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

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Message-ID: <068436c6-ff98-428f-8875-bb1c6f86466b@TX2EHSMHS008.ehs.local>
Date:	Wed, 31 Jul 2013 13:58:52 -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 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
	 next_event:     108080000000 nsecs
	 set_next_event: gt_clockevent_set_next_event
	 set_mode:       gt_clockevent_set_mode
	 event_handler:  tick_handle_oneshot_broadcast
	 retries:        0
	
	tick_broadcast_mask: 00000001
	tick_broadcast_oneshot_mask: 00000000
	
	Tick Device: mode:     1
	Per CPU device: 0
	Clock Event Device: local_timer
	 max_delta_ns:   12884902005
	 min_delta_ns:   1000
	 mult:           715827876
	 shift:          31
	 mode:           3
	 next_event:     106900000000 nsecs
	 set_next_event: twd_set_next_event
	 set_mode:       twd_set_mode
	 event_handler:  hrtimer_interrupt
	 retries:        0

# cat /proc/interrupts 
	           CPU0       
	 27:        252       GIC  27  gt
	 29:        626       GIC  29  twd
	 43:          0       GIC  43  ttc_clockevent
	 82:        410       GIC  82  xuartps
	IPI0:          0  CPU wakeup interrupts
	IPI1:          0  Timer broadcast interrupts
	IPI2:          0  Rescheduling interrupts
	IPI3:          0  Function call interrupts
	IPI4:          0  Single function call interrupts
	IPI5:          0  CPU stop interrupts
	Err:          0


	Sören


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