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 15:58:25 -0800
From:   Jacob Keller <jacob.e.keller@...el.com>
To:     Rahul Rameshbabu <rrameshbabu@...dia.com>
CC:     Jakub Kicinski <kuba@...nel.org>,
        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 1/20/2023 10:00 AM, Rahul Rameshbabu wrote:
> I assumed the phase control word is more than just a simple one-shot
> (atomic) time offset done in the hardware (at least internally what the
> hardware does when implementing this control word).
> 
>   adjtime modifies HW counter with a value to move the 1 PPS abruptly to new location.
>   adjphase modifies the frequency to quickly nudge the 1 PPS to new location and also includes a HW filter to smooth out the adjustments and fine tune frequency.
> 
>   Continuous small offset adjustments using adjtime, likley see sudden shifts of the 1 PPS.  The 1 PPS probably disappears and re-appears.
>   Continuous small offset adjustments using adjphase, should see continuous 1 PPS.
> 

Sure. I guess what I don't understand is why or when one would want to
use adjphase instead of just performing frequency adjustment via the
.adjfine operation...

Especially since "gradual adjustment over time" means it will still be
slow to converge (just like adjusting frequency is today).

We should definitely improve the doc to explain the diff between them
and make sure that its more clear to driver implementations.

It also makes it harder to justify mapping small .adjtime to .adjphase,
as it seems like .adjphase isn't required to adjust the offset
immediately. Perhaps the adjustment size is small enough that isn't a
big problem?

Thanks,
Jake