netdev - Re: [PATCH devicetree v3] dt-bindings: phy: Add `tx-p2p-microvolt` property binding

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Date:   Thu, 20 Jan 2022 20:50:51 +0100
From:   Marc Kleine-Budde <mkl@...gutronix.de>
To:     Marek Behún <kabel@...nel.org>
Cc:     Rob Herring <robh+dt@...nel.org>, devicetree@...r.kernel.org,
        Vladimir Oltean <olteanv@...il.com>,
        Holger Brunck <holger.brunck@...achienergy.com>,
        Andrew Lunn <andrew@...n.ch>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Russell King <rmk+kernel@...linux.org.uk>,
        linux-phy@...ts.infradead.org, Vinod Koul <vkoul@...nel.org>,
        Kishon Vijay Abraham I <kishon@...com>, ore@...gutronix.de,
        alexandru.tachici@...log.com
Subject: Re: [PATCH devicetree v3] dt-bindings: phy: Add `tx-p2p-microvolt`
 property binding

On 20.01.2022 19:01:55, Marek Behún wrote:
> On Thu, 20 Jan 2022 09:49:14 +0100
> Marc Kleine-Budde <mkl@...gutronix.de> wrote:
> 
> > On 19.01.2022 14:11:17, Marek Behún wrote:
> > > Common PHYs and network PCSes often have the possibility to specify
> > > peak-to-peak voltage on the differential pair - the default voltage
> > > sometimes needs to be changed for a particular board.
> > > 
> > > Add properties `tx-p2p-microvolt` and `tx-p2p-microvolt-names` for this
> > > purpose. The second property is needed to specify the mode for the
> > > corresponding voltage in the `tx-p2p-microvolt` property, if the voltage
> > > is to be used only for speficic mode. More voltage-mode pairs can be
> > > specified.
> > > 
> > > Example usage with only one voltage (it will be used for all supported
> > > PHY modes, the `tx-p2p-microvolt-names` property is not needed in this
> > > case):
> > > 
> > >   tx-p2p-microvolt = <915000>;
> > > 
> > > Example usage with voltages for multiple modes:
> > > 
> > >   tx-p2p-microvolt = <915000>, <1100000>, <1200000>;
> > >   tx-p2p-microvolt-names = "2500base-x", "usb", "pcie";
> > > 
> > > Add these properties into a separate file phy/transmit-amplitude.yaml,
> > > which should be referenced by any binding that uses it.  
> > 
> > If I understand your use-case correctly, you need different voltage p2p
> > levels in the connection between the Ethernet MAC and the Ethernet
> > switch or Ethernet-PHY?
> 
> This is a SerDes differential pair amplitude. So yes to your question,
> if the MII interface uses differential pair, like sgmii, 10gbase-r, ...
> 
> > Some of the two wire Ethernet standards (10base-T1S, 10base-T1L,
> > 100base-T1, 1000base-T1) defines several p2p voltage levels on the wire,
> > i.e. between the PHYs. Alexandru has posed a series where you can
> > specify the between-PHY voltage levels:
> > 
> > | https://lore.kernel.org/all/20211210110509.20970-8-alexandru.tachici@analog.com/
> 
> Copper ethernet is something different, so no conflict
> 
> > Can we make clear that your binding specifies the voltage level on the
> > MII interface, in contrast Alexandru's binding?
> 
> The binding explicitly says "common PHY", not ethernet PHY. I don't
> thing there will be any confusion. It can also be specified for USB3+
> differential pairs, or PCIe differential pairs, or DisplayPort
> differential pairs...

Thanks for the clarification.

regards,
Marc

-- 
Pengutronix e.K.                 | Marc Kleine-Budde           |
Embedded Linux                   | https://www.pengutronix.de  |
Vertretung West/Dortmund         | Phone: +49-231-2826-924     |
Amtsgericht Hildesheim, HRA 2686 | Fax:   +49-5121-206917-5555 |

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