[PATCH 1/5] arm: provide a mechanism to reserve performance counters

[Jamie Iles]

On Thu, Dec 17, 2009 at 04:14:22PM -0000, Will Deacon wrote:
> > +const struct pmu_irqs *
> > +reserve_pmu(void)
> > +{
> > +	int ret = down_trylock(&pmu_mutex) ? -EBUSY : 0;
> > +
> > +	return ret ? ERR_PTR(ret) : &pmu_irqs;
> > +}
> 
> I think it would be sensible to return an error (-ENODEV) if
> pmu_irqs.num_irqs == 0. Not doing so can cause applications
> to fail silently when they are running on unsupported boards.
I did think about that, but when the interrupts were in oprofile, it
didn't regard this as an error so I kept this the same.

In the perf events code, we check that num_irqs is >= 1. You _could_ use
the pmu as free running counters that you just periodically sample and
wouldn't need an interrupt so I thought it best to leave this error
checking to the user.

Cheers,

Jamie