linux-kernel - Re: [PATCH] clockevents: Leave broadcast device shtudown only if the current device is always running.

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Date:	Wed, 11 Apr 2012 14:01:12 -0700
From:	Suresh Siddha <suresh.b.siddha@...el.com>
To:	Feng Tang <feng.tang@...el.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>, santosh.shilimkar@...com,
	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 Thu, 2012-04-12 at 00:04 +0800, Feng Tang wrote:
> Thanks for the explaination, I understand the root cause now.
> 
> And regarding the original commit 77b0d60c5
>     clockevents: Leave the broadcast device in shutdown mode when not needed
> 
> @@ -575,11 +575,15 @@ void tick_broadcast_switch_to_oneshot(void)
>  	unsigned long flags;
>  
>  	raw_spin_lock_irqsave(&tick_broadcast_lock, flags);
> +	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);
> +
> +end:
>  	raw_spin_unlock_irqrestore(&tick_broadcast_lock, flags);
>  }
>  
> The tick_broadcast_switch_to_oneshot() get called mostly at boot time or
> during cpu hotplug. And when it is called at boot time, the tick_broadcast_mask
> is alwasys 0 at the point,

On the correctly working platforms, that is the case. But there are
cases where APIC timer calibration fails, in which case APIC is marked
as CLOCK_EVT_FEAT_DUMMY and in this scenario, broadcast mask will be
non-zero.

>  thus this check is always true and skip the
> following oneshot setting up for broadcast device, 

On most platforms yes, but there are some buggy exceptions which the
code is trying to handle.

thanks,
suresh

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