linux-kernel - Re: [PATCH net-next 2/3] dt-bindings: net: micrel: Configure latency values and timestamping check for LAN8814 phy

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Date:   Fri, 11 Mar 2022 15:28:14 +0100
From:   Horatiu Vultur <horatiu.vultur@...rochip.com>
To:     Richard Cochran <richardcochran@...il.com>
CC:     "Russell King (Oracle)" <linux@...linux.org.uk>,
        Andrew Lunn <andrew@...n.ch>, <Divya.Koppera@...rochip.com>,
        <netdev@...r.kernel.org>, <hkallweit1@...il.com>,
        <davem@...emloft.net>, <kuba@...nel.org>, <robh+dt@...nel.org>,
        <devicetree@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <UNGLinuxDriver@...rochip.com>, <Madhuri.Sripada@...rochip.com>,
        <Manohar.Puri@...rochip.com>
Subject: Re: [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