linux-kernel - Re: Enable arm_global_timer for Zynq brakes boot

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Message-ID: <51F8DA38.2060508@linaro.org>
Date:	Wed, 31 Jul 2013 11:34:48 +0200
From:	Daniel Lezcano <daniel.lezcano@...aro.org>
To:	Sören Brinkmann <soren.brinkmann@...inx.com>
CC:	Stephen Boyd <sboyd@...eaurora.org>,
	John Stultz <john.stultz@...aro.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Stuart Menefy <stuart.menefy@...com>,
	Russell King <linux@....linux.org.uk>,
	Michal Simek <michal.simek@...inx.com>,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: Enable arm_global_timer for Zynq brakes boot

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.

Is it possible the global timer driver simply does not work ? So when
the cpuidle driver switches to it, the system stays stuck with no interrupt.

Removing the CPUIDLE_FLAG_TIMER_STOP prevents to use the broadcast timer
(aka arm global timer), so the problem does not appear.

And when the C3STOP feature flag is added to the global timer, this one
can't be a broadcast timer, so another clock is selected for that (I
guess cadence_ttc). So again the problem does not appear.

I am more and more convinced the problem is not coming from the cpuidle
driver. The cpuidle flag has just spotted a problem somewhere else and I
suspect the arm_global_timer is not working for zynq.


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