netdev - Re: [PATCH net-next RFC v4 2/5] net: Expose available time stamping layers to user space.

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Message-ID: <ZPmYAwb0Gz++XCAA@shell.armlinux.org.uk>
Date: Thu, 7 Sep 2023 10:29:39 +0100
From: "Russell King (Oracle)" <linux@...linux.org.uk>
To: Andrew Lunn <andrew@...n.ch>
Cc: Jakub Kicinski <kuba@...nel.org>,
	Richard Cochran <richardcochran@...il.com>,
	Köry Maincent <kory.maincent@...tlin.com>,
	Vladimir Oltean <vladimir.oltean@....com>, netdev@...r.kernel.org,
	glipus@...il.com, maxime.chevallier@...tlin.com,
	vadim.fedorenko@...ux.dev, gerhard@...leder-embedded.com,
	thomas.petazzoni@...tlin.com, krzysztof.kozlowski+dt@...aro.org,
	robh+dt@...nel.org
Subject: Re: [PATCH net-next RFC v4 2/5] net: Expose available time stamping
 layers to user space.

On Tue, Sep 05, 2023 at 10:29:51PM +0200, Andrew Lunn wrote:
> > Maybe we should try to enumerate the use cases, I don't remember now
> > but I think the concern was that there may be multiple PHYs?
> 
> You often see a Marvell 10G PHY between a MAC and an SFP cage. You can
> then get a copper SFP module which has a PHY in it.
> 
> So:
> 
> "Linux" NIC: [DMA MAC][PHY][PHY] 

Let's be clear - one of the reasons that this whole topic was triggered
was because of mvpp2 plus Marvell 1G PHYs. mvpp2 has a good PTP
implementation, where as Marvell 1G PHYs are not quite as good. With
the code as it stood, merely adding PTP support to Marvell PHYs would
result in setups that use a Marvell 1G PHY with mvpp2 to break - some
PTP API calls would end up going to one PTP implementation while other
PTP API calls end up going to the other.

Once this gets solved properly, then I can think about sending the
patches that add support for PTP in Marvell 1G PHYs, and then we will
have the situation where we have a MAC and a PHY on the *same* network
interface that are PTP capable.

People have been asking me about the Marvell PHY PTP patches - and I
have had to tell them that they can't be merged because of the PTP
API crazyness.

So... it would be entirely possible, if PTP were to be implemented for
the Marvell 10G PHYs, that there would be _three_ points with a SFP
module to do PTP, although it probably does not make much sense to
attempt to do so on the SFP PHY. In any case, before we get to that
point, we first need to work out how to support multiple ethernet PHYs
on one MAC.

Even with that solved, the situation you describe above is likely to be
problematical. PHYs that connect to SFPs generally only support fibre
interface modes and not SGMII on that port which limits the usefulness
of copper SFPs - they won't be able to do 10M / 100M unless they're one
of those PHYs that does rate adaption which seem fairly rare at the
moment.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
FTTP is here! 80Mbps down 10Mbps up. Decent connectivity at last!