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

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Date:   Fri, 20 Jan 2023 09:21:09 -0800
From:   Jacob Keller <jacob.e.keller@...el.com>
To:     Rahul Rameshbabu <rrameshbabu@...dia.com>,
        Jakub Kicinski <kuba@...nel.org>
CC:     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>,
        Richard Cochran <richardcochran@...il.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 1/19/2023 8:26 PM, Rahul Rameshbabu wrote:
> On Thu, 19 Jan, 2023 20:03:43 -0800 Jakub Kicinski <kuba@...nel.org> wrote:
>> The other question is about the exact semantics of ->adjphase
>> - do all timecounter based clock implementations support it
>> by definition?
> 
> My understanding is no (though anyone is free to jump in to correct me
> on this). Only implementations with support for precisely handling small
> PPS corrections can support adjphase (being able to adjust small offsets
> without causing same or worse drift).

I guess I'm missing something here? timecounters allow adjusting time in
an atomic way. They don't lose any time when making an adjustment
because its a change to the wrapping around a fixed cycle counter.

How does that not comply with adjphase? and if it doesn't, then whats
the difference between adjphase and just correcting offset using adjfine
for frequency adjustment?

I guess adjusting phase will do the small corrections in hardware
(perhaps by temporarily adjusting the nominal frequency of the clock)
but will then return to the normal frequency once complete?

So adjphase is more than just being atomic...?