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

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Message-ID: <Y2tgHYT+rV1+fxTm@linux-8mug>
Date:   Wed, 9 Nov 2022 16:09:01 +0800
From:   Chester Lin <clin@...e.com>
To:     Andrew Lunn <andrew@...n.ch>
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>,
        Chester Lin <clin@...e.com>
Subject: Re: [PATCH 2/5] dt-bindings: net: add schema for NXP S32CC dwmac
 glue driver

Hi Andrew,

On Fri, Nov 04, 2022 at 02:30:11PM +0100, Andrew Lunn wrote:
> > 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.
> 

Thanks for your correction.

> Does the clock need to change in order to support 2500Base-X? If i

Since I'm not a hardware designer from NXP and I can't find any board that
S32G2 CPUs could integrate SFP, so I am not able to tell how the clock could
be configured while supporting 2500Base-X.

> understand you correctly, Linux does not control the clocks, and so
> cannot change the clocks? So that probably means you cannot actually

To be more precise, the SCMI clock protocol in ATF [ARM Trusted Firmware]
doesn't design any interface to get/set parents of a clock so that re-parenting
clocks via the SCMI clk driver [clk-scmi.c] in Linux Kernel is impossible
for this case. That means, if any board design allows run-time swap on different
phys, the dedicated clks which represent each phy-mode are required in order
to trigger clock re-parenting in ATF since different phy connections would
need different clock sources.

> 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