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Message-ID: <20201007105458.gdbrwyzfjfaygjke@skbuf>
Date: Wed, 7 Oct 2020 13:54:58 +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 Wed, Oct 07, 2020 at 12:39:49PM +0200, Kurt Kanzenbach wrote:
> On Tue Oct 06 2020, Vladimir Oltean wrote:
> > On Tue, Oct 06, 2020 at 03:56:31PM +0200, Kurt Kanzenbach wrote:
> >> 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).
>
> This is sounds like a problem.
Yeah, tell me about it.
> For instance the hellcreek switch has actually three ptp hardware
> clocks and the time stamping can be configured to use either one of
> them.
The sja1105 also has a corrected and an uncorrected PTP clock that can
take timestamps. Initially I had thought I'd be going to spend some time
figuring out multi-PHC support, but now I don't see any practical reason
to use the uncorrected PHC for anything.
> How would the user space distinguish what time stamp is taken by
> which clock? This is not a problem at the moment, because currently
> only the synchronized clock is exported to user space. See change log
> of this patch.
It wouldn't, of course. You'd need to add the plumbing for that.
> > 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.
>
> OK. So what happens if the driver will accept to disable hardware
> timestamping? Is there anything else that needs to be implemented? Are
> there (good) examples?
It needs to not call skb_complete_tx_timestamp() and friends.
For PHY timestamping, it also needs to invoke the correct methods for RX
and for TX, where the PHY timestamping hooks will get called. I don't
think that DSA is compatible yet with PHY timestamping, but it is
probably a trivial modification. Please read
Documentation/networking/timestamping.rst, we try to keep it fairly
comprehensive.
Thanks,
-Vladimir
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