linux-kernel - Re: [RFC PATCH 5/8] dt-bindings: net: pcs: add bindings for MediaTek USXGMII PCS

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Message-ID: <ZWS6Hl2tZ0MPj+OL@shell.armlinux.org.uk>
Date:   Mon, 27 Nov 2023 15:47:42 +0000
From:   "Russell King (Oracle)" <linux@...linux.org.uk>
To:     Daniel Golle <daniel@...rotopia.org>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Conor Dooley <conor+dt@...nel.org>,
        Chunfeng Yun <chunfeng.yun@...iatek.com>,
        Vinod Koul <vkoul@...nel.org>,
        Kishon Vijay Abraham I <kishon@...nel.org>,
        Felix Fietkau <nbd@....name>, John Crispin <john@...ozen.org>,
        Sean Wang <sean.wang@...iatek.com>,
        Mark Lee <Mark-MC.Lee@...iatek.com>,
        Lorenzo Bianconi <lorenzo@...nel.org>,
        Matthias Brugger <matthias.bgg@...il.com>,
        AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@...labora.com>,
        Andrew Lunn <andrew@...n.ch>,
        Heiner Kallweit <hkallweit1@...il.com>,
        Alexander Couzens <lynxis@...0.eu>,
        Philipp Zabel <p.zabel@...gutronix.de>, netdev@...r.kernel.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-mediatek@...ts.infradead.org, linux-phy@...ts.infradead.org
Subject: Re: [RFC PATCH 5/8] dt-bindings: net: pcs: add bindings for MediaTek
 USXGMII PCS

On Thu, Nov 09, 2023 at 09:51:47PM +0000, Daniel Golle wrote:
> MediaTek's USXGMII can be found in the MT7988 SoC. We need to access
> it in order to configure and monitor the Ethernet SerDes link in
> USXGMII, 10GBase-R and 5GBase-R mode. By including a wrapped
> legacy 1000Base-X/2500Base-X/Cisco SGMII LynxI PCS as well, those
> interface modes are also available.

I think this binding is based on the implementation than on hardware.

What I believe you have is this setup:

        .---- LynxI PCS ----.
MAC ---+                     +--- PEXTP --- world
        `--- USXGMII PCS ---'

You are representing the PEXTP as a separate entity in DT, but then
you're representing the LynxI PCS and the USXGMII PCS as a single
block, which seems to be how you've decided to implement it.

Given that the LynxI PCS is already in use elsewhere in the Mediatek
range, I suggest that the LynxI PCS is one block of IP, and the USXGMII
PCS is a separate block of IP.

1) Would it not be better to model the two PCS seperately?

2) The addition of the SGMII reset needs more information - is this
   controlling a reset for the LynxI block? If so, it should be part
   of a LynxI PCS binding.

3) The PEXTP is presumably a separate block which can be shared between
   several devices - for example, the LynxI, USXGMII, and probably SATA
   and PCIe as well. From the 802.3's network model, the PEXTP is the
   PMA/PMD.

   From the point of view of 802.3's model, a network interface has
   various layers such as the MAC, PCS and PMA/PMD, and sitting above
   these layers is the management of the system. Rather than chasing
   the data flow (which in a network device can be complex) wouldn't
   it be better to continue with the 802.3 model as we are doing with
   other devices, rather than trying to go with a new approach here?

-- 
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