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

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Date:   Mon, 7 Mar 2022 05:02:24 +0000
From:   <Divya.Koppera@...rochip.com>
To:     <richardcochran@...il.com>, <andrew@...n.ch>
CC:     <netdev@...r.kernel.org>, <hkallweit1@...il.com>,
        <linux@...linux.org.uk>, <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

Hi Richard,

> -----Original Message-----
> From: Richard Cochran <richardcochran@...il.com>
> Sent: Friday, March 4, 2022 7:26 PM
> To: Andrew Lunn <andrew@...n.ch>
> Cc: Divya Koppera - I30481 <Divya.Koppera@...rochip.com>;
> netdev@...r.kernel.org; hkallweit1@...il.com; linux@...linux.org.uk;
> davem@...emloft.net; kuba@...nel.org; robh+dt@...nel.org;
> devicetree@...r.kernel.org; linux-kernel@...r.kernel.org; UNGLinuxDriver
> <UNGLinuxDriver@...rochip.com>; Madhuri Sripada - I34878
> <Madhuri.Sripada@...rochip.com>; Manohar Puri - I30488
> <Manohar.Puri@...rochip.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