linux-kernel - Re: [PATCH 4/7] clocksource/cadence_ttc: Adjust interval in clock notifier

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Date:	Fri, 22 Nov 2013 10:06:34 -0800
From:	Sören Brinkmann <soren.brinkmann@...inx.com>
To:	Daniel Lezcano <daniel.lezcano@...aro.org>
CC:	Rob Herring <rob.herring@...xeda.com>,
	Pawel Moll <pawel.moll@....com>,
	Mark Rutland <mark.rutland@....com>,
	Stephen Warren <swarren@...dotorg.org>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Russell King <linux@....linux.org.uk>,
	Michal Simek <michal.simek@...inx.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	<linux-kernel@...r.kernel.org>,
	<linux-arm-kernel@...ts.infradead.org>,
	<devicetree@...r.kernel.org>
Subject: Re: [PATCH 4/7] clocksource/cadence_ttc: Adjust interval in clock
 notifier

Hi Daniel,

On Tue, Nov 12, 2013 at 05:29:45PM +0100, Daniel Lezcano wrote:
> On 11/08/2013 10:21 PM, Soren Brinkmann wrote:
> >The clockevent has to be reprogrammed if the timer's input
> >clock frequency changes and the timer is in periodic mode, in order to
> >maintain the correct timer interval.
> >
> >Signed-off-by: Soren Brinkmann <soren.brinkmann@...inx.com>
> >---
> >  drivers/clocksource/cadence_ttc_timer.c | 4 ++++
> >  1 file changed, 4 insertions(+)
> >
> >diff --git a/drivers/clocksource/cadence_ttc_timer.c b/drivers/clocksource/cadence_ttc_timer.c
> >index a92350b55d32..68a336038d8f 100644
> >--- a/drivers/clocksource/cadence_ttc_timer.c
> >+++ b/drivers/clocksource/cadence_ttc_timer.c
> >@@ -338,6 +338,10 @@ static int ttc_rate_change_clockevent_cb(struct notifier_block *nb,
> >  		/* update cached frequency */
> >  		ttc->freq = ndata->new_rate;
> >
> >+		if (ttcce->ce.mode == CLOCK_EVT_MODE_PERIODIC)
> >+			ttc_set_interval(ttc, DIV_ROUND_CLOSEST(ttc->freq,
> >+						PRESCALE * HZ));
> >+
> 
> Couldn't be racy ?
I guess you're right. What would be the proper way to resolve it? If we
look at a little more code things look like this:

	local_irq_save(flags);                                           
	clockevents_update_freq(&ttcce->ce,                              
	                ndata->new_rate / PRESCALE);                     
	local_irq_restore(flags);                                        
	                                                                 
	/* update cached frequency */                                    
	ttc->freq = ndata->new_rate;                                     
	                                                                 
	if (ttcce->ce.mode == CLOCK_EVT_MODE_PERIODIC)                   
	        ttc_set_interval(ttc, DIV_ROUND_CLOSEST(ttc->freq,       
	                                PRESCALE * HZ));

Would it be enough to move the ttc_set_interval() call within the
section where local IRQs are disabled? I have the feeling for this timer
the local_irq_save() may in general not be the right option since the
timer is not CPU local.
Should we probably change this do irq_disable(ttcce->ce.irq)?

	Thanks,
	Sören


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