Enable arm_global_timer for Zynq brakes boot

[Sören Brinkmann]

On Thu, Aug 01, 2013 at 07:29:12PM +0200, Daniel Lezcano wrote:
> On 08/01/2013 01:38 AM, Sören Brinkmann wrote:
> > On Thu, Aug 01, 2013 at 01:01:27AM +0200, Daniel Lezcano wrote:
> >> On 08/01/2013 12:18 AM, Sören Brinkmann wrote:
> >>> On Wed, Jul 31, 2013 at 11:08:51PM +0200, Daniel Lezcano wrote:
> >>>> On 07/31/2013 10:58 PM, Sören Brinkmann wrote:
> >>>>> On Wed, Jul 31, 2013 at 10:49:06PM +0200, Daniel Lezcano wrote:
> >>>>>> On 07/31/2013 12:34 AM, Sören Brinkmann wrote:
> >>>>>>> On Tue, Jul 30, 2013 at 10:47:15AM +0200, Daniel Lezcano wrote:
> >>>>>>>> On 07/30/2013 02:03 AM, Sören Brinkmann wrote:
> >>>>>>>>> Hi Daniel,
> >>>>>>>>>
> >>>>>>>>> On Mon, Jul 29, 2013 at 02:51:49PM +0200, Daniel Lezcano wrote:
> >>>>>>>>> (snip)
> >>>>>>>>>>
> >>>>>>>>>> the CPUIDLE_FLAG_TIMER_STOP flag tells the cpuidle framework the local
> >>>>>>>>>> timer will be stopped when entering to the idle state. In this case, the
> >>>>>>>>>> cpuidle framework will call clockevents_notify(ENTER) and switches to a
> >>>>>>>>>> broadcast timer and will call clockevents_notify(EXIT) when exiting the
> >>>>>>>>>> idle state, switching the local timer back in use.
> >>>>>>>>>
> >>>>>>>>> I've been thinking about this, trying to understand how this makes my
> >>>>>>>>> boot attempts on Zynq hang. IIUC, the wrongly provided TIMER_STOP flag
> >>>>>>>>> would make the timer core switch to a broadcast device even though it
> >>>>>>>>> wouldn't be necessary. But shouldn't it still work? It sounds like we do
> >>>>>>>>> something useless, but nothing wrong in a sense that it should result in
> >>>>>>>>> breakage. I guess I'm missing something obvious. This timer system will
> >>>>>>>>> always remain a mystery to me.
> >>>>>>>>>
> >>>>>>>>> Actually this more or less leads to the question: What is this
> >>>>>>>>> 'broadcast timer'. I guess that is some clockevent device which is
> >>>>>>>>> common to all cores? (that would be the cadence_ttc for Zynq). Is the
> >>>>>>>>> hang pointing to some issue with that driver?
> >>>>>>>>
> >>>>>>>> If you look at the /proc/timer_list, which timer is used for broadcasting ?
> >>>>>>>
> >>>>>>> So, the correct run results (full output attached).
> >>>>>>>
> >>>>>>> The vanilla kernel uses the twd timers as local timers and the TTC as
> >>>>>>> broadcast device:
> >>>>>>> 	Tick Device: mode:     1                                                         
> >>>>>>> 	Broadcast device  
> >>>>>>> 	Clock Event Device: ttc_clockevent
> >>>>>>>
> >>>>>>> When I remove the offending CPUIDLE flag and add the DT fragment to
> >>>>>>> enable the global timer, the twd timers are still used as local timers
> >>>>>>> and the broadcast device is the global timer:
> >>>>>>> 	Tick Device: mode:     1                                                         
> >>>>>>> 	Broadcast device                                                                 
> >>>>>>> 	Clock Event Device: arm_global_timer
> >>>>>>>
> >>>>>>> Again, since boot hangs in the actually broken case, I don't see way to
> >>>>>>> obtain this information for that case.
> >>>>>>
> >>>>>> Can't you use the maxcpus=1 option to ensure the system to boot up ?
> >>>>>
> >>>>> Right, that works. I forgot about that option after you mentioned, that
> >>>>> it is most likely not that useful.
> >>>>>
> >>>>> Anyway, this are those sysfs files with an unmodified cpuidle driver and
> >>>>> the gt enabled and having maxcpus=1 set.
> >>>>>
> >>>>> /proc/timer_list:
> >>>>> 	Tick Device: mode:     1
> >>>>> 	Broadcast device
> >>>>> 	Clock Event Device: arm_global_timer
> >>>>> 	 max_delta_ns:   12884902005
> >>>>> 	 min_delta_ns:   1000
> >>>>> 	 mult:           715827876
> >>>>> 	 shift:          31
> >>>>> 	 mode:           3
> >>>>
> >>>> Here the mode is 3 (CLOCK_EVT_MODE_ONESHOT)
> >>>>
> >>>> The previous timer_list output you gave me when removing the offending
> >>>> cpuidle flag, it was 1 (CLOCK_EVT_MODE_SHUTDOWN).
> >>>>
> >>>> Is it possible you try to get this output again right after onlining the
> >>>> cpu1 in order to check if the broadcast device switches to SHUTDOWN ?
> >>>
> >>> How do I do that? I tried to online CPU1 after booting with maxcpus=1
> >>> and that didn't end well:
> >>> 	# echo 1 > online && cat /proc/timer_list 
> >>
> >> Hmm, I was hoping to have a small delay before the kernel hangs but
> >> apparently this is not the case... :(
> >>
> >> I suspect the global timer is shutdown at one moment but I don't
> >> understand why and when.
> >>
> >> Can you add a stack trace in the "clockevents_shutdown" function with
> >> the clockevent device name ? Perhaps, we may see at boot time an
> >> interesting trace when it hangs.
> > 
> > I did this change:
> > 	diff --git a/kernel/time/clockevents.c b/kernel/time/clockevents.c
> > 	index 38959c8..3ab11c1 100644
> > 	--- a/kernel/time/clockevents.c
> > 	+++ b/kernel/time/clockevents.c
> > 	@@ -92,6 +92,8 @@ void clockevents_set_mode(struct clock_event_device *dev,
> > 	  */
> > 	 void clockevents_shutdown(struct clock_event_device *dev)
> > 	 {
> > 	+       pr_info("ce->name:%s\n", dev->name);
> > 	+       dump_stack();
> > 	        clockevents_set_mode(dev, CLOCK_EVT_MODE_SHUTDOWN);
> > 	        dev->next_event.tv64 = KTIME_MAX;
> > 	 }
> > 
> > It is hit a few times during boot, so I attach a full boot log. I really
> > don't know what to look for, but I hope you can spot something in it. I
> > really appreciate you taking the time.
> 
> Thanks for the traces.

Sure.

> 
> If you try without the ttc_clockevent configured in the kernel (but with
> twd and gt), does it boot ?

Absence of the TTC doesn't seem to make any difference. It hangs at the
same location.

	Thanks,
	Sören