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

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Date:   Tue, 6 Oct 2020 17:01:02 +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 03:56:31PM +0200, Kurt Kanzenbach wrote:
> On Tue Oct 06 2020, Vladimir Oltean wrote:
> > 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.
> 
> Yeah, sure. That use case makes sense. What's the problem exactly?

The SO_TIMESTAMPING / SO_TIMESTAMPNS cmsg socket API simply doesn't have
any sort of identification for a hardware TX timestamp (where it came
from). So when you'll poll for TX timestamps, you'll receive a TX
timestamp from the PHY and another one from the switch, and those will
be in a race with one another, so you won't know which one is which.