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

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Message-ID: <87fsc5u4k7.fsf@nvidia.com>
Date:   Thu, 19 Jan 2023 22:02:16 -0800
From:   Rahul Rameshbabu <rrameshbabu@...dia.com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     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>,
        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 Thu, 19 Jan, 2023 21:45:16 -0800 Jakub Kicinski <kuba@...nel.org> wrote:
> On Thu, 19 Jan 2023 21:22:00 -0800 Rahul Rameshbabu wrote:
>> > Hm, so are you saying that:
>> >
>> > adjtime(delta):
>> > 	clock += delta
>> >
>> > but:
>> >
>> > adjfreq(delta):  
>> 
>> Did you mean adjphase here?
>
> Yes, sorry
>
>> > 	on clock tick & while delta > 0:
>> > 		clock += small_value
>> > 		delta -= small_value
>> >
>> > because from looking at mlx5 driver code its unclear whether the
>> > implementation does a precise but one shot adjustment or gradual
>> > adjustments.  
>> 
>> The pseudo code your drafted is accurate otherwise. The lack of clarity
>> in our driver comes from leaving the responsibility of that smooth
>> gradual transition (to keep in sync with the clock frequency while
>> running) up to the device.
>
> Ah, I see. That makes sense. This is how system clocks react too 
> or is it a local PTP invention? I think it may be worth documenting 
> in ptp_clock_kernel.h, most driver authors may not understand 
> the difference between adjtime and adjphase :(

I think this concept was uniquely proposed for ptp core stack. I do not
think I have seen this explicit differenciation before (but maybe
internal implementations do take a similar approach).

I agree with improving the documentation of adjphase vs adjtime in
ptp_clock_kernel.h

 * @adjphase:  Adjusts the phase offset of the hardware clock.
 *             parameter delta: Desired change in nanoseconds.
 *
 * @adjtime:  Shifts the time of the hardware clock.
 *            parameter delta: Desired change in nanoseconds.

This doesn't really help much in terms of understanding the difference
of adjusting the phase vs shifting the time. The quote from Vincent I
provided in my previous email was in response to a question raised by
Aya Levin who was working on timing related features in the mlx5 driver.

https://lore.kernel.org/netdev/228ceba4-47a8-49ef-994a-fe898cdc7fc1@nvidia.com/