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

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Message-ID: <fba9791a-8e73-398b-5851-689dc1d769f0@gmail.com>
Date:   Fri, 14 Jun 2019 17:06:48 +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 17:02, Dmitry Osipenko пишет:
> 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.
> 

Ah, looks like I see now. So CC7 (CPU idle state) also affects the
arch-timer (like SC7) and hence scheduler clock will be stopped while it
shouldn't, which doesn't sound good.