netdev - Re: Request for feature: force carrier up/on flag

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Date:   Wed, 10 Feb 2021 07:57:46 +0100
From:   Heiner Kallweit <hkallweit1@...il.com>
To:     Anton Hvornum <anton@...rnum.se>
Cc:     netdev@...r.kernel.org,
        Stephen Hemminger <stephen@...workplumber.org>
Subject: Re: Request for feature: force carrier up/on flag

On 10.02.2021 01:32, Stephen Hemminger wrote:
> On Tue, 9 Feb 2021 18:35:54 +0100
> Anton Hvornum <anton@...rnum.se> wrote:
> 
>> Hi.
>>
>> I am a bit new to ethtool, kernel drivers and all the surrounding aspects.
>> I also recognize that my use case is of low priority and a bit niche,
>> but any response would be greatly appreciated.
>>
>> I'm modifying an existing Intel driver, and I'm looking for a way to
>> integrate/add another ethtool hook to force call `netif_carrier_on`.
>> There's plenty of hooks/listeners (not clear as to what you call
>> these) between the Intel driver and ethtool via C API's today that
>> allows for ethtool to control the driver. Many of which are for speed,
>> autonegotiation etc. But I don't see any flags/functions to force a
>> carrier state to up.
>>
>> This would be very useful for many reasons, primarily on fiber optic
>> setups where you are developing hardware such as switches, routers and
>> even developing network cards. Or if you've got a passive device such
>> as IDS or something similar that passively monitors network traffic
>> and can't/shouldn't send out link requests.
>> There are commercial products with modified drivers that support this,
>> but since the Intel hardware in this case seems to support it - just
>> that there's no way controlling it with the tools that exist today. I
>> would therefore request a feature for consideration to ethtool that
>> can force carrier states up/down.
>>
>> A intuitive option I think would be:
>> ethtool --change carrier on
>>
>> Assuming that drivers follow suit and allow this. But a first step
>> would be for the tools to support it in the API so drivers have
>> something to call/listen for. In the meantime, I can probably
>> integrate a private flag and go that route, but that feels hacky and
>> won't foster driver developers to follow suit. My goal is to empower
>> more open source alternatives to otherwise expensive commercials
>> solutions.
>>
>> Best wishes,
>> Anton Hvornum
> 
> Normally, carrier just reflects the state of what the hardware is
> reporting. Why not set admin down which tells the NIC to take
> the device offline, and that drops the fiber link.
> 
> Or maybe what you want is already there.
> Try writing to /sys/class/net/ethX/carrier which forces a carrier change?
> 

This attribute uses callback ndo_change_carrier in struct net_device_ops.
The kerneldoc provides further details.