netdev - Re: [PATCH net-next v6 4/7] net: dsa: hellcreek: Add support for hardware timestamping

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Message-ID: <20201006133222.74w3r2jwwhq5uop5@skbuf>
Date:   Tue, 6 Oct 2020 16:32:22 +0300
From:   Vladimir Oltean <olteanv@...il.com>
To:     Kurt Kanzenbach <kurt@...utronix.de>
Cc:     Andrew Lunn <andrew@...n.ch>,
        Vivien Didelot <vivien.didelot@...il.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>, netdev@...r.kernel.org,
        Rob Herring <robh+dt@...nel.org>, devicetree@...r.kernel.org,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Richard Cochran <richardcochran@...il.com>,
        Kamil Alkhouri <kamil.alkhouri@...offenburg.de>,
        ilias.apalodimas@...aro.org
Subject: Re: [PATCH net-next v6 4/7] net: dsa: hellcreek: Add support for
 hardware timestamping

On Tue, Oct 06, 2020 at 03:30:36PM +0200, Kurt Kanzenbach wrote:
> That's the point. The user (or anybody else) cannot disable hardware
> stamping, because it is always performed. So, why should it be allowed
> to disable it even when it cannot be disabled?

Because your driver's user can attach a PTP PHY to your switch port, and
the network stack doesn't support multiple TX timestamps attached to the
same skb. They'll want the TX timestamp from the PHY and not from your
switch.