Re: [RFC PATCH] perf cs-etm: Handle valid-but-zero timestamps

[Mike Leach]

Hi James,

1) Not sure about this particular target - but previously all 0
timestamps have been the result of not enabling a clock or the
coresight timestamp generator.

2) Given that you can synthesise timestamps when all the values are
all 0 - does this not imply that they could be synthesised when they
are not present at all?

Cheers

Mike

On Fri, 7 May 2021 at 11:02, James Clark <james.clark@xxxxxxx> wrote:
>
>
>
> On 07/05/2021 12:58, James Clark wrote:
> > There is an intermittent issue on Trogdor devices that
> > results in all Coresight timestamps having a value of zero.
>
> I've attached a file here that has the issue. From the dump you
> can see the zero timestamps:
>
>         Idx:69; ID:10;  I_TIMESTAMP : Timestamp.; Updated val = 0x0
>         Idx:71; ID:10;  I_ATOM_F1 : Atom format 1.; E
>         Idx:72; ID:10;  I_ADDR_S_IS0 : Address, Short, IS0.; Addr=0xFFFFFFE723C65824 ~[0x5824]
>
> This doesn't have an impact on decoding as they end up being
> decoded in file order as in with timeless mode.
>
> James
>
> > Because zero is currently treated as "not found" rather
> > than "found but zero", this breaks the decoding flow
> > when it would otherwise work.
> >
> > This patch adds an out parameter and return code so
> > the difference between zero and non-existent timestamps
> > can be determined.
> >
> > There is also a change to fix an underflow.
> >
> > Although this is a workaround, the change is technically
> > a valid way of writing the cs_etm__etmq_get_timestamp()
> > function. It could have been written similarly to this
> > without trying to work around this issue, so it's no less
> > correct. But, because it's a workaround to a problem
> > elsewhere, I will submit this as an RFC for feedback.
> >
> > This patch applies on top of the "[PATCH v2 0/2] perf
> > cs-etm: Set time on synthesised samples to preserve ordering"
> > patchset.
> >
> > Co-developed-by: Denis Nikitin <denik@xxxxxxxxxxxx>
> > Signed-off-by: Denis Nikitin <denik@xxxxxxxxxxxx>
> > Signed-off-by: James Clark <james.clark@xxxxxxx>
> > ---
> >  .../perf/util/cs-etm-decoder/cs-etm-decoder.c |  5 ++++-
> >  tools/perf/util/cs-etm.c                      | 22 +++++++++----------
> >  2 files changed, 15 insertions(+), 12 deletions(-)
> >
> > 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 b01d363b9301..947e44413c6e 100644
> > --- a/tools/perf/util/cs-etm-decoder/cs-etm-decoder.c
> > +++ b/tools/perf/util/cs-etm-decoder/cs-etm-decoder.c
> > @@ -320,7 +320,10 @@ cs_etm_decoder__do_hard_timestamp(struct cs_etm_queue *etmq,
> >        * which instructions started by subtracting the number of instructions
> >        * executed to the timestamp.
> >        */
> > -     packet_queue->cs_timestamp = elem->timestamp - packet_queue->instr_count;
> > +     if (packet_queue->instr_count >= elem->timestamp)
> > +             packet_queue->cs_timestamp = 0;
> > +     else
> > +             packet_queue->cs_timestamp = elem->timestamp - packet_queue->instr_count;
> >       packet_queue->next_cs_timestamp = elem->timestamp;
> >       packet_queue->instr_count = 0;
> >
> > diff --git a/tools/perf/util/cs-etm.c b/tools/perf/util/cs-etm.c
> > index e5c1a1b22a2a..1969921c406a 100644
> > --- a/tools/perf/util/cs-etm.c
> > +++ b/tools/perf/util/cs-etm.c
> > @@ -210,13 +210,14 @@ void cs_etm__etmq_set_traceid_queue_timestamp(struct cs_etm_queue *etmq,
> >       etmq->pending_timestamp_chan_id = trace_chan_id;
> >  }
> >
> > -static u64 cs_etm__etmq_get_timestamp(struct cs_etm_queue *etmq,
> > +static int cs_etm__etmq_get_timestamp(struct cs_etm_queue *etmq,
> > +                                   u64 *timestamp,
> >                                     u8 *trace_chan_id)
> >  {
> >       struct cs_etm_packet_queue *packet_queue;
> >
> >       if (!etmq->pending_timestamp_chan_id)
> > -             return 0;
> > +             return -ENODATA;
> >
> >       if (trace_chan_id)
> >               *trace_chan_id = etmq->pending_timestamp_chan_id;
> > @@ -224,13 +225,15 @@ static u64 cs_etm__etmq_get_timestamp(struct cs_etm_queue *etmq,
> >       packet_queue = cs_etm__etmq_get_packet_queue(etmq,
> >                                                    etmq->pending_timestamp_chan_id);
> >       if (!packet_queue)
> > -             return 0;
> > +             return -ENODATA;
> >
> >       /* Acknowledge pending status */
> >       etmq->pending_timestamp_chan_id = 0;
> >
> >       /* See function cs_etm_decoder__do_{hard|soft}_timestamp() */
> > -     return packet_queue->cs_timestamp;
> > +     if (timestamp)
> > +             *timestamp = packet_queue->cs_timestamp;
> > +     return 0;
> >  }
> >
> >  static void cs_etm__clear_packet_queue(struct cs_etm_packet_queue *queue)
> > @@ -864,11 +867,10 @@ static int cs_etm__setup_queue(struct cs_etm_auxtrace *etm,
> >                * Function cs_etm_decoder__do_{hard|soft}_timestamp() does all
> >                * the timestamp calculation for us.
> >                */
> > -             cs_timestamp = cs_etm__etmq_get_timestamp(etmq, &trace_chan_id);
> > -
> > -             /* We found a timestamp, no need to continue. */
> > -             if (cs_timestamp)
> > +             if (!cs_etm__etmq_get_timestamp(etmq, &cs_timestamp, &trace_chan_id)) {
> > +                     /* We found a timestamp, no need to continue. */
> >                       break;
> > +             }
> >
> >               /*
> >                * We didn't find a timestamp so empty all the traceid packet
> > @@ -2286,9 +2288,7 @@ static int cs_etm__process_queues(struct cs_etm_auxtrace *etm)
> >               if (ret)
> >                       goto out;
> >
> > -             cs_timestamp = cs_etm__etmq_get_timestamp(etmq, &trace_chan_id);
> > -
> > -             if (!cs_timestamp) {
> > +             if (cs_etm__etmq_get_timestamp(etmq, &cs_timestamp, &trace_chan_id)) {
> >                       /*
> >                        * Function cs_etm__decode_data_block() returns when
> >                        * there is no more traces to decode in the current
> >



-- 
Mike Leach
Principal Engineer, ARM Ltd.
Manchester Design Centre. UK