netdev - Re: [PATCH net-next 1/4] dt-bindings: net: ethernet-controller: update descriptions of RGMII modes

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Message-ID: <aAaafd8LZ3Ks-AoT@shell.armlinux.org.uk>
Date: Mon, 21 Apr 2025 20:20:29 +0100
From: "Russell King (Oracle)" <linux@...linux.org.uk>
To: Matthias Schiffer <matthias.schiffer@...tq-group.com>
Cc: Andrew Lunn <andrew+netdev@...n.ch>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Rob Herring <robh@...nel.org>,
	Krzysztof Kozlowski <krzk+dt@...nel.org>,
	Conor Dooley <conor+dt@...nel.org>,
	Andy Whitcroft <apw@...onical.com>,
	Dwaipayan Ray <dwaipayanray1@...il.com>,
	Lukas Bulwahn <lukas.bulwahn@...il.com>,
	Joe Perches <joe@...ches.com>, Jonathan Corbet <corbet@....net>,
	Nishanth Menon <nm@...com>, Vignesh Raghavendra <vigneshr@...com>,
	Siddharth Vadapalli <s-vadapalli@...com>,
	Roger Quadros <rogerq@...nel.org>, Tero Kristo <kristo@...nel.org>,
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
	netdev@...r.kernel.org, devicetree@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, linux@...tq-group.com
Subject: Re: [PATCH net-next 1/4] dt-bindings: net: ethernet-controller:
 update descriptions of RGMII modes

On Tue, Apr 15, 2025 at 12:18:01PM +0200, Matthias Schiffer wrote:
> diff --git a/Documentation/devicetree/bindings/net/ethernet-controller.yaml b/Documentation/devicetree/bindings/net/ethernet-controller.yaml
> index 45819b2358002..2ddc1ce2439a6 100644
> --- a/Documentation/devicetree/bindings/net/ethernet-controller.yaml
> +++ b/Documentation/devicetree/bindings/net/ethernet-controller.yaml
> @@ -74,19 +74,21 @@ properties:
>        - rev-rmii
>        - moca
>  
> -      # RX and TX delays are added by the MAC when required
> +      # RX and TX delays are part of the board design (through PCB traces). MAC
> +      # and PHY must not add delays.
>        - rgmii
>  
> -      # RGMII with internal RX and TX delays provided by the PHY,
> -      # the MAC should not add the RX or TX delays in this case
> +      # RGMII with internal RX and TX delays provided by the MAC or PHY. No
> +      # delays are included in the board design; this is the most common case
> +      # in modern designs.
>        - rgmii-id
>  
> -      # RGMII with internal RX delay provided by the PHY, the MAC
> -      # should not add an RX delay in this case
> +      # RGMII with internal RX delay provided by the MAC or PHY. TX delay is
> +      # part of the board design.
>        - rgmii-rxid
>  
> -      # RGMII with internal TX delay provided by the PHY, the MAC
> -      # should not add an TX delay in this case
> +      # RGMII with internal TX delay provided by the MAC or PHY. RX delay is
> +      # part of the board design.
>        - rgmii-txid
>        - rtbi
>        - smii

Sorry, but I don't think this wording improves the situation - in fact,
I think it makes the whole thing way more confusing.

Scenario 1: I'm a network device driver author. I read the above, Okay,
I have a RGMII interface, but I need delays to be added. I'll detect
when RGMII-ID is used, and cause the MAC driver to add the delays, but
still pass PHY_INTERFACE_MODE_RGMII_ID to phylib.

Scenario 2: I'm writing a DT file for a board. Hmm, so if I specify
rgmii because the delays are implemented in the traces, but I need to
fine-tune them. However, the documentation says that delays must not
be added by the MAC or the PHY so how do I adjust them. I know, I'll
use rgmii-id because that allows delays!

I suspect neither of these two are really want you mean, but given
this wording, that's exactly where it leads - which is more
confusion and less proper understanding.

-- 
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