netdev - Re: [PATCH net-next 1/3] net: phy: add and use phy_check

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Message-ID: <82c01854-4571-4e4f-680d-316f7cd0870f@gmail.com>
Date:   Thu, 19 Mar 2020 18:04:51 +0100
From:   Heiner Kallweit <hkallweit1@...il.com>
To:     Florian Fainelli <f.fainelli@...il.com>,
        Russell King - ARM Linux admin <linux@...linux.org.uk>,
        Andrew Lunn <andrew@...n.ch>
Cc:     David Miller <davem@...emloft.net>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next 1/3] net: phy: add and use phy_check_downshift

On 19.03.2020 17:55, Florian Fainelli wrote:
> Le 2020-03-19 à 09:36, Heiner Kallweit a écrit :
>> *Downshift reporting/warning*
>> In most cases downshift is caused by some problem with the cabling.
>> Users like the typical Ubuntu user in most cases are not familiar
>> with the concept of PHY downshift and what causes a downshift.
>> Therefore it's not sufficient to just report a downshift, we have
>> to provide the user with a hint what to do.
>> Adding the "downshifted" info to phy_print_status() is a good idea,
>> however I'd see it as an optional addition to the mentioned hint
>> to the user what to do.
>> The info "unknown downshift" IMO would just cause confusion. If we
>> have nothing to say, then why say something. Also users may interpret
>> "unknown" as "there's something wrong".
> 
> Ideally we would also have support for cable testing as well which would
> allow users to know what to do next to figure out why their link speed
> is not what they had hoped for. That does require a specific PHY driver
> though as there is no standard way to obtain that information.
> 
> FWIW, a bunch of drivers like tg3, e1000e, igc and possibly others do
> report when downshifting occurs, of course not all of those in that list
> use the PHY library.
> 
Yes, a direct link to cable testing would be nice. With the new
netlink-based ethtool support for cable testing comes closer.
AFAIK Andrew is evaluating what an API could look like.

However for the time being I think a downshift warning in phylib would
be helpful to explain to the user: Check your damned cabling instead
of spamming support forums with blaming the network driver or any other
piece of software or hardware.