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

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Message-ID: <CAMuHMdWYFrx-CUapCQPX=hJtZBHycRTyN-dq8fXSXZwOiPJDMw@mail.gmail.com>
Date: Wed, 22 Nov 2023 08:40:54 +0100
From: Geert Uytterhoeven <geert@...ux-m68k.org>
To: Andrew Lunn <andrew@...n.ch>
Cc: Niklas Söderlund <niklas.soderlund+renesas@...natech.se>, 
	Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>, 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

Hi Andrew,

On Tue, Nov 21, 2023 at 8:54 PM Andrew Lunn <andrew@...n.ch> 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.
>
> Do you have a board that actually requires this?

We have several users of the corresponding properties for EtherAVB:

$ git grep -W "&.*avb" -- "*renesas*dts*" | grep internal-delay-ps
arch/arm64/boot/dts/renesas/beacon-renesom-som.dtsi-
rx-internal-delay-ps = <1800>;
arch/arm64/boot/dts/renesas/beacon-renesom-som.dtsi-
tx-internal-delay-ps = <2000>;
arch/arm64/boot/dts/renesas/hihope-rzg2-ex.dtsi- tx-internal-delay-ps = <2000>;
arch/arm64/boot/dts/renesas/hihope-rzg2-ex.dtsi- rx-internal-delay-ps = <1800>;
arch/arm64/boot/dts/renesas/r8a77970-eagle.dts- rx-internal-delay-ps = <1800>;
arch/arm64/boot/dts/renesas/r8a77970-eagle.dts- tx-internal-delay-ps = <2000>;
arch/arm64/boot/dts/renesas/r8a77970-v3msk.dts- rx-internal-delay-ps = <1800>;
arch/arm64/boot/dts/renesas/r8a77970-v3msk.dts- tx-internal-delay-ps = <2000>;
arch/arm64/boot/dts/renesas/r8a779a0-falcon.dts- tx-internal-delay-ps = <2000>;
arch/arm64/boot/dts/renesas/r8a779g0-white-hawk-cpu.dtsi-
tx-internal-delay-ps = <2000>;
arch/arm64/boot/dts/renesas/salvator-common.dtsi- tx-internal-delay-ps = <2000>;
arch/arm64/boot/dts/renesas/ulcb.dtsi- tx-internal-delay-ps = <2000>;

> In general, we try to have the PHY add the delay, not the MAC. So i
> would start with hard coding the delay to 0ns, and only implement
> these properties if you have a board where the PHY cannot add the
> delay.

If I understand the KSZ9031 bindings correctly, that PHY is limited to
a skew of up to 960 ps, not 1800 or 2000 ps.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds