linux-kernel - [PATCHv3 5/5] arm-cci: CCI-500: Work around PMU counter writes

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Message-Id: <1447783407-18027-6-git-send-email-suzuki.poulose@arm.com>
Date:	Tue, 17 Nov 2015 18:03:27 +0000
From:	"Suzuki K. Poulose" <suzuki.poulose@....com>
To:	linux-arm-kernel@...ts.infradead.org
Cc:	mark.rutland@....com, punit.agrawal@....com, arm@...nel.org,
	linux-kernel@...r.kernel.org,
	"Suzuki K. Poulose" <suzuki.poulose@....com>
Subject: [PATCHv3 5/5] arm-cci: CCI-500: Work around PMU counter writes

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) Save the current event and program the target counter to count an
    invalid event, which by spec is guaranteed to not-generate any events.
 3) Enable the target counter.
 4) Enable the CCI PMU
 5) Write to the target counter.
 6) Disable the CCI PMU and the target counter
 7) Restore the event back on the target counter.
 8) Restore the status of the all the counters

Cc: Punit Agrawal <punit.agrawal@....com>
Cc: Mark Rutland <mark.rutland@....com>
Signed-off-by: Suzuki K. Poulose <suzuki.poulose@....com>
---
 drivers/bus/arm-cci.c |   47 +++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 47 insertions(+)

diff --git a/drivers/bus/arm-cci.c b/drivers/bus/arm-cci.c
index 88b612f..6020a02 100644
--- a/drivers/bus/arm-cci.c
+++ b/drivers/bus/arm-cci.c
@@ -835,6 +835,52 @@ static void __pmu_write_counter(struct cci_pmu *cci_pmu, u32 value, int idx)
 	pmu_write_register(cci_pmu, value, idx, CCI_PMU_CNTR);
 }
 
+#ifdef CONFIG_ARM_CCI500_PMU
+
+/*
+ * CCI-500 has advanced power saving policies, which could gate the
+ * clocks to the PMU counters, which makes the writes to them ineffective.
+ * The only way to write to those counters is when the global counters
+ * are enabled and the particular counter is enabled.
+ *
+ * So we do the following :
+ *
+ * 1) Disable all the PMU counters, saving their current state
+ * 2) Save the programmed event, and write an invalid event code
+ *    to the event control register for the counter, so that the
+ *    counters are not modified.
+ * 3) Enable the counter control for the counter.
+ * 4) Enable the global PMU profiling
+ * 5) Set the counter value
+ * 6) Disable the counter, global PMU.
+ * 7) Restore the event in the target counter
+ * 8) Restore the status of the rest of the counters.
+ *
+ * We choose an event code which has very little chances of getting
+ * assigned a valid code for step(2). We use the highest possible
+ * event code (0x1f) for the master interface 0.
+ */
+#define CCI500_INVALID_EVENT	((CCI500_PORT_M0 << CCI500_PMU_EVENT_SOURCE_SHIFT) | \
+				 (CCI500_PMU_EVENT_CODE_MASK << CCI500_PMU_EVENT_CODE_SHIFT))
+static void cci500_pmu_write_counter(struct cci_pmu *cci_pmu, u32 value, int idx)
+{
+	unsigned long mask[BITS_TO_LONGS(cci_pmu->num_cntrs)];
+	u32 event;
+
+	pmu_disable_counters(cci_pmu, mask);
+	event = pmu_get_event(cci_pmu, idx);
+	pmu_set_event(cci_pmu, idx, CCI500_INVALID_EVENT);
+	pmu_enable_counter(cci_pmu, idx);
+	__cci_pmu_enable();
+	__pmu_write_counter(cci_pmu, value, idx);
+	__cci_pmu_disable();
+	pmu_disable_counter(cci_pmu, idx);
+	pmu_set_event(cci_pmu, idx, event);
+	pmu_restore_counters(cci_pmu, mask);
+}
+
+#endif	/* CONFIG_ARM_CCI500_PMU */
+
 static void pmu_write_counter(struct perf_event *event, u32 value)
 {
 	struct cci_pmu *cci_pmu = to_cci_pmu(event->pmu);
@@ -1422,6 +1468,7 @@ static struct cci_pmu_model cci_pmu_models[] = {
 			},
 		},
 		.validate_hw_event = cci500_validate_hw_event,
+		.write_counter	= cci500_pmu_write_counter,
 	},
 #endif
 };
-- 
1.7.9.5

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