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

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Message-ID: <20230119214516.59adab05@kernel.org>
Date:   Thu, 19 Jan 2023 21:45:16 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Rahul Rameshbabu <rrameshbabu@...dia.com>
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 21:22:00 -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 :(