Re: [PATCH 08/19] coresight: trbe: Use PERF_AUX_FLAG_PARTIAL instead of PERF_AUX_FLAG_COLLISION

On 01/12/25 4:51 PM, Leo Yan wrote:

When tracing stops, there is no collision with other samples, so using PERF_AUX_FLAG_COLLISION does not accurately reflect the trace state and may mislead userspace.

Use PERF_AUX_FLAG_PARTIAL instead to indicate that tracing stopped and the record may contain gaps.

Signed-off-by: Leo Yan leo.yan@arm.com

drivers/hwtracing/coresight/coresight-trbe.c | 19 ++++++------------- 1 file changed, 6 insertions(+), 13 deletions(-)

diff --git a/drivers/hwtracing/coresight/coresight-trbe.c b/drivers/hwtracing/coresight/coresight-trbe.c index b06885a08e082fd34f68d9588518807b5c47c86e..0caa4a6b437a3aa39fc6bcc72a23711b54f7c598 100644 --- a/drivers/hwtracing/coresight/coresight-trbe.c +++ b/drivers/hwtracing/coresight/coresight-trbe.c @@ -669,21 +669,14 @@ static enum trbe_fault_action trbe_get_fault_act(struct perf_output_handle *hand } /*

• * 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.
  

• * Mark the buffer to indicate that the trace is stopped by setting
  

• * the PARTIAL 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
  

• * TRBE event was detected and the IRQ was consumed by the CPU.
  

  */ if (!is_trbe_running(trbsr))

• perf_aux_output_flag(handle, PERF_AUX_FLAG_COLLISION);
  

• perf_aux_output_flag(handle, PERF_AUX_FLAG_PARTIAL);
  

This is interesting. So there is a no possibility for the records to be overridden in the perf ring buffer and incomplete record is all that can happen when TRBE is stopped, without stopping the ETE first ? Could both of these scenarios might be possible as well ?

if (is_trbe_wrap(trbsr)) return TRBE_FAULT_ACT_WRAP;