linux-kernel - [PATCH v2 07/10] coresight: trbe: Do not truncate buffer on IRQ

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Message-Id: <20210723124611.3828908-8-suzuki.poulose@arm.com>
Date:   Fri, 23 Jul 2021 13:46:08 +0100
From:   Suzuki K Poulose <suzuki.poulose@....com>
To:     coresight@...ts.linaro.org
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        tamas.zsoldos@....com, al.grant@....com, leo.yan@...aro.org,
        mike.leach@...aro.org, mathieu.poirier@...aro.org,
        suzuki.poulose@....com, anshuman.khandual@....com,
        jinlmao@....qualcomm.com, James Clark <james.clark@....com>
Subject: [PATCH v2 07/10] 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:

  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 <mathieu.poirier@...aro.org>
Cc: James Clark <james.clark@....com>
Cc: Mike Leach <mike.leach@...aro.org>
Cc: Anshuman Khandual <anshuman.khandual@....com>
Cc: Leo Yan <leo.yan@...aro.org>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@....com>
---
 drivers/hwtracing/coresight/coresight-trbe.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/hwtracing/coresight/coresight-trbe.c b/drivers/hwtracing/coresight/coresight-trbe.c
index 503bea0137ae..d50f142e86d1 100644
--- a/drivers/hwtracing/coresight/coresight-trbe.c
+++ b/drivers/hwtracing/coresight/coresight-trbe.c
@@ -615,7 +615,7 @@ 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);
+		perf_aux_output_flag(handle, PERF_AUX_FLAG_COLLISION);
 	}
 
 	offset = write - base;
@@ -708,7 +708,7 @@ static void trbe_handle_overflow(struct perf_output_handle *handle)
 	 * collection upon the WRAP event, without stopping the source.
 	 */
 	perf_aux_output_flag(handle, PERF_AUX_FLAG_CORESIGHT_FORMAT_RAW |
-				     PERF_AUX_FLAG_TRUNCATED);
+				     PERF_AUX_FLAG_COLLISION);
 	perf_aux_output_end(handle, size);
 	event_data = perf_aux_output_begin(handle, event);
 	if (!event_data) {
-- 
2.24.1