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

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Message-ID: <4E5E9E16.6060502@candelatech.com>
Date:	Wed, 31 Aug 2011 13:48:22 -0700
From:	Ben Greear <greearb@...delatech.com>
To:	Nicolas de Pesloüan 
	<nicolas.2p.debian@...il.com>
CC:	Ben Hutchings <bhutchings@...arflare.com>,
	Jiri Pirko <jpirko@...hat.com>,
	Michał Mirosław <mirqus@...il.com>,
	netdev@...r.kernel.org, davem@...emloft.net,
	eric.dumazet@...il.com, shemminger@...tta.com
Subject: Re: [patch net-next-2.6 1/2] net: allow to change carrier via sysfs

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.

Actually dropping link, and letting that naturally propagate up the stack seems
more reasonable than lying about the status half way up the stack.

Thanks,
Ben

-- 
Ben Greear <greearb@...delatech.com>
Candela Technologies Inc  http://www.candelatech.com
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