linux-kernel - Re: [PATCH 4/6] net: ethernet: ti: cpts: add ptp pps support

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Date:   Fri, 2 Dec 2016 11:58:34 -0600
From:   Grygorii Strashko <grygorii.strashko@...com>
To:     Richard Cochran <richardcochran@...il.com>
CC:     Murali Karicheri <m-karicheri2@...com>,
        Wingman Kwok <w-kwok2@...com>,
        "David S. Miller" <davem@...emloft.net>, <netdev@...r.kernel.org>,
        Mugunthan V N <mugunthanvnm@...com>,
        Sekhar Nori <nsekhar@...com>, <linux-kernel@...r.kernel.org>,
        <linux-omap@...r.kernel.org>, Rob Herring <robh+dt@...nel.org>,
        <devicetree@...r.kernel.org>
Subject: Re: [PATCH 4/6] net: ethernet: ti: cpts: add ptp pps support

Hi Richard,

On 12/02/2016 03:58 AM, Richard Cochran wrote:
> On Wed, Nov 30, 2016 at 11:17:38PM +0100, Richard Cochran wrote:
>> On Wed, Nov 30, 2016 at 02:43:57PM -0600, Grygorii Strashko wrote:
>>> Sry, but this is questionable - code for pps comes from TI internal
>>> branches (SDK releases) where it survived for a pretty long time.
> 
> Actually, there is a way to get an accurate PPS from the am335x.  See
> this recent thread:
> 
>    https://www.mail-archive.com/linuxptp-devel@lists.sourceforge.net/msg01726.html
> 
> That is the way to go, and so, please drop this present patch.
> 

thanks for the links - it sounds very interesting.

As I understood, people trying to enable PPS on am335 device with the
goal to have PPS signal generated on some SoC pin and therefore they use DMtimer.
Also, as i understood, the Timer Load Register (TLDR) is corrected once
a second at each HW_TS_PUSH - as result, if freq was corrected during current sec
there will be some HW_TS_PUSH generation jitter any way.

Above solution is a bit complex for keystone 2 SoCs, as CPTS itself on these SoCs has
output pin (ts_comp) which can be used for PPS signal generation. So, I think,
similar results can be achieved by removing PPS correction code from cpts_ptp_adjfreq()
and updating CPTS_TS_LOAD_VAL once a sec in cpts_overflow_check().

or I missed smth?

-- 
regards,
-grygorii