linux-kernel - Re: [RFC PATCH 0/3] clocksource: exynos_mct: allow mct to use 64-bit counter from coprocessor

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Message-ID: <CALW4P+LqjhQ+Puadz4W-n_v=KEnwvQiUkk5jkDLRxhjVQGwFyw@mail.gmail.com>
Date:	Tue, 28 Jul 2015 19:20:25 +0300
From:	Alexey Klimov <klimov.linux@...il.com>
To:	Doug Anderson <dianders@...omium.org>
Cc:	linux-samsung-soc <linux-samsung-soc@...r.kernel.org>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	Chirantan Ekbote <chirantan@...omium.org>,
	Tarek Dakhran <t.dakhran@...il.com>,
	Krzysztof Kozłowski <k.kozlowski@...sung.com>,
	kgene <kgene@...nel.org>, Thomas Gleixner <tglx@...utronix.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Yury Norov <yury.norov@...il.com>,
	Alim Akhtar <alim.akhtar@...il.com>,
	Mark Rutland <mark.rutland@....com>, marc.zyngier@....com
Subject: Re: [RFC PATCH 0/3] clocksource: exynos_mct: allow mct to use 64-bit
 counter from coprocessor

Hi Doug,

On Tue, Jul 28, 2015 at 6:24 PM, Doug Anderson <dianders@...omium.org> wrote:
> Alexey,
>
> On Mon, Jul 27, 2015 at 2:28 PM, Alexey Klimov <klimov.linux@...il.com> wrote:
>> Hi all,
>>
>> year(s) ago it was discovered that MCT timer and ARM architectured
>> timer
>> are the same hardware with different interface. Here [1].
>>
>> I followed mail-list discussions about removing MCT and using arch
>> timer for Exynos5-based SoCs but things aren't moving at least latest
>> upstream kernel on odroid-xu3 will use MCT as default timers.
>> Maybe the reason are some power-management related things that very
>> specific to Samsung. I don't know.
>>
>>
>> Idea of this draft patchset comes from Doug patches when he tried to
>> optimize read of 64-bit counter located in mmio. [2]
>> Why not using cp15 counter instead if possible?
>
> I hate to burst your bubble here, but...
>
> ...I think it would be a bad idea to use the cp15 counter on exynos
> 5422.  According to Samsung, there are issues where using cp15 could
> sometimes return the wrong value, especially if you happen to read it
> while on an A7 instead of an A15.  It will tend to work pretty well,
> but Samsung claimed that it might not work right on some CPUs or at
> some temperatures.

I think this is right time when I can say "If only I had known about
it before!".
That's quite interesting especially about temperature issues.
Maybe it's worth to insert comments in driver/docs about this.

> I've added Alim who provided this information to me (in the private
> http://crosbug.com/p/29556).  He may be able to elaborate more.

Alim, could you please add details if any?

> Generally I'd say that if the arch timer works for you: use the arch
> timer.  If the arch timer doesn't work for you, use the MCT.
>
> -Doug

I possess no knowledge if there're any different hw revisions of
odroid-xu3 on the field
but looks like arch timer works fine for me on odroid-xu3 Exynos5422
SoC with upstream kernel.
Is it right direction to switch xu3 to arch timer?
Do you have any info or ideas about enabling MCT G_TCON register to
start arch timer
and what's the best way to do it?

Thanks.

-- 
Best regards, Klimov Alexey
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