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

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Date:	Mon, 12 Aug 2013 12:53:56 +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 08/09/2013 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 ?

>  	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...


> +
> +	/*
>  	 * Use the higher rated one, but prefer a CPU local device with a lower
>  	 * rating than a non-CPU local device
>  	 */
> 


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