linux-kernel - Re: timers & suspend

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Date:	Thu, 3 Jul 2014 09:09:55 -0700
From:	Sören Brinkmann <soren.brinkmann@...inx.com>
To:	Daniel Lezcano <daniel.lezcano@...aro.org>
CC:	Thomas Gleixner <tglx@...utronix.de>,
	<linux-kernel@...r.kernel.org>,
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: timers & suspend

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).

> 
> >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.

	Sören
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