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
