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Date:   Mon, 20 Nov 2023 19:09:55 +0000
From:   "Russell King (Oracle)" <linux@armlinux.org.uk>
To:     Andrew Lunn <andrew@lunn.ch>
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Subject: Re: [PATCH net-next v7 15/16] net: ethtool: ts: Let the active time
 stamping layer be selectable
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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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