netdev - Re: [PATCH] dt-bindings: net: renesas,ethertsn: Add bindings for Ethernet TSN

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Message-ID: <ZVymJF2dd7XssGaQ@oden.dyn.berto.se>
Date: Tue, 21 Nov 2023 13:44:20 +0100
From: Niklas Söderlund <niklas.soderlund+renesas@...natech.se>
To: Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
Cc: Rob Herring <robh+dt@...nel.org>,
	Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
	devicetree@...r.kernel.org, netdev@...r.kernel.org,
	linux-renesas-soc@...r.kernel.org
Subject: Re: [PATCH] dt-bindings: net: renesas,ethertsn: Add bindings for
 Ethernet TSN

Hello Krzysztof,

On 2023-11-21 13:20:54 +0100, Krzysztof Kozlowski wrote:
> On 21/11/2023 13:10, Niklas Söderlund wrote:
> >>> +
> >>> +  renesas,rx-internal-delay:
> >>> +    type: boolean
> >>> +    description:
> >>> +      Enable internal Rx clock delay, typically 1.8ns.
> >>
> >> Why this is bool, not delay in ns?
> > 
> > The TSN is only capable of enabling or disable internal delays, not set 
> > how long the delay is. The documentation states that the delay depends 
> > on the electronic characteristics of the particular board, but states 
> > that they typically are 1.8ns for Rx and 2.0ns for Tx.
> 
> I don't understand that part. If you cannot configure the internal
> delay, how could it depend on the board characteristics?

Each of these two properties reflect a single bit in the device 
configuration space. If the bit is set the {Rx,Tx} delay mode is active 
or disabled. The documentation for the bit simply states,

    Tx clock internal Delay Mode

    This bit can add internal Tx clock delay typ 2.0ns*.

    *Refer to Electrical Characteristics for details.

Same paragraph for Rx but a typical 1.8ns delay.

> 
> > 
> > I looked at the generic properties {rx,tx}-internal-delay-ps but they 
> > are of int type. So I opted for a vendor specific bool property. Do you 
> > think a better route is to use the generic property and force the value 
> > to be either 0 or the typical delay?
> > 
> >> Why this is property of a board (not SoC)?
> > 
> > I'm sorry I don't understand this question.
> 
> Why setting internal delay is specific to a board, not to a SoC? Why
> each board would need to configure it? On which parts of hardware on the
> board does this depend?

Ahh, I think I understand. It is per board as I understand the 
documentation. It depends on the electrical characteristics of the 
board.

Maybe updating the description would help, how about?

    Enable internal Rx clock delay. Typically 1.8ns but depends on 
    electrical characteristics of the board.

-- 
Kind Regards,
Niklas Söderlund