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

From: Vladimir Oltean <olteanv@gmail.com>
To: Kurt Kanzenbach <kurt@linutronix.de>
Cc: Andrew Lunn <andrew@lunn.ch>,
	Vivien Didelot <vivien.didelot@gmail.com>,
	Florian Fainelli <f.fainelli@gmail.com>,
	"David S. Miller" <davem@davemloft.net>,
	Jakub Kicinski <kuba@kernel.org>,
	netdev@vger.kernel.org, Rob Herring <robh+dt@kernel.org>,
	devicetree@vger.kernel.org,
	Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
	Richard Cochran <richardcochran@gmail.com>,
	Kamil Alkhouri <kamil.alkhouri@hs-offenburg.de>,
	ilias.apalodimas@linaro.org
Subject: Re: [PATCH net-next v6 4/7] net: dsa: hellcreek: Add support for hardware timestamping
Date: Wed, 7 Oct 2020 13:54:58 +0300	[thread overview]
Message-ID: <20201007105458.gdbrwyzfjfaygjke@skbuf> (raw)
In-Reply-To: <87lfgiqpze.fsf@kurt>

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