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

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Date:	Mon, 12 Aug 2013 18:53:10 +0200
From:	Daniel Lezcano <daniel.lezcano@...aro.org>
To:	Stephen Boyd <sboyd@...eaurora.org>
CC:	srinivas.kandagatla@...com,
	S??ren Brinkmann <soren.brinkmann@...inx.com>,
	Russell King <linux@....linux.org.uk>,
	Michal Simek <michal.simek@...inx.com>,
	linux-kernel@...r.kernel.org, Stuart Menefy <stuart.menefy@...com>,
	John Stultz <john.stultz@...aro.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: Enable arm_global_timer for Zynq brakes boot

On 08/12/2013 06:23 PM, Stephen Boyd wrote:
> On 08/12/13 03:53, Daniel Lezcano wrote:
>> On 08/09/2013 07:27 PM, Stephen Boyd wrote:
>>> On 08/09, Daniel Lezcano wrote:
>>>> yes, but at least the broadcast mechanism should send an IPI to cpu0 to
>>>> wake it up, no ? As Stephen stated this kind of configuration should has
>>>> never been tested before so the tick broadcast code is not handling this
>>>> case properly IMHO.
>>>>
>>> If you have a per-cpu tick device that isn't suffering from
>>> FEAT_C3_STOP why wouldn't you use that for the tick versus a
>>> per-cpu tick device that has FEAT_C3_STOP? It sounds like there
>>> is a bug in the preference logic or you should boost the rating
>>> of the arm global timer above the twd. Does this patch help? It
>>> should make the arm global timer the tick device and whatever the
>>> cadence timer you have into the broadcast device.
>>>
>>> ---8<----
>>> diff --git a/kernel/time/tick-broadcast.c b/kernel/time/tick-broadcast.c
>>> index 218bcb5..d3539e5 100644
>>> --- a/kernel/time/tick-broadcast.c
>>> +++ b/kernel/time/tick-broadcast.c
>>> @@ -77,6 +77,9 @@ static bool tick_check_broadcast_device(struct clock_event_device *curdev,
>>>  	    !(newdev->features & CLOCK_EVT_FEAT_ONESHOT))
>>>  		return false;
>>>  
>>> +	if (cpumask_equal(newdev->cpumask, cpumask_of(smp_processor_id())))
>>> +		return false;
>> Yes, that makes sense to prevent local timer devices to be a broadcast one.
>>
>> In the case of the arm global timer, each cpu will register their timer,
>> so the test will be ok but is it possible the cpu0 registers the timers
>> for the other cpus ?
> 
> As far as I know every tick device has to be registered on the CPU that
> it will be used on. See tick_check_percpu().

Ah, ok I see. Thx for the pointer.

>>>  	return !curdev || newdev->rating > curdev->rating;
>>>  }
>>>  
>>> diff --git a/kernel/time/tick-common.c b/kernel/time/tick-common.c
>>> index 64522ec..1628b9f 100644
>>> --- a/kernel/time/tick-common.c
>>> +++ b/kernel/time/tick-common.c
>>> @@ -245,6 +245,15 @@ static bool tick_check_preferred(struct clock_event_device *curdev,
>>>  	}
>>>  
>>>  	/*
>>> +	 * Prefer tick devices that don't suffer from FEAT_C3_STOP
>>> +	 * regardless of their rating
>>> +	 */
>>> +	if (curdev && cpumask_equal(curdev->cpumask, newdev->cpumask) &&
>>> +	    !(newdev->features & CLOCK_EVT_FEAT_C3STOP) &&
>>> +	    (curdev->features & CLOCK_EVT_FEAT_C3STOP))
>>> +		return true;
>> That sounds reasonable, but what is the acceptable gap between the
>> ratings ? I am wondering if there isn't too much heuristic in the tick
>> code...
> 
> Yes I wonder if we should just change the ratings of the clockevents.
> But it feels to me like the rating should only matter if the two are
> equal in features. Otherwise we can use the features to determine what
> we want.

Is it desirable for real time system ? (I am not expert in this area, so
may be this question has no sense :)


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