netdev - Re: [RFC 0/3] ptp: Add esterror support

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Message-ID: <4fb35444-3508-4f77-9c66-22acf808b93c@linux.dev>
Date: Wed, 14 Aug 2024 09:44:29 +0100
From: Vadim Fedorenko <vadim.fedorenko@...ux.dev>
To: Andrew Lunn <andrew@...n.ch>,
 Maciek Machnikowski <maciek@...hnikowski.net>
Cc: netdev@...r.kernel.org, richardcochran@...il.com,
 jacob.e.keller@...el.com, darinzon@...zon.com, kuba@...nel.org
Subject: Re: [RFC 0/3] ptp: Add esterror support

On 13/08/2024 21:05, Andrew Lunn wrote:
> On Tue, Aug 13, 2024 at 12:55:59PM +0000, Maciek Machnikowski wrote:
>> This patch series implements handling of timex esterror field
>> by ptp devices.
>>
>> Esterror field can be used to return or set the estimated error
>> of the clock. This is useful for devices containing a hardware
>> clock that is controlled and synchronized internally (such as
>> a time card) or when the synchronization is pushed to the embedded
>> CPU of a DPU.
> 
> How can you set the estimated error of a clock? Isn't it a properties
> of the hardware, and maybe the network link? A 10BaseT/Half duplex
> link is going to have a bigger error than a 1000BaseT link because the
> bits take longer on the wire etc.

AFAIU, it's in the spec of the hardware, but it can change depending on
the environment, like temperature. The link speed doesn't matter here,
this property can be used to calculate possible drift of the clock in
the micro holdover mode (between sync points).

> What is the device supposed to do with the set value?

It can be used to report the value back to user-space to calculate the
boundaries of "true time" returned by the hardware.

Thanks!