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: <87ttwizkki.fsf@nvidia.com>
Date: Thu, 11 May 2023 13:20:45 -0700
From: Rahul Rameshbabu <rrameshbabu@...dia.com>
To: Richard Cochran <richardcochran@...il.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 Wed, 10 May, 2023 19:09:40 -0700 Richard Cochran <richardcochran@...il.com> wrote:
> On Wed, May 10, 2023 at 01:52:58PM -0700, Rahul Rameshbabu wrote:
>> +PTP hardware clock requirements for '.adjphase'
>> +-----------------------------------------------
>> +
>> +   The 'struct ptp_clock_info' interface has a '.adjphase' function.
>> +   This function has a set of requirements from the PHC in order to be
>> +   implemented.
>> +
>> +     * The PHC implements a servo algorithm internally that is used to
>> +       correct the offset passed in the '.adjphase' call.
>> +     * When other PTP adjustment functions are called, the PHC servo
>> +       algorithm is disabled,
>
> I disagree with this part:
>
>> and the frequency prior to the '.adjphase'
>> +       call is restored internally in the PHC.
>
> That seems like an arbitrary rule that doesn't make much sense.

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? 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/

>
> Thanks,
> Richard

-- Rahul Rameshbabu