linux-kernel - Re: [PATCH v2] clocksource: arm_global

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Date:	Fri, 20 Nov 2015 21:28:07 +0200
From:	Grygorii Strashko <grygorii.strashko@...com>
To:	John Stultz <john.stultz@...aro.org>
CC:	santosh shilimkar <santosh.shilimkar@...cle.com>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Srinivas Kandagatla <srinivas.kandagatla@...il.com>,
	Maxime Coquelin <maxime.coquelin@...com>,
	lkml <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>, <kernel@...inux.com>,
	"linux-omap@...r.kernel.org" <linux-omap@...r.kernel.org>,
	Arnd Bergmann <arnd@...db.de>, Felipe Balbi <balbi@...com>,
	Tony Lindgren <tony@...mide.com>,
	Santosh Shilimkar <ssantosh@...nel.org>,
	Marc Zyngier <marc.zyngier@....com>
Subject: Re: [PATCH v2] clocksource: arm_global_timer: fix suspend resume

On 11/20/2015 09:09 PM, John Stultz wrote:
> On Fri, Nov 20, 2015 at 10:35 AM, Grygorii Strashko
> <grygorii.strashko@...com> wrote:
>> Hi Santosh,
>>
>> On 11/20/2015 07:23 PM, santosh shilimkar wrote:
>>> + Thomas, Marc
>>>
>>> On 11/20/2015 5:57 AM, Grygorii Strashko wrote:
>>>> Now the System stall is observed on TI AM437x based board
>>>> (am437x-gp-evm) during resuming from System suspend when ARM Global
>>>> timer is selected as clocksource device - SysRq are working, but
>>>> nothing else. The reason of stall is that ARM Global timer loses its
>>>> contexts.
>>>>
>>>> The reason of stall is that ARM Global timer loses its contexts during
>>>> System suspend:
>>>>      GT_CONTROL.TIMER_ENABLE = 0 (unbanked)
>>>>      GT_COUNTERx = 0
>>>>
>>>> Hence, update ARM Global timer driver to reflect above behaviour
>>>> - re-enable ARM Global timer on resume GT_CONTROL.TIMER_ENABLE = 1
>>>> - ensure clocksource and clockevent devices have coresponding flags
>>>>     (CLOCK_SOURCE_SUSPEND_NONSTOP and CLOCK_EVT_FEAT_C3STOP) set
>>>>     depending on presence of "always-on" DT property.
>>>>
>>> Something which loses context in low power states can't be
>>> called "always-on"
>>
>> Sry, it's kinda new area for me and I could make mistakes.
>>
>> While working on this patch I've:
>>   - re-used implementation from ARM arch timer
>> commit 82a5619410d4c4df65c04272db198eca5a867c18
>> Author: Lorenzo Pieralisi <lorenzo.pieralisi@....com>
>> Date:   Tue Apr 8 10:04:32 2014 +0100
>>
>>      clocksource: arch_arm_timer: Fix age-old arch timer C3STOP detection issue
>>
>>
>> - and followed timekeeping.txt:
>> "Typically the clock source is a monotonic, atomic counter which will provide
>>   n bits which count from 0 to 2^(n-1) and then wraps around to 0 and start over.
>> It will ideally NEVER stop ticking as long as the system is running. It
>> may stop during system suspend."
>> ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>>
>> And that exactly my use-case: I'd like to use ARM GT as clocksource
>> and with CPUIdle = n. But System suspend has to be allowed.
>>
>>
>>>
>>> Also if the clock-soucre can't tick in the low power states
>>> then that device shouldn't be used as a clock-source.
>>
>> Agree. clocksource can't (except with suspend). Have I missed something?
> 
> I think the point Stantosh is making is that if the clocksource stops
> in suspend (which is allowed) you should not be setting
> CLOCK_SOURCE_SUSPEND_NONSTOP (which promises the clocksource doesn't
> stop in suspend, so it can be used for suspend timing as well).
> 

Ok. Thanks. I got it now. Adding CLOCK_SOURCE_SUSPEND_NONSTOP is mistake.

> The contradictory part in your patch is that you're also setting the
> clockevent logic as  CLOCK_EVT_FEAT_C3STOP, which flags that the
> clockevent hardware might stop in low-power idle states (ie: not
> suspend, but while the system is running).  Usually hardware that
> halts in low-power mode (like the apic on some x86 machines) is not
> also used for timekeeping (instead we use the hpet/acpi there).

Sry, I've set CLOCK_EVT_FEAT_C3STOP if "always-on" = *false*

^ this, I thought, might be valid because it will stop in low-power idle states
  and "always-on" was expected to indicate case when CPUIdle = n (no CPU PM),
  same way as Lorenzo did.
  Would it be correct to just set it always?

and CLOCK_SOURCE_SUSPEND_NONSTOP if "always-on" = *true*

^ this part is mistake

> 
> So its unlikely that the hardware both stays running through suspend,
> but also might halt in idle. That would be "unique".
> 

Thanks a lot.


-- 
regards,
-grygorii
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