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

Suzuki K. Poulose
2016-01-11T10:48:23Z

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