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

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Date:	Tue, 5 Jan 2016 10:51:47 +0000
From:	"Suzuki K. Poulose" <Suzuki.Poulose@....com>
To:	Mark Rutland <mark.rutland@....com>
Cc:	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	arm@...nel.org, punit.agrawal@....com, peterz@...radead.org
Subject: Re: [PATCH v5 03/11] arm-cci: Group writes to counter

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.

>
>> +static void __maybe_unused
>> +pmu_write_counters(struct cci_pmu *cci_pmu, unsigned long *mask, u32 value)
>> +{
>> +	__pmu_write_counters(cci_pmu, mask, value);
>> +}
>
> Why are these not just one function for now?

Yes, this could be just one function for now, until we introduce the hooks. This was
a written to avoid another refactoring in the later patch.

Thanks
Suzuki
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