netdev - Re: [PATCH net-next 1/9] ptp: Clarify ptp_clock_info .adjphase expects an internal servo to be used

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Message-ID: <ZF2No6gW3HlzZscV@hoboy.vegasvil.org>
Date: Thu, 11 May 2023 17:51:47 -0700
From: Richard Cochran <richardcochran@...il.com>
To: Rahul Rameshbabu <rrameshbabu@...dia.com>
Cc: netdev@...r.kernel.org, Saeed Mahameed <saeed@...nel.org>,
	Gal Pressman <gal@...dia.com>, Tariq Toukan <tariqt@...dia.com>,
	"David S. Miller" <davem@...emloft.net>,
	Jakub Kicinski <kuba@...nel.org>,
	Jacob Keller <jacob.e.keller@...el.com>
Subject: Re: [PATCH net-next 1/9] ptp: Clarify ptp_clock_info .adjphase
 expects an internal servo to be used

On Thu, May 11, 2023 at 01:20:45PM -0700, Rahul Rameshbabu wrote:

> If the PHC does not restore the frequency, won't the value cached in the
> ptp stack in the kernel become inaccurate compared to the frequency
> change induced by the '.adjphase' call?

If the HW implements a PI controller, and if it has converged, then
the current frequency will be close to the remote time server's.

> This concern is why I added this
> clause in the documentation. Let me know if my understanding is off with
> regards to this. I think we had a similar conversation on this
> previously in the mailing list.
> 
> https://lore.kernel.org/netdev/Y88L6EPtgvW4tSA+@hoboy.vegasvil.org/

I guess it depends on the HW algorithm and the situation.  But I don't
think there is a "rule" that always gets the best result.

Thanks,
Richard