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Date:   Thu, 23 Sep 2021 15:39:17 +0100
From:   Suzuki K Poulose <>
        Suzuki K Poulose <>,
        James Clark <>
Subject: [PATCH v4 3/5] coresight: trbe: Do not truncate buffer on IRQ

The TRBE driver marks the AUX buffer as TRUNCATED when we get an IRQ
on FILL event. This has rather unwanted side-effect of the event
being disabled when there may be more space in the ring buffer.

So, instead of TRUNCATE we need a different flag to indicate
that the trace may have lost a few bytes (i.e from the point of
generating the FILL event until the IRQ is consumed). Anyways, the
userspace must use the size from RECORD_AUX headers to restrict
the "trace" decoding.

Using PARTIAL flag causes the perf tool to generate the
following warning:

  AUX data had gaps in it XX times out of YY!

  Are you running a KVM guest in the background?

which is pointlessly scary for a user. The other remaining options
are :
  - COLLISION - Use by SPE to indicate samples collided
  - Add a new flag - Specifically for CoreSight, doesn't sound
    so good, if we can re-use something.

Given that we don't already use the "COLLISION" flag, the above
behavior can be notified using this flag for CoreSight.

Cc: Mathieu Poirier <>
Cc: James Clark <>
Cc: Mike Leach <>
Cc: Anshuman Khandual <>
Cc: Leo Yan <>
Signed-off-by: Suzuki K Poulose <>
Changes since v3:
 - Moved setting the COLLISION flag to a helper and added
   comments to explain.
 drivers/hwtracing/coresight/coresight-trbe.c | 27 +++++++++++++++-----
 1 file changed, 21 insertions(+), 6 deletions(-)

diff --git a/drivers/hwtracing/coresight/coresight-trbe.c b/drivers/hwtracing/coresight/coresight-trbe.c
index 47120d263639..0a9106c15639 100644
--- a/drivers/hwtracing/coresight/coresight-trbe.c
+++ b/drivers/hwtracing/coresight/coresight-trbe.c
@@ -120,6 +120,25 @@ static void trbe_reset_local(void)
 	write_sysreg_s(0, SYS_TRBSR_EL1);
+static void trbe_report_wrap_event(struct perf_output_handle *handle)
+	/*
+	 * Mark the buffer to indicate that there was a WRAP event by
+	 * setting the COLLISION flag. This indicates to the user that
+	 * the TRBE trace collection was stopped without stopping the
+	 * ETE and thus there might be some amount of trace that was
+	 * lost between the time the WRAP was detected and the IRQ
+	 * was consumed by the CPU.
+	 *
+	 * Setting the TRUNCATED flag would move the event to STOPPED
+	 * state unnecessarily, even when there is space left in the
+	 * ring buffer. Using the COLLISION flag doesn't have this side
+	 * effect. We only set TRUNCATED flag when there is no space
+	 * left in the ring buffer.
+	 */
+	perf_aux_output_flag(handle, PERF_AUX_FLAG_COLLISION);
 static void trbe_stop_and_truncate_event(struct perf_output_handle *handle)
 	struct trbe_buf *buf = etm_perf_sink_config(handle);
@@ -612,7 +631,7 @@ static unsigned long arm_trbe_update_buffer(struct coresight_device *csdev,
 		 * for correct size. Also, mark the buffer truncated.
 		write = get_trbe_limit_pointer();
-		perf_aux_output_flag(handle, PERF_AUX_FLAG_TRUNCATED);
+		trbe_report_wrap_event(handle);
 	offset = write - base;
@@ -708,11 +727,7 @@ static void trbe_handle_overflow(struct perf_output_handle *handle)
 	if (buf->snapshot)
 		handle->head += size;
-	/*
-	 * Mark the buffer as truncated, as we have stopped the trace
-	 * collection upon the WRAP event, without stopping the source.
-	 */
-	perf_aux_output_flag(handle, PERF_AUX_FLAG_TRUNCATED);
+	trbe_report_wrap_event(handle);
 	perf_aux_output_end(handle, size);
 	event_data = perf_aux_output_begin(handle, event);
 	if (!event_data) {

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