[PATCH] perf/arm-cmn: Reset DTM_PMU_CONFIG at probe

[PATCH] perf/arm-cmn: Reset DTM_PMU_CONFIG at probe

[Robin Murphy]

Although we treat the DTM counters as free-running such that we're not
too concerned about the initial DTM state, it's possible for a previous
user to have left DTM counters enabled and paired with DTC counters.
Thus if the first events are scheduled using some, but not all, DTMs,
the as-yet-unused ones could end up adding spurious increments to the
event counts at the DTC. Make sure we sync our initial DTM_PMU_CONFIG
state to all the DTMs at probe time to avoid that possibility.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
---
 drivers/perf/arm-cmn.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/drivers/perf/arm-cmn.c b/drivers/perf/arm-cmn.c
index 0b2df72cee9f..81b0dfd511aa 100644
--- a/drivers/perf/arm-cmn.c
+++ b/drivers/perf/arm-cmn.c
@@ -1862,6 +1862,7 @@ static void arm_cmn_init_dtm(struct arm_cmn_dtm *dtm, struct arm_cmn_node *xp, i
 
 	dtm->base = xp->pmu_base + CMN_DTM_OFFSET(idx);
 	dtm->pmu_config_low = CMN_DTM_PMU_CONFIG_PMU_EN;
+	writeq_relaxed(dtm->pmu_config_low, dtm->base + CMN_DTM_PMU_CONFIG);
 	for (i = 0; i < 4; i++) {
 		dtm->wp_event[i] = -1;
 		writeq_relaxed(0, dtm->base + CMN_DTM_WPn_MASK(i));
-- 
2.36.1.dirty

Re: [PATCH] perf/arm-cmn: Reset DTM_PMU_CONFIG at probe

[Will Deacon]

On Mon, 5 Dec 2022 19:46:13 +0000, Robin Murphy wrote:
> Although we treat the DTM counters as free-running such that we're not
> too concerned about the initial DTM state, it's possible for a previous
> user to have left DTM counters enabled and paired with DTC counters.
> Thus if the first events are scheduled using some, but not all, DTMs,
> the as-yet-unused ones could end up adding spurious increments to the
> event counts at the DTC. Make sure we sync our initial DTM_PMU_CONFIG
> state to all the DTMs at probe time to avoid that possibility.
> 
> [...]

Applied to will (for-next/perf), thanks!

[1/1] perf/arm-cmn: Reset DTM_PMU_CONFIG at probe
      https://git.kernel.org/will/c/bb21ef19a3d8

Cheers,
-- 
Will

https://fixes.arm64.dev
https://next.arm64.dev
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