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

[Dmitry Osipenko]

14.06.2019 16:22, Thierry Reding ÐÐÑÐÑ:
> On Fri, Jun 14, 2019 at 03:24:07PM +0300, Dmitry Osipenko wrote:
>> 14.06.2019 13:47, Thierry Reding ÐÐÑÐÑ:
>>> From: Thierry Reding <treding@xxxxxxxxxx>
>>>
>>> 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@xxxxxxxxxx>
>>> ---
>>>  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.