linux-kernel - Re: [GIT PULL 1/3] ARM: soc: exynos: Drivers for v4.9

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Date:   Tue, 11 Oct 2016 09:46:49 +0900
From:   Kukjin Kim <kgene.kim@...il.com>
To:     Krzysztof Kozlowski <krzk@...nel.org>
Cc:     Olof Johansson <olof@...om.net>, Arnd Bergmann <arnd@...db.de>,
        Kevin Hilman <khilman@...nel.org>,
        "arm@...nel.org" <arm@...nel.org>,
        Krzysztof Kozlowski <k.kozlowski@...sung.com>,
        Kukjin Kim <kgene@...nel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-samsung-soc@...r.kernel.org" 
        <linux-samsung-soc@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
        Javier Martinez Canillas <javier@....samsung.com>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        kgene.kim@...sung.com
Subject: Re: [GIT PULL 1/3] ARM: soc: exynos: Drivers for v4.9

2016. 10. 3. 21:19 Kukjin Kim <kgene.kim@...il.com> wrote:

+ my samsung email

> 2016. 10. 3. 15:48 Krzysztof Kozlowski <krzk@...nel.org> wrote:
> 
>>>> On Sun, Oct 02, 2016 at 05:25:07PM -0700, Olof Johansson wrote:
>>>>> On Mon, Sep 19, 2016 at 8:53 AM, Krzysztof Kozlowski <krzk@...nel.org> wrote:
>>>>>> On Mon, Sep 19, 2016 at 05:02:40PM +0200, Arnd Bergmann wrote:
>>>>>> On Sunday, September 18, 2016 6:39:46 PM CEST Krzysztof Kozlowski wrote:
>>>>>> Samsung drivers/soc update for v4.9:
>>>>>> 1. Allow compile testing of exynos-mct clocksource driver on ARM64.
>>>>>> 2. Document Exynos5433 PMU compatible (already used by clkout driver and more
>>>>>>  will be coming soon).
>>>>> 
>>>>> Pulled into next/drivers, thanks
>>>>> 
>>>>> Just for my understanding: why do we need the exynos-mct driver on ARM64
>>>>> but not the delay-timer portion of it?
>>>> 
>>>> I think we want all of it but Doug's optimization 3252a646aa2c
>>>> ("clocksource: exynos_mct: Only use 32-bits where possible") is not
>>>> ARM64 friendly. One way of dealing with it would be to prepare two
>>>> versions of exynos4_read_current_timer(). One reading only lower 32-bit
>>>> value for ARMv7 and second (slow) reading lower and upper for ARMv8.
>>>> 
>>>>> 
>>>>> Is there an advantage in using MCT over the architected timer on these
>>>>> chips? If so, should we also have a way to use it as the delay timer?
>>>> 
>>>> No, there is no real advantage... except that the SoC has some
>>>> interesting "characteristics"... The timers are tightly coupled. Very
>>>> tightly. I spent a lot of time and failed to boot my ARMv8 board without
>>>> some MCT magic.
>>> 
>>> What kind of magic is that?
>> 
>> Most notably: the arch timer starts when MCT forward running counter
>> starts. Without kicking MCT, the arch timer seems to be frozen.
>> 
>>> I can understand that needing the MCT for
>>> some system-level timer functionality might be true (wakeups, etc),
>>> but for system timesource avoiding the MMIO timer and using the arch
>>> ones is a substantial performance improvement for gettimeofday() and
>>> friends.
>>> 
>>> There was extensive discussion last year over using arch timers on
>>> 5420/5422, and it fizzled out with vague comments about something not
>>> working right between A15/A7 on b.L. hardware. I'm presuming whatever
>>> implementation details of that SoC has since been fixed on later chips
>>> (including v8). Any chance you can confirm? It'd be very nice to leave
>>> MCT behind on v8 as a system time source.
>> 
>> Unfortunately, I cannot confirm this, at least on Exynos5433 (ARMv8). I
>> played with arch and MCT timers on it and failed to get the
>> arch-timer-only setup working. I did not have access to newer Exynos
>> designs (Exynos 7) so I do not know how it works there.
> 
> Hi guys,
> 
> I know what Olof want to know and actually several days ago someone asked me about that. As you guys talked, a couple of years ago there were some discussions...BTW I need to contact to hardware designer before let you guys know because something needs to be confirmed by them even I know roughly.
> 
> Note I'm in vacation with my family. Will be back on this in several days with exact information.
> 
> BRs,
> Kukjin