timers & suspend

[Sören Brinkmann]

On Thu, 2014-07-03 at 07:46PM +0200, Daniel Lezcano wrote:
> On 07/03/2014 07:40 PM, Sören Brinkmann wrote:
> >On Thu, 2014-07-03 at 07:26PM +0200, Daniel Lezcano wrote:
> >>On 07/03/2014 06:09 PM, Sören Brinkmann wrote:
> >>>On Thu, 2014-07-03 at 02:21PM +0200, Daniel Lezcano wrote:
> >>>>On 06/30/2014 08:39 PM, Sören 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.
> >I tested on the current master branch. I see TTC interrupts on CPU0 and
> >timer_list shows the TTC to be the broadcast device. Removing the TTC
> >nodes from my DT results in boot hanging - shortly after cpuidle is
> >started.
> >
> >Removing cpuidle from my kernel makes the system boot. And it has no
> >broadcast device anymore.
> 
> You can safely remove the CPUIDLE_FLAG_TIMER_STOP in the cpuidle
> drivers and I am pretty sure it will boot without problem.
Yes, that works.

	Thanks,
	Sören