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

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Date:	Wed, 31 Jul 2013 22:49:06 +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.

Can't you use the maxcpus=1 option to ensure the system to boot up ?


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