linux-kernel - Re: [PATCH v2 1/2] clocksource/drivers/exynos_mct_v2: introduce Exynos MCT version 2 driver for next Exynos SoC

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Message-ID: <83f6574b-bbab-f0c3-7198-f773c3dcfc63@canonical.com>
Date:   Wed, 3 Nov 2021 09:18:07 +0100
From:   Krzysztof Kozlowski <krzysztof.kozlowski@...onical.com>
To:     Youngmin Nam <youngmin.nam@...sung.com>,
        Mark Rutland <mark.rutland@....com>
Cc:     will@...nel.org, daniel.lezcano@...aro.org, tglx@...utronix.de,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-samsung-soc@...r.kernel.org, pullip.cho@...sung.com,
        hoony.yu@...sung.com, hajun.sung@...sung.com,
        myung-su.cha@...sung.com, kgene@...nel.org
Subject: Re: [PATCH v2 1/2] clocksource/drivers/exynos_mct_v2: introduce
 Exynos MCT version 2 driver for next Exynos SoC

On 03/11/2021 01:09, Youngmin Nam wrote:
> On Tue, Nov 02, 2021 at 10:28:10AM +0000, Mark Rutland wrote:
>> On Tue, Nov 02, 2021 at 09:11:21AM +0900, Youngmin Nam wrote:
>>> Exynos MCT version 2 is composed of 1 FRC and 12 comparators.
>>> There are no global timer and local timer anymore.
>>> The 1 of 64bit FRC serves as "up-counter"(not "comparators").
>>> The 12 comaprators(not "counter") can be used as per-cpu event timer
>>> so that it can support upto 12 cores.
>>> And a RTC source can be used as backup clock source.
>>
>> [...]
>>
>>> +static int exynos_mct_starting_cpu(unsigned int cpu)
>>> +{
>>> +	struct mct_clock_event_device *mevt = per_cpu_ptr(&percpu_mct_tick, cpu);
>>> +	struct clock_event_device *evt = &mevt->evt;
>>> +
>>> +	snprintf(mevt->name, sizeof(mevt->name), "mct_comp%d", cpu);
>>> +
>>> +	evt->name = mevt->name;
>>> +	evt->cpumask = cpumask_of(cpu);
>>> +	evt->set_next_event = exynos_comp_set_next_event;
>>> +	evt->set_state_periodic = mct_set_state_periodic;
>>> +	evt->set_state_shutdown = mct_set_state_shutdown;
>>> +	evt->set_state_oneshot = mct_set_state_shutdown;
>>> +	evt->set_state_oneshot_stopped = mct_set_state_shutdown;
>>> +	evt->tick_resume = mct_set_state_shutdown;
>>> +	evt->features = CLOCK_EVT_FEAT_PERIODIC | CLOCK_EVT_FEAT_ONESHOT;
>>> +	evt->rating = 500;	/* use value higher than ARM arch timer */
>>
>> Previously Will asked you to try CLOCK_EVT_FEAT_PERCPU here, and to set
>> the C3STOP flag on the arch timer via the DT when necessary, rather than
>> trying to override the arch timer like this:
>>
>>   https://protect2.fireeye.com/v1/url?k=72526080-2dc9598b-7253ebcf-002590f5b904-ca603717c6462908&q=1&e=be56aa83-dbac-4639-913d-d388620fe3fc&u=https%3A%2F%2Flore.kernel.org%2Fr%2F20211027073458.GA22231%40willie-the-truck
>>
>> There are a bunch of things that depend on the architected timer working
>> as a clocksource (e.g. vdso, kvm), and it *should* work as a lock
>> clockevent_device if configured correctly, and it's much more consistent
>> with *everyone else* to use the arhcitected timer by default.
>>
>> Please try as Will suggested above, so that this works from day one.
>>
>> Thanks,
>> Mark.
>>
> 
> Hi Mark.
> It looks like you missed my previous mail.
> https://lore.kernel.org/all/20211029035422.GA30523@perf/#t
> 
> Yes, I believe Will's suggestion definitely will work.
> But that is for performance not functionality.
> As a driver for new H/W IP I would like to confirm functionality first.
> 
> We need more time to test this feature with our exynos core power down feature.
> And we need to do a various regression test whether there is another corner case or not.
> So, how about we apply Will's suggetion later after the current patchset is merged first?
> After doing our regression test with our exynos core power down feature, we can confirm this.
> 

Not really, because once it is merged there is no incentive to fix it or
simply changing it can be forgotten. Also similarly to commit
6282edb72bed ("clocksource/drivers/exynos_mct: Increase priority over
ARM arch timer"), there should be a valid and serious reason to
prioritize Exynos MCT.


Best regards,
Krzysztof