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

On Wed Oct 07 2020, Vladimir Oltean wrote:

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
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.

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.

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.

Thanks,
Kurt