Re: [PATCH v5 03/11] arm-cci: Group writes to counter

[Suzuki K. Poulose]

On 11/01/16 10:44, Mark Rutland wrote:
> On Tue, Jan 05, 2016 at 10:51:47AM +0000, Suzuki K. Poulose wrote:
> > On 04/01/16 19:03, Mark Rutland wrote:
> > > On Mon, Jan 04, 2016 at 11:54:42AM +0000, Suzuki K. Poulose wrote:
> > > > Add a helper to group the writes to PMU counter, this will be
> > > > used to delay setting the event period to pmu::pmu_enable()
> >
> > > > +/* Write a value to a given set of counters */
> > > > +static void __pmu_write_counters(struct cci_pmu *cci_pmu, unsigned long *mask, u32 value)
> > > > +{
> > > > +	int i;
> > > > +
> > > > +	for_each_set_bit(i, mask, cci_pmu->num_cntrs)
> > > > +		__pmu_write_counter(cci_pmu, value, i);
> > > > +}
> > >
> > > I don't understand this as-is. Why do all the counters have the same
> > > value?
> >
> > The only value we write to the counters is the period. This routine writes
> > a given value to a set of counters specified by the mask (not to be confused
> > with the PMU->hw_events->mask). This will help to group the writes to the counters,
> > especially since preparatory steps to write to a single counter itself is costly.
> > So, we do all the preparation only once for a batch of counters.
> >
> > The other option is to use hw_events->prev_count (which should be set before calling
> > the function) for each counter specified in the mask. I am fine with either of the
> > two.
>
> I think this would be clearer using prev_count.
>
> I guess it doesn't matter since we won't support sampling, but it would
> match the shape of other PMU drivers.

OK, will use that.

Thanks
Suzuki