netdev - Re: signal quality and cable diagnostic

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Message-ID: <20200524224215.GE1551@shell.armlinux.org.uk>
Date:   Sun, 24 May 2020 23:42:16 +0100
From:   Russell King - ARM Linux admin <linux@...linux.org.uk>
To:     Pavel Machek <pavel@...x.de>
Cc:     Christian Herber <christian.herber@....com>,
        Oleksij Rempel <o.rempel@...gutronix.de>,
        Andrew Lunn <andrew@...n.ch>,
        "David S. Miller" <davem@...emloft.net>,
        Florian Fainelli <f.fainelli@...il.com>,
        Heiner Kallweit <hkallweit1@...il.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Jonathan Corbet <corbet@....net>,
        Michal Kubecek <mkubecek@...e.cz>,
        David Jander <david@...tonic.nl>,
        "kernel@...gutronix.de" <kernel@...gutronix.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "mkl@...gutronix.de" <mkl@...gutronix.de>,
        Marek Vasut <marex@...x.de>
Subject: Re: signal quality and cable diagnostic

On Sun, May 24, 2020 at 11:27:57PM +0200, Pavel Machek wrote:
> > > The SNR seems to be most universal value, when it comes to comparing
> > > different situations (different links and different PHYs). The
> > > resolution of BER is not that detailed, for the NXP PHY is says only
> > > "BER below 1e-10" or not.
> > 
> > The point I was trying to make is that SQI is intentionally called SQI and NOT SNR, because it is not a measure for SNR. The standard only suggest a mapping of SNR to SQI, but vendors do not need to comply to that or report that. The only mandatory requirement is linking to BER. BER is also what would be required by a user, as this is the metric that determines what happens to your traffic, not the SNR.
> > 
> > So when it comes to KAPI parameters, I see the following options
> > - SQI only
> > - SQI + plus indication of SQI level at which BER<10^-10 (this is the only required and standardized information)
> > - SQI + BER range (best for users, but requires input from the silicon vendors)
> 
> Last option looks best to me... and it will mean that hopefully silicon vendors standartize
> something in future.

It already has been for > 1G PHYs, but whether they implement it is
another question altogether.  It's a 22-bit limiting counter in the
PCS.  There's also indications of "high BER".

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
FTTC for 0.8m (est. 1762m) line in suburbia: sync at 13.1Mbps down 424kbps up