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SubjectRe: [PATCH net-next 2/3] dt-bindings: net: micrel: Configure latency values and timestamping check for LAN8814 phy
The 03/09/2022 11:52, Richard Cochran wrote:
>
> On Wed, Mar 09, 2022 at 02:55:49PM +0000, Russell King (Oracle) wrote:
>
> > I think we understand this, and compensating for the delay in the PHY
> > is quite reasonable, which surely will be a fixed amount irrespective
> > of the board.
>
> The PHY delays are not fixed. They can be variable, even packet to packet.
>
> https://www.researchgate.net/publication/260434179_Measurement_of_egress_and_ingress_delays_of_PTP_clocks
>
> https://www.researchgate.net/publication/265731050_Experimental_verification_of_the_egress_and_ingress_latency_correction_in_PTP_clocks
>
> Some PHYs are well behaved. Some are not.

What about adding only some sane values in the driver like here [1].
And the allow the user to use linuxptp to fine tune all this.

>
> In any case, the linuxptp user space stack supports the standardized
> method of correcting a system's delay asymmetry. IMO it makes no
> sense to even try to let kernel device drivers correct these delays.
> Driver authors will get it wrong, and indeed they have already tried
> and failed. And when the magic numbers change from one kernel release
> to another, it only makes the end user's job harder, because they will
> have to update their scripts to correct the bogus numbers.
>
> Thanks,
> Richard
>

[1] https://elixir.bootlin.com/linux/v5.17-rc7/source/drivers/net/phy/mscc/mscc_ptp.c#L245

--
/Horatiu

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