linux-kernel - Re: [PATCH 5/6] ARM: AM43xx: clk: Change the cpts ref clock source to dpll_core

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Date:	Mon, 28 Apr 2014 18:25:56 +0530
From:	George Cherian <george.cherian@...com>
To:	Richard Cochran <richardcochran@...il.com>
CC:	<netdev@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<linux-arm-kernel@...ts.infradead.org>,
	<devicetree@...r.kernel.org>, <linux-omap@...r.kernel.org>,
	<davem@...emloft.net>, <jeffrey.t.kirsher@...el.com>,
	<dborkman@...hat.com>, <ast@...mgrid.com>, <tklauser@...tanz.ch>,
	<mpa@...gutronix.de>, <bhutchings@...arflare.com>,
	<zonque@...il.com>, <balbi@...com>, <mugunthanvnm@...com>,
	<t-kristo@...com>, <mturquette@...aro.org>,
	<linux@....linux.org.uk>, <galak@...eaurora.org>,
	<ijc+devicetree@...lion.org.uk>, <mark.rutland@....com>,
	<pawel.moll@....com>, <robh+dt@...nel.org>, <tony@...mide.com>,
	<bcousson@...libre.com>
Subject: Re: [PATCH 5/6] ARM: AM43xx: clk: Change the cpts ref clock source
 to dpll_core_m5 clk

On 4/28/2014 12:40 PM, Richard Cochran wrote:
> On Mon, Apr 28, 2014 at 09:40:24AM +0530, George Cherian wrote:
>> cpsw_cpts_rft_clk has got the choice of 3 clocksources
>>   -dpll_core_m4_ck
>>   -dpll_core_m5_ck
>>   -dpll_disp_m2_ck
>>
>> By default dpll_core_m4_ck is selected, witn this as clock
>> source the CPTS doesnot work properly. It gives clockcheck errors
>> while running PTP.
>>
>>   clockcheck: clock jumped backward or running slower than expected!
> It is strange that I have never seen this error, since I have often
> tested linuxptp on a beagle bone white.
In beagle bone white (AM335x)  CPTS has a choice of 2 clocksource
-dpll_core_m5_ck
-dpll_core_m4_ck
and by default  dpll_core_m5_ck is used. Where as in AM437x the default 
clocksource used is dpll_core_m4_ck .

You can change the clocksource in beagle bone white by writing  1 to 
0x44e00520 (By default its 0).
>
> Can you please explain why this clock doesn't work correctly?
>
>> By selecting dpll_core_m5_ck as the clocksource fixes this issue.
>> In AM335x dpll_core_m5_ck is the default clocksource.
> The choice of clock source in the CPTS driver originally came from
> TI. It would be nice to know why that was the wrong choice.
>
> Thanks,
> Richard


-- 
-George

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