linux-kernel - Re: [patch/rfc] perf on raspberry-pi without overflow interrupt

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Date:	Thu, 16 Jan 2014 11:13:29 -0500 (EST)
From:	Vince Weaver <vincent.weaver@...ne.edu>
To:	Will Deacon <will.deacon@....com>
cc:	Vince Weaver <vincent.weaver@...ne.edu>,
	Chad Paradis <chad.paradis@...t.maine.edu>,
	Peter Zijlstra <peterz@...radead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Paul Mackerras <paulus@...ba.org>,
	Ingo Molnar <mingo@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	Stephane Eranian <eranian@...il.com>
Subject: Re: [patch/rfc] perf on raspberry-pi without overflow interrupt

On Wed, 15 Jan 2014, Will Deacon wrote:

> Hi Vince,
> 
> On Tue, Jan 14, 2014 at 04:42:13AM +0000, Vince Weaver wrote:
> > On Fri, 10 Jan 2014, Will Deacon wrote:
> > > In the absence of a core change, I think I'd rather have something like your
> > > second patch, but without the extra no_overflow_irq field (you can check the
> > > platform device, as I mentioned previously).
> > 
> > Something like the following?  It works on my rasp-pi, still waiting for
> > the compile to finish on the pandaboard so I haven't verified that the
> > has-working-interrupt case still works.
> 
> [...]
> 
> > diff --git a/arch/arm/kernel/perf_event.c b/arch/arm/kernel/perf_event.c
> > index bc3f2ef..e2c4aa2 100644
> > --- a/arch/arm/kernel/perf_event.c
> > +++ b/arch/arm/kernel/perf_event.c
> > @@ -397,7 +397,7 @@ __hw_perf_event_init(struct perf_event *event)
> >  	 */
> >  	hwc->config_base	    |= (unsigned long)mapping;
> >  
> > -	if (!hwc->sample_period) {
> > +	if (!is_sampling_event(event)) {
> >  		/*
> >  		 * For non-sampling runs, limit the sample_period to half
> >  		 * of the counter width. That way, the new counter value
> > @@ -407,6 +407,14 @@ __hw_perf_event_init(struct perf_event *event)
> >  		hwc->sample_period  = armpmu->max_period >> 1;
> >  		hwc->last_period    = hwc->sample_period;
> >  		local64_set(&hwc->period_left, hwc->sample_period);
> > +	} else {
> > +
> > +		/*
> > +		 * If we have no PMU interrupt we cannot sample.
> > +		 */
> > +		if (platform_get_irq(armpmu->plat_device, 0) < 0)
> > +			return -EOPNOTSUPP;
> 
> I think this should be <= 0, but apart from that this looks alright to me in
> the absence of a core change.

It's a shame platform_get_irq() doesn't have better documentation.  
I always forget what the result of the "is 0 a valid IRQ" flamewar was.
A grep through the kernel shows more or less an even split of using !, 
using <0, and using <=0.

I'll put together a patch with this change and send it off.  I'm also 
investigating the proper core change, but I'm guessing that's going to 
take a bit longer to get together, and it would be nice to have the 
rasp-pi counters working sooner rather than later.

Vince
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