DHCP or whatever.
> 
> Of course, if you've configured a static IP address for your ethernet
> interface, NM will happily bring up the non-carrier-capable ethernet
> interface with the static IP, because there's no way to know whether or
> not there's a cable plugged in when the driver reports carrier=on
> 
> Of course, if you've *not* selected "Connect automatically", then NM
> will wait for you to do something before configuring the interface, as
> expected.
> 
> This matches the behavior of most distro network systems where, if the
> card didn't support carrier detection and since it would then report
> IFF_UP/IFF_LOWER_UP/IFF_RUNNING/etc, the distro scripts would configure
> the device anyway, because the driver reports carrier=on.

(to be 100% clear, I believe that Pantelis' patch to add minimal ethtool
support is *correct*, and it will fix the issue with NetworkManager)

Dan

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