Re: Enable arm_global_timer for Zynq brakes boot

[Mark Rutland <mark.rutland@arm.com>]

On Thu, Aug 08, 2013 at 06:11:26PM +0100, Sören Brinkmann wrote:
> Hi Daniel,
> 
> On Thu, Aug 01, 2013 at 07:48:04PM +0200, Daniel Lezcano wrote:
> > On 08/01/2013 07:43 PM, Sören Brinkmann wrote:
> > > 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.
> >
> > Ok, IMO there is a problem with the broadcast device registration (may
> > be vs twd).
> 
> I have an idea, but no real evidence to prove it:
> Some of the registers in the arm_global_timer are banked per CPU. I.e.
> some code must be executed on the CPU the timer is associated with
> (struct clock_event_device.cpumask) to have the intended effect
> As far as I can tell, there is no guarantee, that the set_mode()
> and program_next_event() calls execute on the correct CPU.

I believe the core clockevents code enforces that, or all other percpu
clockevent_device drivers would be horrifically broken.

Thanks,
Mark.

> If this was correct, shutting down the timer for the CPU entering
> idle might actually shut down the timer for the running CPU, if
> set_mode() executes on the CPU which is _not_ about to enter idle.
> 
> I tried to prove this by adding some really ugly smp_call_any() wrappers
> in kernel/time/clockevents.c for the calls to set_mode() and
> program_net_event() but that ends in all kinds of dead locks.
> 
>         Sören
> 
> 
> 
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