Subject: Enable arm_global_timer for Zynq brakes boot

From: [email-masked] (Daniel Lezcano)
Date: Thu, 01 Aug 2013 19:29:12 +0200

On 08/01/2013 01:38 AM, S?ren Brinkmann wrote:
> On Thu, Aug 01, 2013 at 01:01:27AM +0200, Daniel Lezcano wrote:
>> On 08/01/2013 12:18 AM, S?ren Brinkmann wrote:
>>> On Wed, Jul 31, 2013 at 11:08:51PM +0200, Daniel Lezcano wrote:
>>>> On 07/31/2013 10:58 PM, S?ren Brinkmann wrote:
>>>>> On Wed, Jul 31, 2013 at 10:49:06PM +0200, Daniel Lezcano wrote:
>>>>>> On 07/31/2013 12:34 AM, S?ren Brinkmann wrote:
>>>>>>> On Tue, Jul 30, 2013 at 10:47:15AM +0200, Daniel Lezcano wrote:
>>>>>>>> On 07/30/2013 02:03 AM, S?ren 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.

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

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