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

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Message-ID: <20210209163242.7ce62140@hermes.local>
Date:   Tue, 9 Feb 2021 16:32:42 -0800
From:   Stephen Hemminger <stephen@...workplumber.org>
To:     Anton Hvornum <anton@...rnum.se>
Cc:     netdev@...r.kernel.org
Subject: Re: Request for feature: force carrier up/on flag

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?