netdev - Re: Setting link down or up in software

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Date:   Fri, 13 Jan 2017 10:20:25 +0100
From:   Zefir Kurtisi <zefir.kurtisi@...atec.com>
To:     Mason <slash.tmp@...e.fr>, netdev <netdev@...r.kernel.org>
Cc:     Mans Rullgard <mans@...sr.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        Andrew Lunn <andrew@...n.ch>,
        Thibaud Cornic <thibaud_cornic@...madesigns.com>
Subject: Re: Setting link down or up in software

On 01/12/2017 04:16 PM, Mason wrote:
> On 12/01/2017 14:05, Mason wrote:
> 
>> I'm wondering what are the semantics of calling
>>
>> 	ip link set dev eth0 down
>>
>> I was expecting that to somehow instruct the device's ethernet driver
>> to shut everything down, have the PHY tell the peer that it's going
>> away, maybe even put the PHY in some low-power mode, etc.
>>
>> But it doesn't seem to be doing any of that on my HW.
>>
>> So what exactly is it supposed to do?
>>
>>
>> And on top of that, I am seeing random occurrences of
>>
>> 	nb8800 26000.ethernet eth0: Link is Down
>>
>> Sometimes it is printed immediately.
>> Sometimes it is printed as soon as I run "ip link set dev eth0 up" (?!)
>> Sometimes it is not printed at all.
>>
>> I find this erratic behavior very confusing.
>>
>> Is it the symptom of some deeper bug?
> 
> Here's an example of "Link is Down" printed when I set link up:
> 
> At [   62.750220] I run ip link set dev eth0 down
> Then leave the system idle for 10 minutes.
> At [  646.263041] I run ip link set dev eth0 up
> At [  647.364079] it prints "Link is Down"
> At [  649.417434] it prints "Link is Up - 1Gbps/Full - flow control rx/tx"
> 
> I think whether I set up the PHY to use interrupts or polling
> does have an influence on the weirdness I observe.
> 
> AFAICT, changing the interface flags is done in dev_change_flags
> which calls __dev_change_flags and __dev_notify_flags
> 
> Is one of these supposed to call the device driver through a
> callback at some point?
> 
> How/when is the phy_state_machine notified of the change in
> interface flags?
> 
> Regards.
> 
Hm, reminds me of something at my side that I recently fixed with [1]. For me it
was pulling the cable got randomly unnoticed at PHY layer - but might be related.

Do you by chance have some component that polls the link states over the ethtool
interface very often (like once per second)? At my side it was a snmpd agent that
pro-actively updated the interface states every second and with that 'stole' the
link change information from the phy link state machine. What you need to have to
run in such a failing situation is:
1) an ETH driver that updates link status in ethtool GSET path (e.g. dsa does)
2) some component that continuously polls states via ethtool GSET


Cheers,
Zefir


[1] https://patchwork.ozlabs.org/patch/711839/