netdev - RE: [EXT] Re: signal quality and cable diagnostic

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Date:   Thu, 14 May 2020 07:13:30 +0000
From:   Christian Herber <christian.herber@....com>
To:     Oleksij Rempel <o.rempel@...gutronix.de>,
        Andrew Lunn <andrew@...n.ch>
CC:     "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>,
        Russell King <linux@...linux.org.uk>,
        "mkl@...gutronix.de" <mkl@...gutronix.de>,
        Marek Vasut <marex@...x.de>
Subject: RE: [EXT] Re: signal quality and cable diagnostic

On Tue, May 12, 2020 at 10:22:01AM +0200, Oleksij Rempel wrote:

> So I think we should pass raw SQI value to user space, at least in the
> first implementation.

> What do you think about this?

Hi Oleksij,

I had a check about the background of this SQI thing. The table you reference with concrete SNR values is informative only and not a requirement. The requirements are rather loose.

This is from OA:
- Only for SQI=0 a link loss shall occur.
- The indicated signal quality shall monotonic increasing /decreasing with noise level.
- It shall be indicated in the datasheet at which level a BER<10^-10 (better than 10^-10) is achieved (e.g. "from SQI=3 to SQI=7 the link has a BER<10^-10 (better than 10^-10)")

I.e. SQI does not need to have a direct correlation with SNR. The fundamental underlying metric is the BER.
You can report the raw SQI level and users would have to look up what it means in the respective data sheet. There is no guaranteed relation between SQI levels of different devices, i.e. SQI 5 can have lower BER than SQI 6 on another device.
Alternatively, you could report BER < x for the different SQI levels. However, this requires the information to be available. While I could provide these for NXP, it might not be easily available for other vendors.
If reporting raw SQI, at least the SQI level for BER<10^-10 should be presented to give any meaning to the value.

Regards,

Christian