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Subject[PATCH v3 03/10] coresight: tmc-etf: Add comment for store ordering
Date
AUX ring buffer is required to separate the data store and aux_head
store, since the function CS_LOCK() has contained memory barrier mb(),
mb() is a more conservative barrier than smp_wmb() on Arm32/Arm64, thus
it's needless to add any explicit barrier anymore.

Add comment to make clear for the barrier usage for ETF.

Signed-off-by: Leo Yan <leo.yan@linaro.org>
---
drivers/hwtracing/coresight/coresight-tmc-etf.c | 6 ++++++
1 file changed, 6 insertions(+)

diff --git a/drivers/hwtracing/coresight/coresight-tmc-etf.c b/drivers/hwtracing/coresight/coresight-tmc-etf.c
index 45b85edfc690..9a42ee689921 100644
--- a/drivers/hwtracing/coresight/coresight-tmc-etf.c
+++ b/drivers/hwtracing/coresight/coresight-tmc-etf.c
@@ -553,6 +553,12 @@ static unsigned long tmc_update_etf_buffer(struct coresight_device *csdev,
if (buf->snapshot)
handle->head += to_read;

+ /*
+ * AUX ring buffer requires to use memory barrier to separate the trace
+ * data store and aux_head store, because CS_LOCK() contains mb() which
+ * gives more heavy barrier than smp_wmb(), it's not necessary to
+ * explicitly invoke any barrier.
+ */
CS_LOCK(drvdata->base);
out:
spin_unlock_irqrestore(&drvdata->spinlock, flags);
--
2.25.1
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