[PATCH v4 3/5] coresight: trbe: Do not truncate buffer on IRQ

From: Suzuki K Poulose
Date: Thu Sep 23 2021 - 10:39:43 EST


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@xxxxxxxxxx>
Cc: James Clark <james.clark@xxxxxxx>
Cc: Mike Leach <mike.leach@xxxxxxxxxx>
Cc: Anshuman Khandual <anshuman.khandual@xxxxxxx>
Cc: Leo Yan <leo.yan@xxxxxxxxxx>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@xxxxxxx>
---
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) {
--
2.24.1