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

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Message-ID: <71c3e9a0-d14f-4748-dfe6-6a41b323818f@intel.com>
Date:   Mon, 23 Jan 2023 10:14:03 -0800
From:   Jacob Keller <jacob.e.keller@...el.com>
To:     Richard Cochran <richardcochran@...il.com>
CC:     Rahul Rameshbabu <rrameshbabu@...dia.com>,
        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>,
        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 1/22/2023 6:15 PM, Richard Cochran wrote:
> Wow, lots of confusion in this thread!
> 
> On Fri, Jan 20, 2023 at 03:58:25PM -0800, Jacob Keller wrote:
> 
>> 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...
> 
> The difference is in where the frequency adjust is calculated.  There
> are two possibilities:
> 
> 1. It may be done in user space.
>    This is NTP timex's ADJ_FREQUENCY.
>    PHC implements .adjfine
> 
> 2. It may be left to kernel space.
>    This is NTP timex's ADJ_OFFSET.
>    PHC implements .adjphase
> 
> In case of #2, the hardware implements some kind of clock servo.  The
> inputs to the servo are the reported phase offsets.  The output of the
> servo is a frequency adjustment.
> 
>> Especially since "gradual adjustment over time" means it will still be
>> slow to converge (just like adjusting frequency is today).
> 
> It depends on the servo parameters.
>  
>> We should definitely improve the doc to explain the diff between them
>> and make sure that its more clear to driver implementations.
> 
> Sure.
> 
>> It also makes it harder to justify mapping small .adjtime to .adjphase,
> 
> adjtime is totally different.  Don't mix those two up:
> 
> - adjtime  sets the time (phase), plain and simple.
> 
> - adjphase feeds the phase offsets into a servo algorithm in hardware.
> 
> FWIW, the PHC subsystem handles the semantics of timex ADJ_FREQUENCY
> and ADJ_OFFSET in exactly the same way as the NTP subsystem.
> 
> 
> Thanks,
> Richard

Thanks for a succinct summary!