netdev - Re: [PATCH net-next] net: ethtool: allow MAC drivers to override ethtool get_ts

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Message-ID: <20210114125506.GC3154@hoboy.vegasvil.org>
Date:   Thu, 14 Jan 2021 04:55:06 -0800
From:   Richard Cochran <richardcochran@...il.com>
To:     Russell King <rmk+kernel@...linux.org.uk>
Cc:     Andrew Lunn <andrew@...n.ch>,
        Heiner Kallweit <hkallweit1@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>, netdev@...r.kernel.org
Subject: Re: [PATCH net-next] net: ethtool: allow MAC drivers to override
 ethtool get_ts_info

On Sun, Jan 10, 2021 at 11:13:44AM +0000, Russell King wrote:
> This allows network drivers such as mvpp2 to use their more accurate
> timestamping implementation than using a less accurate implementation
> in the PHY. Network drivers can opt to defer to phylib by returning
> -EOPNOTSUPP.

My expectation is that PHY time stamping is more accurate than MAC
time stamping.
 
> This change will be needed if the Marvell PHY drivers add support for
> PTP.

Huh?  The mvpp2 appears to be a MAC.  If this device has integrated
PHYs then I don't see the issue.  If your board has the nvpp2 device
with the dp83640 PHYTER, then don't you want to actually use the
PHYTER?

>From my observation of the product offerings, I have yet to see a new
PHY (besides the dp83640 PHYTER) that implement time stamping.  The
PHYTER is 100 megabit only, and my understanding that it is too
difficult or even impossible to provide time stamps from within a
gigabit+ PHY.  So you needn't fear new time stamping PHYs to spoil
your setup!

> Note: this may cause a change for any drivers that use phylib and
> provide get_ts_info(). It is not obvious if any such cases exist.

Up until now, the code always favored PHY devices and devices external
to the MAC that snoop on the MII bus.  The assumption is that anyone
who builds a board with such specialty devices really wants to use
them.

Thanks,
Richard