netdev - Re: [net-next 03/15] net/mlx5: Add adjphase function to support hardware-only offset control

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Date:   Sun, 22 Jan 2023 18:22:58 -0800
From:   Richard Cochran <richardcochran@...il.com>
To:     Rahul Rameshbabu <rrameshbabu@...dia.com>
Cc:     Jakub Kicinski <kuba@...nel.org>,
        Jacob Keller <jacob.e.keller@...el.com>,
        Saeed Mahameed <saeed@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Paolo Abeni <pabeni@...hat.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Saeed Mahameed <saeedm@...dia.com>, netdev@...r.kernel.org,
        Tariq Toukan <tariqt@...dia.com>,
        Gal Pressman <gal@...dia.com>,
        Vincent Cheng <vincent.cheng.xh@...esas.com>
Subject: Re: [net-next 03/15] net/mlx5: Add adjphase function to support
 hardware-only offset control

On Sun, Jan 22, 2023 at 01:11:57PM -0800, Rahul Rameshbabu wrote:

> The way NVIDIA devices internally handle adjphase is to adjust the
> frequency for a short period of time to make the small time offset
> adjustments smooth (using some internal calculations) where the time
> offset "nudge" is applied but frequency is also adjusted to prevent
> immediate drift after that time adjustment.

Whatever floats your boat.

> However, we aren't sure if
> this is the only approach possible to achieve accurate corrections for
> small offset adjustments with adjphase,

Typically one would implement a PI controller.

> so I would suggest that the
> documentation be updated to state something discussing that adjphase is
> expected to support small jumps in offset precisely without necessarily
> bringing up frequency manipulation potentially done to achieve this.

Sorry if the docs aren't clear.  (However, the use of NTP timex
ADJ_FREQUENCY and ADJ_OFFSET really should be clear to everyone
familiar with NTP.)

Bottom line: PHC that offers adjphase should implement a clock servo.

Thanks,
Richard