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SubjectRE: [PATCH net-next 2/3] dt-bindings: net: micrel: Configure latency values and timestamping check for LAN8814 phy
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Hi Richard,

> -----Original Message-----
> From: Richard Cochran <richardcochran@gmail.com>
> Sent: Friday, March 4, 2022 7:26 PM
> To: Andrew Lunn <andrew@lunn.ch>
> Cc: Divya Koppera - I30481 <Divya.Koppera@microchip.com>;
> netdev@vger.kernel.org; hkallweit1@gmail.com; linux@armlinux.org.uk;
> davem@davemloft.net; kuba@kernel.org; robh+dt@kernel.org;
> devicetree@vger.kernel.org; linux-kernel@vger.kernel.org; UNGLinuxDriver
> <UNGLinuxDriver@microchip.com>; Madhuri Sripada - I34878
> <Madhuri.Sripada@microchip.com>; Manohar Puri - I30488
> <Manohar.Puri@microchip.com>
> Subject: Re: [PATCH net-next 2/3] dt-bindings: net: micrel: Configure latency
> values and timestamping check for LAN8814 phy
>
> EXTERNAL EMAIL: Do not click links or open attachments unless you know the
> content is safe
>
> On Fri, Mar 04, 2022 at 01:50:47PM +0100, Andrew Lunn wrote:
> > Why does this need to be configured, rather than hard coded? Why would
> > the latency for a given speed change? I would of thought though you
> > would take the average length of a PTP packet and divide is by the
> > link speed.
>
> Latency is unrelated to frame length.
>
> My understanding is that it is VERY tricky to measure PHY latency.
> Studies have shown that some PHYs vary by link speed, and some vary
> randomly, frame by frame.
>
> So I can understand wanting to configure it. However, DTS is probably the
> wrong place. The linuxptp user space stack has configuration variables for this
> purpose:
>
> egressLatency
> Specifies the difference in nanoseconds between the actual
> transmission time at the reference plane and the reported trans‐
> mit time stamp. This value will be added to egress time stamps
> obtained from the hardware. The default is 0.
>
> ingressLatency
> Specifies the difference in nanoseconds between the reported re‐
> ceive time stamp and the actual reception time at reference
> plane. This value will be subtracted from ingress time stamps
> obtained from the hardware. The default is 0.
>

I will check this and come back with fix if it is applicable.

Thanks,
Divya

> Thanks,
> Richard
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