Re: [PATCH v2 4/6] perf cs-etm: Treat NO_SYNC element as trace discontinuity

Mathieu Poirier
2018-12-10T22:53:07Z

On Mon, Dec 10, 2018 at 04:52:59PM +0800, Leo Yan wrote:
> CoreSight tracer driver might insert barrier packet between different
> buffers, thus the decoder can spot the boundaries based on the barrier
> packet; the decoder is possible to hit a barrier packet and emit a
> NO_SYNC element, then the decoder will find a periodic synchronisation
> point inside that next trace block that starts trace again but does not
> have the TRACE_ON element as indicator - usually because this block of
> trace has wrapped the buffer so we have lost the original point that
> trace was enabled.
>
> In upper case, it results in the trace stream only inserts the
> OCSD_GEN_TRC_ELEM_NO_SYNC element in the middle of tracing stream, but
> we don't handle NO_SYNC element properly and at the end users miss to
> see the info for trace discontinuity.
>
> Though OCSD_GEN_TRC_ELEM_NO_SYNC is different from CS_ETM_TRACE_ON when
> output from the decoder, but both of them indicate the trace data is
> discontinuous; this patch treats OCSD_GEN_TRC_ELEM_NO_SYNC as trace
> discontinuity and generates CS_ETM_DISCONTINUITY packet for it, so
> cs-etm can handle discontinuity for this case, finally it saves the last
> trace data for previous trace block and restart samples for new block.
>
> Credit to Mike Leach and Robert Walker who made me clear for underlying
> mechanism for NO_SYNC element, Mike also shared with me for detailed
> explanation for why we can treat NO_SYNC and TRACE_ON elements as the
> same, so this commit log reused most of his explanation.

Credit and thank you note such as this one go in the cover letter. With this
change:

Reviewed-by: Mathieu Poirier <mathieu.poirier@linaro.org>

>
> Cc: Mathieu Poirier <mathieu.poirier@linaro.org>
> Cc: Mike Leach <mike.leach@linaro.org>
> Cc: Robert Walker <robert.walker@arm.com>
> Signed-off-by: Leo Yan <leo.yan@linaro.org>
> ---
> tools/perf/util/cs-etm-decoder/cs-etm-decoder.c | 2 ++
> 1 file changed, 2 insertions(+)
>
> diff --git a/tools/perf/util/cs-etm-decoder/cs-etm-decoder.c b/tools/perf/util/cs-etm-decoder/cs-etm-decoder.c
> index a3994f1..46b67f1 100644
> --- a/tools/perf/util/cs-etm-decoder/cs-etm-decoder.c
> +++ b/tools/perf/util/cs-etm-decoder/cs-etm-decoder.c
> @@ -411,6 +411,8 @@ static ocsd_datapath_resp_t cs_etm_decoder__gen_trace_elem_printer(
> case OCSD_GEN_TRC_ELEM_UNKNOWN:
> break;
> case OCSD_GEN_TRC_ELEM_NO_SYNC:
> + resp = cs_etm_decoder__buffer_discontinuity(decoder,
> + trace_chan_id);
> decoder->trace_on = false;
> break;
> case OCSD_GEN_TRC_ELEM_TRACE_ON:
> --
> 2.7.4
>