netdev - Re: [PATCH] net:cpsw: Pass unhandled ioctl's on to generic phy ioctl

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Date:	Wed, 05 Feb 2014 12:06:29 +0100
From:	Christian Riesch <christian.riesch@...cron.at>
To:	Ben Hutchings <ben@...adent.org.uk>,
	Richard Cochran <richardcochran@...il.com>
CC:	"Sørensen, Stefan" 
	<Stefan.Sorensen@...ctralink.com>,
	"davem@...emloft.net" <davem@...emloft.net>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"mugunthanvnm@...com" <mugunthanvnm@...com>
Subject: Re: [PATCH] net:cpsw: Pass unhandled ioctl's on to generic phy ioctl

On 2014-02-05 11:23, Ben Hutchings wrote:
> On Wed, 2014-02-05 at 08:12 +0100, Richard Cochran wrote:
>> On Tue, Feb 04, 2014 at 09:51:59PM +0000, Ben Hutchings wrote:
>>>
>>> Right.  If all versions of CPSW include hardware timestamping then
>>> bother with PHY timestamping at all?  And why make CONFIG_TI_CPTS
>>> configurable?
>>
>> On the one hand, PHY time stamping is more accurate and offers
>> synchronization performance that is measurably better than MAC time
>> stamping.
>
> I suppose that depends on how much jitter there is in the PHY?

Not only jitter, asymmetries of the PHY delays must be well known, too, 
and these data are not always available from datasheets.

If someone is interested in measurement results: We did a few 
experiments last year for 100 Mbit Ethernet and compared MAC 
timestamping of a P2020 with the PHY timestamping in the DP83640 (see 
Table III in [1]).

Christian


[1] C. Riesch, C. Marinescu, and M. Rudigier, "Measurement of egress and 
ingress delays of PTP clocks", in 2013 International IEEE Symposium on 
Precision Clock Synchronization for Measurement Control and 
Communication (ISPCS), Lemgo, Germany, Sep. 22–27, 2013, pp. 113–118.
http://www.riesch.at/christian/ISPCS2013_Measurement_of_egress_and_ingress_delays_of_PTP_clocks.pdf
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