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

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Date:	Wed, 31 Jul 2013 09:43:15 -0700
From:	Sören Brinkmann <soren.brinkmann@...inx.com>
To:	Daniel Lezcano <daniel.lezcano@...aro.org>
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 Wed, Jul 31, 2013 at 11:34:48AM +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.
> 
> 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.

I made the following experiment:
I removed all TTC and twd nodes from my dts, leaving the gt as only
timer source and the system boots. Interestingly, in this case no
broadcast device is available.

/proc/timer_list (shortended):
	Tick Device: mode:     1
	Broadcast device
	Clock Event Device: <NULL>
	tick_broadcast_mask: 00000000
	tick_broadcast_oneshot_mask: 00000000
	
	Tick Device: mode:     1
	Per CPU device: 0
	Clock Event Device: arm_global_timer
	 max_delta_ns:   12884902005
	 min_delta_ns:   1000
	 mult:           715827876
	 shift:          31
	 mode:           3
	 next_event:     25654916518 nsecs
	 set_next_event: gt_clockevent_set_next_event
	 set_mode:       gt_clockevent_set_mode
	 event_handler:  hrtimer_interrupt
	 retries:        0
	
	Tick Device: mode:     1
	Per CPU device: 1
	Clock Event Device: arm_global_timer
	 max_delta_ns:   12884902005
	 min_delta_ns:   1000
	 mult:           715827876
	 shift:          31
	 mode:           3
	 next_event:     25660000000 nsecs
	 set_next_event: gt_clockevent_set_next_event
	 set_mode:       gt_clockevent_set_mode
	 event_handler:  hrtimer_interrupt
	 retries:        0

/proc/interrupts:
	# cat /proc/interrupts 
	           CPU0       CPU1       
	 27:       1483       1623       GIC  27  gt
	 82:        507          0       GIC  82  xuartps
	IPI0:          0          0  CPU wakeup interrupts
	IPI1:          0          0  Timer broadcast interrupts
	IPI2:       1211       1238  Rescheduling interrupts
	IPI3:          0          0  Function call interrupts
	IPI4:         42         51  Single function call interrupts
	IPI5:          0          0  CPU stop interrupts
	Err:          0


	Sören


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