RE: [PATCH v3 net-next 3/3] net: ena: Add PHC documentation

["Arinzon, David" <darinzon@amazon.com>]

Thank you Rahul for the detailed explanations

Hi Jakub,

> > Just wanted to clarify that this feature and the associated
> > documentation are specifically intended for reading a HW timestamp,
> > not for TX/RX packet timestamping.
> 
> Oh, so you're saying you can only read the clock from the device?
> The word timestamp means time associated with an event.
> 

Based on the documentation of gettimex64 API
The ts parameter holds the PHC timestamp.
We are using the same terminology
https://elixir.bootlin.com/linux/v6.11.6/source/include/linux/ptp_clock_kernel.h#L97

 * @gettimex64:  Reads the current time from the hardware clock and optionally
 *               also the system clock.
 *               parameter ts: Holds the PHC timestamp.
 *               parameter sts: If not NULL, it holds a pair of timestamps from
 *               the system clock. The first reading is made right before
 *               reading the lowest bits of the PHC timestamp and the second
 *               reading immediately follows that.

> In the doc you talk about:
> 
> > +PHC support and capabilities can be verified using ethtool:
> > +
> > +.. code-block:: shell
> > +
> > +  ethtool -T <interface>
> 
> which is for packet timestamping
> 

ethtool -T shows all timestamping capabilities, which indeed include
packet timestamping but also the PTP Hardware Clock (PHC) index
If the value is `none`, it means that there's no PHC support
This is done by implementing the `get_ts_info` hook, which is
part of this patchset.

https://elixir.bootlin.com/linux/v6.11.6/source/include/linux/ethtool.h#L720

> also:
> 
> > ENA Linux driver supports PTP hardware clock providing timestamp
> > reference to achieve nanosecond accuracy.
> 
> You probably want to double check the definitions of accuracy and
> resolution.
> 

Thank you, will be changed in the next patchset

> We recently merged an Amazon PTP clock driver from David Woodhouse,
> see commit 20503272422693. If you're not timestamping packets why not use
> that driver?

The AMZNC10C vmclock device driver is intended to be used in systems where there's an hypervisor.
The PHC driver in this patchset is intended for virtual and non-virtual (metal) instances in AWS.
The AMZNC10C might not be available in the future on the same instances where PHC is available.