Re: [PATCH v2 23/23] coresight: trbe: Report trace gaps with PARTIAL instead of COLLISION

[James Clark <james.clark@linaro.org>]

On 16/10/2026 17:55, Leo Yan wrote:
> When a fill-mode wrap stops TRBE, the trace source can continue running
> until software handles the stop. Trace generated during this interval
> may be lost.
> 
> The overflow path now stops and restarts the trace source around buffer
> reprogramming. This emits ASYNC packets so the decoder can synchronize
> with the next trace segment. Use PERF_AUX_FLAG_PARTIAL to report the
> possible gap instead of PERF_AUX_FLAG_COLLISION.
> 
> Set PARTIAL in both the overflow handler and the buffer update callback.
> This also preserves the gap indication when event shutdown handles a
> pending wrap, even if no further trace segment follows. Limit the flag
> to stopped buffers.
> 
> Signed-off-by: Leo Yan <leo.yan@arm.com>
> ---
>   drivers/hwtracing/coresight/coresight-trbe.c | 29 +++++++---------------------
>   1 file changed, 7 insertions(+), 22 deletions(-)
> 
> diff --git a/drivers/hwtracing/coresight/coresight-trbe.c b/drivers/hwtracing/coresight/coresight-trbe.c
> index 6bc82d5b55539795059da37039d2929b4ea504e3..29a1882b1f3c2eb7166f71eeef37badece67039c 100644
> --- a/drivers/hwtracing/coresight/coresight-trbe.c
> +++ b/drivers/hwtracing/coresight/coresight-trbe.c
> @@ -285,25 +285,6 @@ static void trbe_reset_local(struct trbe_cpudata *cpudata)
>   	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_truncate_event(struct perf_output_handle *handle)
>   {
>   	perf_aux_output_flag(handle, PERF_AUX_FLAG_TRUNCATED);
> @@ -917,10 +898,12 @@ static unsigned long arm_trbe_update_buffer(struct coresight_device *csdev,
>   		perf_aux_output_flag(handle, PERF_AUX_FLAG_TRUNCATED);
>   		size = 0;
>   		goto done;
> -	} else if (act == TRBE_FAULT_ACT_WRAP) {
> -		trbe_report_wrap_event(handle);
>   	}
>   
> +	/* Trace may have been lost before the source was stopped. */
> +	if (act == TRBE_FAULT_ACT_WRAP && !is_trbe_running(status))
> +		perf_aux_output_flag(handle, PERF_AUX_FLAG_PARTIAL);
> +
>   	size = trbe_get_trace_size(handle, buf, act == TRBE_FAULT_ACT_WRAP);
>   
>   done:
> @@ -1246,7 +1229,9 @@ static int trbe_handle_overflow(struct trbe_cpudata *cpudata,
>   	if (buf->snapshot)
>   		handle->head += size;
>   
> -	trbe_report_wrap_event(handle);
> +	if (is_buffer_stopped)
> +		perf_aux_output_flag(handle, PERF_AUX_FLAG_PARTIAL);
> +
>   	perf_aux_output_end(handle, size);
>   	event_data = perf_aux_output_begin(handle, event);
>   	/*
> 
Reviewed-by: James Clark <james.clark@linaro.org>