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Date:   Fri, 29 Oct 2021 12:54:22 +0900
From:   Youngmin Nam <youngmin.nam@samsung.com>
To:     Will Deacon <will@kernel.org>
Cc:     krzysztof.kozlowski@canonical.com, mark.rutland@arm.com,
        daniel.lezcano@linaro.org, tglx@linutronix.de,
        linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
        linux-samsung-soc@vger.kernel.org, pullip.cho@samsung.com,
        hoony.yu@samsung.com, hajun.sung@samsung.com,
        myung-su.cha@samsung.com, kgene@kernel.org
Subject: Re: [PATCH v1 1/2] clocksource/drivers/exynos_mct_v2: introduce
 Exynos MCT version 2 driver for next Exynos SoC
Message-ID: <20211029035422.GA30523@perf>
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References: <1b93aaf3-ed64-b105-dec4-07b6f27b385b@canonical.com>
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        <CGME20211027011125epcas2p2916524051416ede854b750c91a19073b@epcas2p2.samsung.com>
        <20211027013709.GA17353@perf> <20211027073458.GA22231@willie-the-truck>
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On Wed, Oct 27, 2021 at 08:34:58AM +0100, Will Deacon wrote:
> On Wed, Oct 27, 2021 at 10:38:37AM +0900, Youngmin Nam wrote:
> > On Tue, Oct 26, 2021 at 01:00:51PM +0200, Krzysztof Kozlowski wrote:
> > > On 26/10/2021 12:45, Youngmin Nam wrote:
> > > > On Tue, Oct 26, 2021 at 09:10:28AM +0200, Krzysztof Kozlowski wrote:
> > > >> On 26/10/2021 03:47, Youngmin Nam wrote:
> > > >>>> If everyone added a new driver to avoid integrating with existing code,
> > > >>>> we would have huge kernel with thousands of duplicated solutions. The
> > > >>>> kernel also would be unmaintained.
> > > >>>>
> > > >>>> Such arguments were brought before several times - "I don't want to
> > > >>>> integrating with existing code", "My use case is different", "I would
> > > >>>> need to test the other cases", "It's complicated for me".
> > > >>>>
> > > >>>> Instead of pushing a new vendor driver you should integrate it with
> > > >>>> existing code.
> > > >>>>
> > > >>> Let me ask you one question.
> > > >>> If we maintain as one driver, how can people who don't have the new MCT test the new driver?
> > > >>
> > > >> I assume you talk about a case when someone else later changes something
> > > >> in the driver. Such person doesn't necessarily have to test it. The same
> > > >> as in all other cases (Exynos MCT is not special here): just ask for
> > > >> testing on platform one doesn't have.
> > > >>
> > > >> Even if you submit this as separate driver, there is the exact same
> > > >> problem. People will change the MCTv2 driver without access to hardware.
> > > >>
> > > > Yes, I can test the new MCT driver if someone ask for testing after modifying the new driver.
> > > > But in this case, we don't need to test the previous MCT driver. We have only to test the new MCT driver.
> > > 
> > > Like with everything in Linux kernel. We merge instead of duplicate.
> > > It's not an argument.
> > > 
> > > >> None of these differ for Exynos MCT from other drivers, e.g. mentioned
> > > >> Samsung PMIC drivers, recently modified (by Will and Sam) the SoC clock
> > > >> drivers or the ChipID drivers (changed by Chanho).
> > > > From HW point of view, the previous MCT is almost 10-year-old IP without any major change and
> > > > it will not be used on next new Exynos SoC.
> > > > MCTv2 is the totally newly designed IP and it will replace the Exynos system timer.
> > > > Device driver would be dependent with H/W. We are going to apply a lot of changes for this new MCT.
> > > > For maintenance, I think we should separate the new MCT driver for maintenance.
> > > > 
> > > 
> > > There are several similarities which actually suggest that you
> > > exaggerate the differences.
> > > 
> > > The number of interrupts is the same (4+8 in older one, 12 in new one...).
> > 
> > I didn't "exaggerate" at all.
> > The numer of interrups is the same. But their usage is completely different.
> > The type of each timer is different.
> > And previous MCT can only support upto 8 cores.
> > 
> > * MCTv1 (Let me call previous MCT as MCTv1)
> >  - 4 global timer + 8 local timer
> >  - Global timer and local timer are totally different.
> >  - 4 global timer have only one 64bit FRC that serves as the "up-counter" with 4 "comparators"
> >  - 8 local timer have 8 of 32bit FRC that serves as the "down-counter" without any "comparators".(just expire timer)
> >  - local timer can be used as per-cpu event timer, so it can only support upto 8 cores.
> > 
> > * MCTv2
> >  - There are no global timer and local timer anymore.
> >  - 1 of 64bit FRC that serves as "up-counter" (just counter without "comparators")
> >  - 12 comaprators (These are not "counter") can be used as per-cpu event timer so that it can support upto 12 cores.
> >  - RTC source can be used as backup source.
> > 
> > > You assign the MCT priority also as higher than Architected Timer
> > > (+Cc Will and Mark - is it ok for you?)
> > >     evt->rating = 500;      /* use value higher than ARM arch timer *
> > > 
> > Yes, this is absolutely correct on event timer.
> > We cannot use arm arch timer which is operating based on PPI as per-cpu event timer because of poewr mode.
> 
> You should be able to now that I've added support for per-cpu wakeup timers.
> 
> As long as the Arm arch timer is marked as C3STOP (e.g. by sticking the
> "local-timer-stop" property in the DT notes), then the MCT will be used
> as the wakeup source if you set the CLOCK_EVT_FEAT_PERCPU feature flag.
> 
> Give it a try.
> 
> Will
> 
Hi Will. Thanks for sharing information.

In MCTv2, we need more time to test because this patchset is the start for MCTv2 new driver.
This feature is for better performance not functionality.
How about considering this later after the current patchset is merged first ?
After doing our regression test, we will be able to consider applying this.

Thanks.


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