[PATCH v6 10/12] arm-cci: CCI-500: Work around PMU counter writes

On Mon, Jan 25, 2016 at 11:21:15AM +0000, Suzuki K. Poulose wrote:

The CCI PMU driver sets the event counter to the half of the maximum
value(2^31) it can count before we start the counters via
pmu_event_set_period(). This is done to give us the best chance to
handle the overflow interrupt, taking care of extreme interrupt latencies.

However, CCI-500 comes with advanced power saving schemes, which
disables the clock to the event counters unless the counters are enabled to
count (PMCR.CEN). This prevents the driver from writing the period to the
counters before starting them.  Also, there is no way we can reset the
individual event counter to 0 (PMCR.RST resets all the counters, losing
their current readings). However the value of the counter is preserved and
could be read back, when the counters are not enabled.

So we cannot reliably use the counters and compute the number of events
generated during the sampling period since we don't have the value of the
counter at start.

This patch works around this issue by changing writes to the counter
with the following steps.

 1) Disable all the counters (remembering any counters which were enabled)
 2) Enable the PMU, now that all the counters are disabled.

 For each counter to be programmed, repeat steps 3-7
 3) Save the current event and program the target counter to count an
    invalid event, which by spec is guaranteed to not-generate any events.
 4) Enable the target counter.
 5) Write to the target counter.
 6) Disable the target counter
 7) Restore the event back on the target counter.

 8) Disable the PMU
 9) Restore the status of the all the counters

[...]

Steps 6 and 7 are the wrong way around here. They are correct in the
commit message and the code.

[...]

Let's just say:

	We choose an event which for CCI-500 is guaranteed not to count.
	We use the highest possible event code (0x1f) for master
	interface 0.

Otherwise this looks fine.

Mark.