netdev - Re: [PATCH 3/4] net: dsa: microchip: Disable RGMII in-band status on KSZ9893

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Message-ID: <20200905160647.GJ3164319@lunn.ch>
Date:   Sat, 5 Sep 2020 18:06:47 +0200
From:   Andrew Lunn <andrew@...n.ch>
To:     Paul Barker <pbarker@...sulko.com>
Cc:     Woojung Huh <woojung.huh@...rochip.com>,
        Microchip Linux Driver Support <UNGLinuxDriver@...rochip.com>,
        Vivien Didelot <vivien.didelot@...il.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        "David S . Miller" <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: [PATCH 3/4] net: dsa: microchip: Disable RGMII in-band status on
 KSZ9893

On Sat, Sep 05, 2020 at 04:53:20PM +0100, Paul Barker wrote:
> On Sat, 5 Sep 2020 at 16:32, Andrew Lunn <andrew@...n.ch> wrote:
> >
> > On Sat, Sep 05, 2020 at 03:03:24PM +0100, Paul Barker wrote:
> > > We can't assume that the link partner supports the in-band status
> > > reporting which is enabled by default on the KSZ9893 when using RGMII
> > > for the upstream port.
> >
> > What do you mean by RGMII inband status reporting? SGMII/1000BaseX has
> > in band signalling, but RGMII?
> >
> >    Andrew
> 
> I'm referencing page 56 of the KSZ9893 datasheet
> (http://ww1.microchip.com/downloads/en/DeviceDoc/KSZ9893R-Data-Sheet-DS00002420D.pdf).
> The datasheet says "The RGMII port will not function properly if IBS
> is enabled in the switch, but it is not receiving in-band status from
> a connected PHY." Since we can't guarantee all possible link partners
> will support this it should be disabled. In particular, the IMX6 SoC
> we're using with this switch doesn't support this on its Ethernet
> port.
> 
> I don't really know much about how this is implemented or how widely
> it's supported.

I never knew RGMII had this. What i did find was:

http://web.archive.org/web/20160303171328/http://www.hp.com/rnd/pdfs/RGMIIv2_0_final_hp.pdf

which does talk about it, section 3.4.1. It is clearly optional, and
since this is the first time i've heard of it, i suspect not many
systems actually implement it. So turning it off seems wise.

Reviewed-by: Andrew Lunn <andrew@...n.ch>

    Andrew