netdev - Re: [PATCH v2 03/11] dt-bindings: arm: mediatek: add 'mediatek,pn

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Message-ID: <d789c9bd-82a6-5a63-cd98-b9e1e68daa2d@kernel.org>
Date:   Fri, 10 Feb 2023 13:35:02 +0100
From:   Krzysztof Kozlowski <krzk@...nel.org>
To:     "Russell King (Oracle)" <linux@...linux.org.uk>
Cc:     Daniel Golle <daniel@...rotopia.org>, netdev@...r.kernel.org,
        linux-mediatek@...ts.infradead.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Heiner Kallweit <hkallweit1@...il.com>,
        Lorenzo Bianconi <lorenzo@...nel.org>,
        Mark Lee <Mark-MC.Lee@...iatek.com>,
        John Crispin <john@...ozen.org>, Felix Fietkau <nbd@....name>,
        AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@...labora.com>,
        Matthias Brugger <matthias.bgg@...il.com>,
        DENG Qingfang <dqfext@...il.com>,
        Landen Chao <Landen.Chao@...iatek.com>,
        Sean Wang <sean.wang@...iatek.com>,
        Paolo Abeni <pabeni@...hat.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        "David S. Miller" <davem@...emloft.net>,
        Vladimir Oltean <olteanv@...il.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        Andrew Lunn <andrew@...n.ch>,
        Jianhui Zhao <zhaojh329@...il.com>,
        Bjørn Mork <bjorn@...k.no>
Subject: Re: [PATCH v2 03/11] dt-bindings: arm: mediatek: add
 'mediatek,pn_swap' property

On 10/02/2023 13:23, Russell King (Oracle) wrote:
> On Fri, Feb 10, 2023 at 10:34:17AM +0000, Russell King (Oracle) wrote:
>> On Thu, Feb 09, 2023 at 12:30:27PM +0100, Krzysztof Kozlowski wrote:
>>> On 08/02/2023 23:30, Daniel Golle wrote:
>>>> Hm, none of the current PCS (or PHY) drivers are represented by a
>>>> syscon node... (and maybe that's the mistake in first place?)
>>>
>>> Yes.
>>
>> Nos, it isn't.
> 
> To expand on this - I have no idea why you consider it a mistake that
> apparently all PCS aren't represented by a syscon node.
> 
> PCS is a sub-block in an ethernet system, just the same as a MAC is a
> sub-block. PCS can appear in several locations of an ethernet system,
> but are generally found either side of a serial ethernet link such
> as 1000base-X, SGMII, USXGMII, 10Gbase-R etc.
> 
> So, one can find PCS within an ethernet PHY - and there may be one
> facing the MAC connection, and there will be another facing the media.
> We generally do not need to separate these PCS from the PHY itself
> because we view the PHY as one whole device.
> 
> The optional PCS on the MAC side of the link is something that we
> need to know about, because this has to be configured to talk to the
> PHY, or to configure and obtain negotiation results from in the case of
> fibre links.
> 
> PCS on the MAC side are not a system level device, they are very much a
> specific piece of ethernet hardware in the same way that the MAC is,
> and we don't represent the MAC as a syscon node. There is no reason
> to do so with PCS.
> 
> These PCS on the MAC side tend to be accessed via direct MMIO accesses,
> or over a MDIO bus.
> 
> There's other blocks in the IEEE 802.3 ethernet layering, such as the
> PMA/PMD module (which for the MAC side we tend to model with the
> drivers/phy layer) - but again, these also appear in ethernet PHYs
> in order to produce the electrical signals for e.g. twisted pair
> ethernet.
> 
> So, to effectively state that you consider that PCS should always be
> represented as a syscon node is rather naieve, and really as a DT
> reviewer you should not be making such decisions, but soliciting
> opinions from those who know this subject area in detail _whether_
> they are some kind of system controller before making such a
> decision.

Daniel switched to private emails, so unfortunately our talk is not
visible here, nevertheless thanks for the feedback. Much appreciated!

Best regards,
Krzysztof