netdev - Re: [patch net-next-2.6 1/2] net: allow to change carrier via sysfs

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Date:	Wed, 31 Aug 2011 14:40:23 -0700
From:	Stephen Hemminger <shemminger@...tta.com>
To:	Ben Hutchings <bhutchings@...arflare.com>
Cc:	Ben Greear <greearb@...delatech.com>,
	Nicolas de Pesloüan 
	<nicolas.2p.debian@...il.com>, Jiri Pirko <jpirko@...hat.com>,
	Michał Mirosław <mirqus@...il.com>,
	netdev@...r.kernel.org, davem@...emloft.net, eric.dumazet@...il.com
Subject: Re: [patch net-next-2.6 1/2] net: allow to change carrier via sysfs

On Wed, 31 Aug 2011 22:36:45 +0100
Ben Hutchings <bhutchings@...arflare.com> wrote:

> On Wed, 2011-08-31 at 13:48 -0700, Ben Greear wrote:
> > On 08/31/2011 01:31 PM, Nicolas de Pesloüan wrote:
> > > Le 31/08/2011 22:12, Ben Hutchings a écrit :
> > >> On Wed, 2011-08-31 at 22:03 +0200, Nicolas de Pesloüan wrote:
> > >>> Le 31/08/2011 10:45, Jiri Pirko a écrit :
> > >>>
> > >>>>>>> Do you expect drivers using implementation different than just calling
> > >>>>>>> netif_carrier_on/off? Or is it supposed to also e.g. power down PHYs?
> > >>>>>> Yes, generally it can be used also for en/disable phy, for testing
> > >>>>>> purposes if hw and driver would support it.
> > >>>>>
> > >>>>> I'd like to see this working for GRE tunnel devices (for keepalive
> > >>>>> daemon to be able to indicate to routing daemons whether tunnel is
> > >>>>> really working) - implementation would be identical to dummy's case.
> > >>>>> Should I prepare a patch or can I leave it to you?
> > >>>>
> > >>>> Ok, I can include it to this patchset (I'm going to repost first patch
> > >>>> anyway)
> > >>>
> > >>> Can't we assume that the dummy's case is the default behavior and
> > >>> register this default
> > >>> ndo_change_carrier callback for every device ?
> > >>
> > >> You have got to be joking. No device driver that has real link
> > >> monitoring should use this implementation.
> > >
> > > Well, why not? Arguably, this is probably not the feature one would use every day, but...
> > >
> > > Testing a cluster reaction to a link down event would be easier if one doesn't need to unplug the cable for the test. I understand that one can turn off the
> > > switch port (physical or virtual), but echo 0 > /sys/class/net/eth0/carrier would be nice too.
> > 
> > There is special hardware out there that can do bypass, and often it also has a mode
> > that will programatically cut link by throwing some relays.  We use this for our
> > testing equipment...
> > 
> > If there is some way to twiddle standard-ish hardware to actually drop link, that
> > would be neat.  I'd think it should be an ethtool type of thing, however.
> 
> We need to be able to control this as part of our driver test suite (on
> the peer, not the device under test).  There are various MDIO bits that
> look like they should do this but unfortunately they don't have
> consistent effects.  Besides that, many PHYs are not MDIO-manageable.
> So this would have to be a device-specific operation, whether it's
> exposed through ethtool or sysfs.
> 
> > Actually dropping link, and letting that naturally propagate up the stack seems
> > more reasonable than lying about the status half way up the stack.
> 

For testing clustering, there are hooks in vmware and QEMU/KVM to allow
dropping carrier on the VM side.
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