linux-kernel - RE: [PATCH net-next v1 1/2] ethtool: provide UAPI for PHY Signal Quality Index (SQI)

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Date:   Tue, 19 May 2020 11:21:57 +0000
From:   Christian Herber <christian.herber@....com>
To:     Oleksij Rempel <o.rempel@...gutronix.de>,
        Michal Kubecek <mkubecek@...e.cz>
CC:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        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>,
        David Jander <david@...tonic.nl>,
        "kernel@...gutronix.de" <kernel@...gutronix.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Russell King <linux@...linux.org.uk>,
        "mkl@...gutronix.de" <mkl@...gutronix.de>,
        Marek Vasut <marex@...x.de>
Subject: RE: [PATCH net-next v1 1/2] ethtool: provide UAPI for PHY Signal
 Quality Index (SQI)

On Tue, May 19, 2020 at 12:58:55PM +0200, Oleksij Rempel wrote:
> On Tue, May 19, 2020 at 10:55:20AM +0200, Michal Kubecek wrote:
> > On Tue, May 19, 2020 at 09:51:59AM +0200, Oleksij Rempel wrote:

> > I'm also a bit worried about hardcoding the 0-7 value range. While I
> > understand that it's defined by standard for 100base-T1, we my want to
> > provide such information for other devices in the future. I tried to
> > search if there is something like that for 1000base-T1 and found this:
> >
> > The screenshot on page 10 shows "SQI Value: 00015".
>
> Nice, screenshot based reverse engineering :)
>
> > It's probably not
> > standardized (yet?) but it seems to indicate we may expect other devices
> > providing SQI information with different value range.
>
> what maximal range do we wont to export? u8, u16 or u32?
>
> > Would it make sense to add ETHTOOL_A_LINKSTATE_SQI_MAX attribute telling
> > userspace what the range is?
>
> sounds plausible.

SQI is not yet standardized for 1000BASE-T1 to my knowledge. But looking at the definition of SQI register for 100BASE-T1 gives some good clues.
[0] reserved
[3:1] current SQI value
[4] reserved
[7:5] worst case SQI value since last read

So, the registers are implemented in a way that another LSB can be added.
This would increase the resolution of SQI measurement, not the range.
I.e. the resulting value range could be seen as [0, 0.5, 1, ..., 7.5] or [0, 1, ..., 15].
For the screenshot, I wouldn’t assume the reserved bit is implemented, it could just be that the SQI value was not shifted.

Regards,

Christian