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

Hi Kamil,

On Thu, Oct 08, 2020 at 02:55:57PM +0200, Kamil Alkhouri wrote:

Hello dears,

On Thu, 2020-10-08 at 12:01 +0200, Kurt Kanzenbach wrote:

quoted

On Thu Oct 08 2020, Vladimir Oltean wrote:

quoted

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

quoted

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.

It doesn't has to be, it *should* be. That's at least the outcome we
had
after lots of discussions. Actually Kamil (on Cc) is the expert on
this
topic.

According to 802.1AS-2011 (10.1.1): "The LocalClock entity is a free-
running clock (see 3.3) that provides a common time to the time-aware
system, relative to an arbitrary epoch.", "... All timestamps are taken
relative to the LocalClock entity". The same statement holds true for
802.1AS-2020 (10.1.2.1).

Nice having you part of the discussion.

IEEE 802.1AS-rev draft 8.0, clause F.3 PTP options:

	The physical adjustment of the frequency of the LocalClock
	entity (i.e., physical syntonization) is allowed but not
	required.

In fact, even if that wasn't explicitly written, I am having a hard time
understanding how the "B.1.1 Frequency accuracy" requirement for the
LocalClock could be satisfied as long as it is kept free-running.
Otherwise said, what should I do as a system designer if the
LocalClock's frequency is not within +/- 100 ppm offset to the TAI
frequency, and I'm not allowed to correct it.

By the way, how would you see the split between an unsynchronized and a
synchronized PHC be implemented in the Linux kernel?

Thanks,
-Vladimir