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

On Thu, Oct 08, 2020 at 10:34:11AM +0200, Kurt Kanzenbach wrote:

On Wed Oct 07 2020, Vladimir Oltean wrote:

quoted

On Wed, Oct 07, 2020 at 12:39:49PM +0200, Kurt Kanzenbach wrote:

quoted

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.

Just out of curiosity: How do you implement 802.1AS then? My
understanding is that the free-running clock has to be used for the

Has to be? I couldn't find that wording in IEEE 802.1AS-2011.

calculation of the peer delays and such meaning there should be a way to
get access to both PHCs or having some form of cross timestamping
available.

The hellcreek switch can take cross snapshots of all three ptp clocks in
hardware for that purpose.

Well, at the end of the day, all the other TSN offloads (tc-taprio,
tc-gate) will still have to use the synchronized PTP clock, so what
we're doing is we're simply letting that clock be synchronized by ptp4l.

quoted

quoted

quoted

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.

Hmm? If DSA doesn't support PHY timestamping how are other DSA drivers
dealing with it then? I'm getting really confused.

They aren't dealing with it, of course.

Furthermore, there is no hellcreek hardware available with timestamping
capable PHYs. How am I supposed to even test this?

For now, until there is hardware available, PHY timestamping is not
supported with the hellcreek switch.

I was just pointing out that this is something you'll certainly have to
change if somebody will want PHY timestamping.

Even without hardware, you _could_ probably test that DSA is doing the
right thing by simply adding the PTP timestamping ops to a PHY driver
that you own, and inject dummy timestamps. The expectation becomes that
user space gets those dummy timestamps, and not the ones emitted by your
switch.