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Date:   Sat, 5 Jun 2021 01:18:54 +0200
From:   Pali =?utf-8?B?Um9ow6Fy?= <pali@kernel.org>
To:     "Russell King (Oracle)" <linux@armlinux.org.uk>
Cc:     Madalin Bucur <madalin.bucur@nxp.com>,
        Andrew Lunn <andrew@lunn.ch>,
        Igal Liberman <Igal.Liberman@freescale.com>,
        Shruti Kanetkar <Shruti@freescale.com>,
        Emil Medve <Emilian.Medve@freescale.com>,
        Scott Wood <oss@buserror.net>,
        Rob Herring <robh+dt@kernel.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        "netdev@vger.kernel.org" <netdev@vger.kernel.org>,
        "devicetree@vger.kernel.org" <devicetree@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "Camelia Alexandra Groza (OSS)" <camelia.groza@oss.nxp.com>
Subject: Re: Unsupported phy-connection-type sgmii-2500 in
 arch/powerpc/boot/dts/fsl/t1023rdb.dts
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On Friday 04 June 2021 20:27:33 Russell King (Oracle) wrote:
> 802.3 defined 1000base-X which is a fixed 1G speed interface using a
> 16-bit control word. Implementations of this exist where the control
> word can be disabled.
> 
> Cisco came along, took 1000base-X and augmented it to allow speeds of
> 10M and 100M by symbol repetition, and changing the format of the
> 16-bit control word. Otherwise, it is functionally compatible - indeed
> SGMII with the control word disabled will connect with 1000base-X with
> the control word disabled. I've done it several times.
> 
> There exists 2500base-X, which is 1000base-X clocked faster, and it
> seems the concensus is that it has the AN disabled - in other words,
> no control word.

Thank you for a nice explanation! I think that this information should
be part of documentation as it could help also other people to
understand differences between these modes.