netdev - Re: [PATCH net-next v7 15/16] net: ethtool: ts: Let the active time stamping layer be selectable

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Date: Mon, 20 Nov 2023 19:39:35 +0100
From: Andrew Lunn <andrew@...n.ch>
To: Jakub Kicinski <kuba@...nel.org>
Cc: Vladimir Oltean <vladimir.oltean@....com>,
	Köry Maincent <kory.maincent@...tlin.com>,
	Florian Fainelli <florian.fainelli@...adcom.com>,
	Broadcom internal kernel review list <bcm-kernel-feedback-list@...adcom.com>,
	Heiner Kallweit <hkallweit1@...il.com>,
	Russell King <linux@...linux.org.uk>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>, Paolo Abeni <pabeni@...hat.com>,
	Richard Cochran <richardcochran@...il.com>,
	Radu Pirea <radu-nicolae.pirea@....nxp.com>,
	Jay Vosburgh <j.vosburgh@...il.com>,
	Andy Gospodarek <andy@...yhouse.net>,
	Nicolas Ferre <nicolas.ferre@...rochip.com>,
	Claudiu Beznea <claudiu.beznea@...on.dev>,
	Willem de Bruijn <willemdebruijn.kernel@...il.com>,
	Jonathan Corbet <corbet@....net>,
	Horatiu Vultur <horatiu.vultur@...rochip.com>,
	UNGLinuxDriver@...rochip.com, Simon Horman <horms@...nel.org>,
	Thomas Petazzoni <thomas.petazzoni@...tlin.com>,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-doc@...r.kernel.org,
	Maxime Chevallier <maxime.chevallier@...tlin.com>
Subject: Re: [PATCH net-next v7 15/16] net: ethtool: ts: Let the active time
 stamping layer be selectable

> What about my use case of having a NIC which can stamp at a low rate
> at the PHY (for PTP) and high rate at the DMA block (for congestion
> control)? Both stamp points have the same PHC index.

How theoretical is that? To me, it seems more likely you have two PHC.

The PHY stamping tends to be slow because of the MDIO bus. If the MAC
has fast access to the PHC, it means its not on the MDIO bus. It
probably means you have the PHY integrated into the MAC/SoC, so the
MAC can access it. But if its integrated like this, i don't see why
PHY stamping should be particularly slow. So you can probably use it
for congestion control. And then you don't need DMA stamping.

Do you know of real hardware with a MAC and a PHY sharing a PHC?

   Andrew