linux-kernel - Re: [PATCH 1/2] clocksource: tegra: Use rating when registering clock source

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Date:   Fri, 14 Jun 2019 17:02:45 +0300
From:   Dmitry Osipenko <digetx@...il.com>
To:     Thierry Reding <thierry.reding@...il.com>
Cc:     Daniel Lezcano <daniel.lezcano@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Alessandro Zummo <a.zummo@...ertech.it>,
        Alexandre Belloni <alexandre.belloni@...tlin.com>,
        Jonathan Hunter <jonathanh@...dia.com>,
        linux-tegra@...r.kernel.org, linux-rtc@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] clocksource: tegra: Use rating when registering clock
 source

14.06.2019 16:53, Thierry Reding пишет:
> On Fri, Jun 14, 2019 at 04:29:17PM +0300, Dmitry Osipenko wrote:
>> 14.06.2019 16:22, Thierry Reding пишет:
>>> On Fri, Jun 14, 2019 at 03:24:07PM +0300, Dmitry Osipenko wrote:
>>>> 14.06.2019 13:47, Thierry Reding пишет:
>>>>> From: Thierry Reding <treding@...dia.com>
>>>>>
>>>>> The rating is parameterized depending on SoC generation to make sure it
>>>>> takes precedence on implementations where the architected timer can't be
>>>>> used. This rating is already used for the clock event device. Use the
>>>>> same rating for the clock source to be consistent.
>>>>>
>>>>> Signed-off-by: Thierry Reding <treding@...dia.com>
>>>>> ---
>>>>>  drivers/clocksource/timer-tegra.c | 2 +-
>>>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>>>
>>>>> diff --git a/drivers/clocksource/timer-tegra.c b/drivers/clocksource/timer-tegra.c
>>>>> index f6a8eb0d7322..e6608141cccb 100644
>>>>> --- a/drivers/clocksource/timer-tegra.c
>>>>> +++ b/drivers/clocksource/timer-tegra.c
>>>>> @@ -318,7 +318,7 @@ static int __init tegra_init_timer(struct device_node *np, bool tegra20,
>>>>>  	sched_clock_register(tegra_read_sched_clock, 32, TIMER_1MHz);
>>>>>  
>>>>>  	ret = clocksource_mmio_init(timer_reg_base + TIMERUS_CNTR_1US,
>>>>> -				    "timer_us", TIMER_1MHz, 300, 32,
>>>>> +				    "timer_us", TIMER_1MHz, rating, 32,
>>>>>  				    clocksource_mmio_readl_up);
>>>>>  	if (ret)
>>>>>  		pr_err("failed to register clocksource: %d\n", ret);
>>>>>
>>>>
>>>> Looks good. Although, could you please clarify whether arch-timer stops on T210 when CPU
>>>> enters deepest (powerdown) idle state? I'm starting to lose track a bit already. Because
>>>> if arch-timer stops in the deepest idle state, then it's a bit odd that Joseph didn't add
>>>> the clocksource for T210 in the first place and v5.1 probably shouldn't work well because
>>>> of that already.
>>>
>>> Yes, the architected timer doesn't work across an SC7 (which is what the
>>> deepest idle state is called on Tegra210) transition, hence why we can't
>>> use it as a suspend clocksource. I actually sent out a patch to do that,
>>> earlier.
>>>
>>> And yes, it's entirely possible that v5.1 doesn't work in this regard,
>>> but we're not noticing that because we don't have suspend/resume support
>>> for Tegra210 anyway. There are a couple of missing pieces that we need
>>> in order to make it work.
>>>
>>> This change in particular is only going to affect the CPU idle state
>>> (CC7). Since the architected timer doesn't survive that either, we need
>>> the Tegra timer to be preferred over the architected timer for normal
>>> operation.
>>>
>>> All of these issues go away on Tegra186 and later, where the architected
>>> timer is in an always-on partition and has a PLL that remains on during
>>> SC7 (and CC7).
>>
>> Thank you very much for the clarification. But then what about the
>> sched_clock? I suppose sched_clock will suffer on T210 as well and it's
>> a bit trickier case because apparently arch-timer always wins since it
>> has a higher precision. I guess the best solution will be to just bail
>> out from arch-timer's driver probe in a case of T210.
>>
>> if (of_machine_is_compatible("nvidia,tegra210"))
>> 	return 0.
> 
> I don't think there's any issue with the scheduler clock on Tegra210.
> Before the CPU can be turned off, all tasks scheduled on that CPU have
> to be migrated to another CPU, right? Conversely, before any tasks can
> be scheduled on a CPU that CPU needs to be brought online, at which
> point the architected timer should work fine again.

Is SC7 a CPU-idle state that cpuidle driver may enter or it's a
system-wide suspend state? It's still not clear to me.