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

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Message-ID: <ZVuvA5V6IWa4fIs3@shell.armlinux.org.uk>
Date:   Mon, 20 Nov 2023 19:09:55 +0000
From:   "Russell King (Oracle)" <linux@...linux.org.uk>
To:     Andrew Lunn <andrew@...n.ch>
Cc:     Jakub Kicinski <kuba@...nel.org>,
        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>,
        "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

On Mon, Nov 20, 2023 at 07:39:35PM +0100, Andrew Lunn wrote:
> > 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?

Not so much a MAC and PHY sharing a PHC, but I notice in USXGMII-M,
there is the ability for the PHY to pass the timestamp back to the MAC
through the USXGMII-M connection - which would eliminate the problems
of accessing the PHY to get that the timestamps.

-- 
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