timers & suspend

[daniel.lezcano@linaro.org (Daniel Lezcano)]

On 07/03/2014 06:09 PM, S?ren Brinkmann wrote:
> On Thu, 2014-07-03 at 02:21PM +0200, Daniel Lezcano wrote:
>> On 06/30/2014 08:39 PM, S?ren Brinkmann wrote:
>>> Hi,
>>>
>>> I'm currently working on suspend for Zynq and try to track down some
>>> spurious wakes. It looks like the spurious wakes are caused by timers,
>>> hence I was wondering whether there are any special requirements for
>>> timer drivers when it comes to suspend support or if I just missed
>>> something.
>>>
>>> Zynq sets the 'IRQCHIP_MASK_ON_SUSPEND' flag, which should mask all
>>> interrupts but the wake source. Reading through kernel/irq/pm.c
>>> indicates, that timer interrupts get some special treatment though.
>>> Therefore I implemented some suspend/resume callbacks for the
>>> cadence_ttc which disable and clear the timer's interrupts when going
>>> into suspend. That seems to mitigate the issue quite a bit, but I still
>>> saw spurious wakes - just a lot less often.
>>> Digging a little deeper revealed, the spurious wakes are caused by the
>>> ARM's smp_twd timer now. Given that that driver is probably used by a few
>>> more ARM platforms, I get the feeling that I'm missing something.
>>
>> Do you receive any interrupt from the cadence_ttc ? (/proc/interrupts)
>>
>> That's funny because I realize that you cadence ttc timer is never
>> used as there are the architected timers. The cadence ttc would be
>> only useful if there were an idle state powering down the smp_twd
>> timers but it is not the case on this board, IIUC.
> Yes they are used. They TTC is the only broadcast capable timer for
> Zynq. In my experience, I can not even boot without it (may have
> dependencies on CPUidle or something).

Actually the cpuidle driver is wrong. It assumes the state #1 will power 
off the different cores with their architected timers and then switch to 
the broadcast timer. But this one is not needed as we don't power down 
the core with the twd timers, so no need to switch to a backup timer device.

The implementation of the DDR self refresh idle state (incoming 
patchset) removes the cpu_pm notifiers + the flag TIMER_STOP. The result 
is 0 interrupts for ttc cadence timer. I removed in the dts the cadence 
ttc and my board booted without problem (it is a zynq 702).

Except I missed something, the cadence ttc is actually not used at all.

>>> It's probably worth mentioning that the suspend state in Zynq does not
>>> power off the CPU cores. It just asserts the resets on secondary cores
>>> and the primary one waits in wfi.
>>
>> Why do you need to reset them ?
> CPU0 is not reset, but all secondary cores are. That is as close to
> power off as we get and works well with hotplug.

Ok, I was not aware of this approach. Is there any patchset available in 
the net ?

Thanks

   -- Daniel

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