linux-kernel - RE: [PATCH net 1/2] ptp: idt82p33: optimize idt82p33

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Date:   Thu, 24 Jun 2021 14:38:46 +0000
From:   Min Li <min.li.xe@...esas.com>
To:     Richard Cochran <richardcochran@...il.com>
CC:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH net 1/2] ptp: idt82p33: optimize idt82p33_adjtime

> -----Original Message-----
> From: Richard Cochran <richardcochran@...il.com>
> Sent: June 23, 2021 11:40 PM
> To: Min Li <min.li.xe@...esas.com>
> Cc: netdev@...r.kernel.org; linux-kernel@...r.kernel.org
> Subject: Re: [PATCH net 1/2] ptp: idt82p33: optimize idt82p33_adjtime
> 
> On Wed, Jun 23, 2021 at 10:46:24AM -0400, min.li.xe@...esas.com wrote:
> > From: Min Li <min.li.xe@...esas.com>
> >
> > The current adjtime implementation is read-modify-write and
> > immediately triggered, which is not accurate due to slow i2c bus
> > access. Therefore, we will use internally generated 1 PPS pulse as
> > trigger, which will improve adjtime accuracy significantly. On the
> > other hand, the new trigger will not change TOD immediately but delay it
> to the next 1 PPS pulse.
> 
> Delaying the adjustment by one second (in the worst case) will cause
> problems.  User space expects the adjustment to happen before the call to
> adjtimex() returns.
> 
> In the case of PTP, if new Sync messages arrive before the delayed
> adjustment completes, there will be a HUGE offset error, and that will hurt
> the PI servo.
> 
> So it is better to accept a less accurate jump then to delay the adjustment.
> 
> Thanks,
> Richard

Hi Richard

Thanks for the review

For linuxptp/ptp4l, we can use step_window to adapt to the slow adjtime.

I have tested this change with ptp4l for by setting step_window to 48 (assuming 16 packets per second)
for both 8265.2/8275.1 and they performed well.

Min