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

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Message-ID: <e5fa3847-bb3d-4b32-bd7f-5162a10980b7@lunn.ch>
Date: Wed, 14 Aug 2024 15:08:07 +0200
From: Andrew Lunn <andrew@...n.ch>
To: Vadim Fedorenko <vadim.fedorenko@...ux.dev>
Cc: Maciek Machnikowski <maciek@...hnikowski.net>, 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 Wed, Aug 14, 2024 at 09:44:29AM +0100, Vadim Fedorenko wrote:
> 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).

Is there a clear definition then? Could you reference a standard
indicating what is included and excluded from this?

> > 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.

So the driver itself does not know its own error? It has to be told
it, so it can report it back to user space. Then why bother, just put
it directly into the ptp4l configuration file?

Maybe this is all obvious to somebody who knows PTP inside out, but to
me it is not. Please could you put a better explanation and
justification into the commit message. We need PHY driver writers who
have limited idea about PTP can implement these new calls.

	Andrew