Date:	Tue, 17 Apr 2012 13:44:37 +0530
From:	Santosh Shilimkar <santosh.shilimkar@...com>
To:	Suresh Siddha <suresh.b.siddha@...el.com>
CC:	tglx@...utronix.de, linux-kernel@...r.kernel.org,
	johnstul@...ibm.com
Subject: Re: [PATCH] clockevents: Leave broadcast device shtudown only if
 the current device is always running.

On Tuesday 17 April 2012 03:33 AM, Suresh Siddha wrote:
> On Sun, 2012-04-15 at 18:27 +0530, Santosh Shilimkar wrote:
>> On Tuesday 10 April 2012 04:11 AM, Suresh Siddha wrote:
>>> @@ -575,10 +575,12 @@ void tick_broadcast_switch_to_oneshot(void)
>>>  	unsigned long flags;
>>>  
>>>  	raw_spin_lock_irqsave(&tick_broadcast_lock, flags);
>>> +
>>> +	tick_broadcast_device.mode = TICKDEV_MODE_ONESHOT;
>>> +
>>>  	if (cpumask_empty(tick_get_broadcast_mask()))
>>>  		goto end;
>>>  
>>> -	tick_broadcast_device.mode = TICKDEV_MODE_ONESHOT;
>>>  	bc = tick_broadcast_device.evtdev;
>>>  	if (bc)
>>>  		tick_broadcast_setup_oneshot(bc);
>>>
>>>
>> Last time when I tried your patch, some how he ethernet IRQ masked
>> the issue.
>>
>> The problem is still as before. If you make a minimal kernel
>> configuration and possibly have only local timer, broadcast timer
>> and say console device as CPU wakeup sources, you should hang in
>> the boot process itself(Assuming CPUIDLE driver is built-in)
>>
>> The main issue is, you are not letting the one time setup
>> needed for the broadcast device. Even with above change,
>> the 'tick_broadcast_device.evtdev' is still in SHUTDOWN
>> mode and the event handler is pointing to clockevents_handle_noop()
> 
> On x86, idle driver (like drivers/idle/intel_idle.c:
> __setup_broadcast_timer()) calls clockevents_notify() with
> CLOCK_EVT_NOTIFY_BROADCAST_ON, during which we do
> tick_broadcast_setup_oneshot(). See tick_do_broadcast_on_off() which
> does this. That should setup the evtdev handler, mode etc.
> 
> And during the next CLOCK_EVT_NOTIFY_BROADCAST_ENTER we program the next
> broadcast event.
>
I see.

>> So the very first 'CLOCK_EVT_NOTIFY_BROADCAST_ENTER' call won't
>> program the broadcast device with next event and CPU won't wakeup
>> from low power. if you are lucky and have some other wakeup source,
>> system will move further and eventually the handler and
>> mode of the broad-cast device get set correctly.
> 
> I am confused. Can you elaborate on how on the next spurious wakeup
> event, broad-cast device handler, mode is set?
> 
No it doesn't set it. I was wrong in quoting that. The system was
working because of other wakeup sources. I checked the broadcast
events and they were not happening.

> 
> Anyways, can you add CLOCK_EVT_NOTIFY_BROADCAST_ON/OFF notifications in
> your idle driver to see if it addresses the problem. That is the correct
> thing to do here.
> 
Before your commit 77b0d60c{clockevents: Leave the broadcast device..},
my idle driver was working without CLOCK_EVT_NOTIFY_BROADCAST_ON
notifier. Now it's clear that it was relying on the default
'tick_broadcast_setup_oneshot()' which was happening during boot.

And ofcource notifying explicitly with BROADCAST_ON does the
tick setup. Will update my idle driver accordingly but can
you please explain me why the proposed patch [1] is not correct
approach ? It should be also do what you intended to do
with minimal change, right ?

Regards
Santosh
[1] https://lkml.org/lkml/2012/4/9/13

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