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

From: Suzuki K. Poulose
Date: Mon Jan 11 2016 - 05:48:30 EST

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

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

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.