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

[Leo Yan]

On Tue, May 11, 2021 at 04:53:35PM +0300, James Clark wrote:

[...]

>         /* First get the packet queue for this traceID */
>         packet_queue = cs_etm__etmq_get_packet_queue(etmq, trace_chan_id);
> @@ -320,7 +323,20 @@ 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->timestamp = elem->timestamp - packet_queue->instr_count;
> +       if (!elem->timestamp) {
> +               packet_queue->timestamp = 0;
> +               if (!warned_timestamp_zero) {
> +                       pr_err("Zero Coresight timestamp found at Idx:%" OCSD_TRC_IDX_STR
> +                               ". Decoding may be improved with --itrace=Z...\n", indx);
> +                       warned_timestamp_zero = true;
> +               }
> +       }
> +       else if (packet_queue->instr_count >= elem->timestamp) {

Nitpick: I personally think should use the condition ">" rather than ">=".

> +               packet_queue->timestamp = 0;
> +               pr_err("Timestamp calculation underflow at Idx:%" OCSD_TRC_IDX_STR "\n", indx);
> +       }
> +       else
> +               packet_queue->timestamp = elem->timestamp - packet_queue->instr_count;

Nitpick for coding style, as described in
Documentation/process/coding-style.rst, section "3) Placing Braces and
Spaces", so here should use braces with the format:

  if (!elem->timestamp) {
    /* print warning */
  } else if {packet_queue->instr_count >= elem->timestamp) {
    /* print warning */
  } else {
    packet_queue->timestamp = elem->timestamp - packet_queue->instr_count;
  }

Thanks,
Leo

>         packet_queue->next_timestamp = elem->timestamp;
>         packet_queue->instr_count = 0;
>  
> @@ -542,7 +558,7 @@ cs_etm_decoder__set_tid(struct cs_etm_queue *etmq,
>  
>  static ocsd_datapath_resp_t cs_etm_decoder__gen_trace_elem_printer(
>                                 const void *context,
> -                               const ocsd_trc_index_t indx __maybe_unused,
> +                               const ocsd_trc_index_t indx,
>                                 const u8 trace_chan_id __maybe_unused,
>                                 const ocsd_generic_trace_elem *elem)
>  {
> @@ -579,7 +595,8 @@ static ocsd_datapath_resp_t cs_etm_decoder__gen_trace_elem_printer(
>                 break;
>         case OCSD_GEN_TRC_ELEM_TIMESTAMP:
>                 resp = cs_etm_decoder__do_hard_timestamp(etmq, elem,
> -                                                        trace_chan_id);
> +                                                        trace_chan_id,
> +                                                         indx);
>                 break;
>         case OCSD_GEN_TRC_ELEM_PE_CONTEXT:
>                 resp = cs_etm_decoder__set_tid(etmq, packet_queue,
> 
> 
> James
> 
> > 
> > Thanks,
> > Leo
> >