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

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Message-ID: <20160111104407.GD6499@leverpostej>
Date:	Mon, 11 Jan 2016 10:44:07 +0000
From:	Mark Rutland <mark.rutland@....com>
To:	"Suzuki K. Poulose" <Suzuki.Poulose@....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 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.

> >>+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.

Ok. Either way is fine.

Thanks,
Mark.