netdev - Re: [PATCH 1/3] net: phy: add support to detect 100BASE-T1 capability

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Date:   Thu, 14 Dec 2017 11:30:59 +0100
From:   Lucas Stach <l.stach@...gutronix.de>
To:     Andrew Lunn <andrew@...n.ch>,
        Florian Fainelli <f.fainelli@...il.com>
Cc:     netdev@...r.kernel.org, kernel@...gutronix.de,
        patchwork-lst@...gutronix.de
Subject: Re: [PATCH 1/3] net: phy: add support to detect 100BASE-T1
 capability

Am Donnerstag, den 14.12.2017, 10:46 +0100 schrieb Andrew Lunn:
> > > Hi Lucas
> > > 
> > > Why did you decide to do this, and not add a SUPPORTED_100baseT1?
> > > 
> > > Could a device support both 100-BASE-T and 100-BASE-T1?  If at
> > > some
> > > point we need to differentiate between them, it is going to be
> > > hard. Especially since this is part of the kernel ABI.
> > 
> > Networking and especially PHY isn't really my primary area of
> > expertise, so excuse my ignorance. My reasoning was that we don't
> > differentiate between 100BASE-T2 and 100BASE-T4 in the kernel
> > today, so
> > I thought it was fine to handle T1 the same way.
> > 
> > There are PHYs that can both do regular 100/1000 MBit Ethernet and
> > 100BASE-T1, but definitely not at the same time or over the same
> > electrical wiring. 100BASE-T1 is really different in that it uses
> > capacitive coupling, instead of magnetic like on regular Ethernet.
> > So
> > it is really a board level decision what gets used and is not
> > something
> > I would expect to change at runtime.
> 
> Hi Lucus
> 
> http://www.marvell.com/docs/automotive/assets/marvell-automotive-ethe
> rnet-88Q5050-product-brief-2017-07.pdf
> 
> This is a Marvell 8-port switch. It appears it can switch some of its
> ports between T1, TX, xMII, GMII and SGMII.
> 
> So maybe an end device is fixed to 100BASE-T1, but it looks like
> switches could be more flexible.

If you need this for the configuration of the switch in userspace, then
yes I agree that we should be able to differentiate between TX and T1. 

I'll just note that even while you can switch the PHY mode it won't
make much sense at runtime, as you won't be able to connect T1 to a
switch port that has standard Ethernet magnetics at this PHY port.

> So i think we should be able to differentiate between T1 and TX.
> We might also need an PHY_INTERFACE_MODE_100BASE_T1.

At least the PHYs I've looked at expose regular RGMII or (R)MII to the
MAC.

Again, if you need this for switch configuration, I'm happy to add it.

Regards,
Lucas