Re: [PATCH 2/2] net: fec: Reload PTP registers after link-state change

[Csókás Bence]

Hi!

On 9/16/24 17:05, Frank Li wrote:
> On Mon, Sep 16, 2024 at 04:19:31PM +0200, Csókás, Bence wrote:
> > On link-state change, the controller gets reset,
> > which clears all PTP registers, including PHC time,
> > calibrated clock correction values etc. For correct
> > IEEE 1588 operation we need to restore these after
> > the reset.
>
> I am not sure if it necessary. timer will be big offset after reset. ptpd
> should set_time then do clock frequency adjust, supposed just few ms, ptp
> time will get resync.
>
> of course, restore these value may reduce the resync time.
>
> Frank

There's 3 problems with that:
1. ATCORR, ATINC and ATPER will not be restored, therefore precision 
will be immediately lost.
2. ptpd does NOT set the time, only once, on startup. Currently, on 
link-down, ptpd tries to correct for the missing 54 years by making the 
PHC tick 3% faster (therefore the PPS signal will have a frequency error 
as well), which will never get it there. One work-around is to 
periodically re-start ptpd, but this is obviously sub-optimal.
3. If the PTP server goes away, there's no way to restore the time. 
Whereas if you save and reload it, you can continue, although with 
degraded precision.

Bence