netdev - Re: [PATCH 2/5] dt-bindings: net: add schema for NXP S32CC dwmac glue driver

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Date:   Fri, 4 Nov 2022 14:30:11 +0100
From:   Andrew Lunn <andrew@...n.ch>
To:     Chester Lin <clin@...e.com>
Cc:     Andreas Färber <afaerber@...e.de>,
        Rob Herring <robh@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Jan Petrous <jan.petrous@....com>, netdev@...r.kernel.org,
        s32@....com, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        Matthias Brugger <mbrugger@...e.com>
Subject: Re: [PATCH 2/5] dt-bindings: net: add schema for NXP S32CC dwmac
 glue driver

> Here I just focus on GMAC since there are other LAN interfaces that S32 family
> uses [e.g. PFE]. According to the public GMACSUBSYS ref manual rev2[1] provided
> on NXP website, theoretically GMAC can run SGMII in 1000Mbps and 2500Mbps so I
> assume that supporting 1000BASE-X could be achievable. I'm not sure if any S32
> board variant might have SFP ports but RJ-45 [1000BASE-T] should be the major
> type used on S32G-EVB and S32G-RDB2.

SGMII at 2500Mbps does not exist. Lots of people get this wrong. It
will be 2500Base-X.

Does the clock need to change in order to support 2500Base-X? If i
understand you correctly, Linux does not control the clocks, and so
cannot change the clocks? So that probably means you cannot actually
support 2500Base-X? Once you have Linux actually controlling the
hardware, you can then make use of an SFP or a copper PHY which
supports 2.5G. The PHY will swap its host side between SGMII and
2500Base-X depending on what the line side negotiates, 1000Base-T or
2500Base-T. The MAC driver then needs to change its configuration to
suite.

	Andrew