On Mon, Mar 07, 2022 at 06:54:19PM +0100, Gerhard Engleder wrote:

ktime_to_cycles uses hwtstamp as key for the cache lookup. As long as
the PHC is monotonic, the key is unique. If the time of the PHC is set, then
the cache would be invalidated. I'm afraid that setting the PHC could lead to
wrong or missing timestamps.

"Wrong" timestamps happen with normal PHCs anyhow.  Not a big deal.
Your driver can drop the cache and force racing vclock frames to omit
the Rx timestamp.  User space must deal with this in any case.

For RX both timestamps are already available within skbuff, because they are
stored in front of the Ethernet header by the hardware. So I have to find a way
to detect the RX case and copy the right timestamp to hwtstamp.

This works for your driver, but not for the generic vclock.  Maybe you
could let the ptp_convert_timestamp() method pass the skb back into your
driver.

Thanks,
Richard