[PATCH v3 07/10] coresight: trbe: Do not truncate buffer on IRQ

[Mathieu Poirier]

On Tue, Sep 14, 2021 at 11:26:38AM +0100, Suzuki K Poulose wrote:
> The TRBE driver marks the AUX buffer as TRUNCATED when we get an IRQ
> on FILL event. This has rather unwanted side-effect of the event
> being disabled when there may be more space in the ring buffer.
> 
> So, instead of TRUNCATE we need a different flag to indicate
> that the trace may have lost a few bytes (i.e from the point of
> generating the FILL event until the IRQ is consumed). Anyways, the
> userspace must use the size from RECORD_AUX headers to restrict
> the "trace" decoding.
> 
> Using PARTIAL flag causes the perf tool to generate the
> following warning:
> 
>   Warning:
>   AUX data had gaps in it XX times out of YY!
> 
>   Are you running a KVM guest in the background?
> 
> which is pointlessly scary for a user. The other remaining options
> are :
>   - COLLISION - Use by SPE to indicate samples collided
>   - Add a new flag - Specifically for CoreSight, doesn't sound
>     so good, if we can re-use something.
> 
> Given that we don't already use the "COLLISION" flag, the above
> behavior can be notified using this flag for CoreSight.
> 
> Cc: Mathieu Poirier <mathieu.poirier at linaro.org>
> Cc: James Clark <james.clark at arm.com>
> Cc: Mike Leach <mike.leach at linaro.org>
> Cc: Anshuman Khandual <anshuman.khandual at arm.com>
> Cc: Leo Yan <leo.yan at linaro.org>
> Signed-off-by: Suzuki K Poulose <suzuki.poulose at arm.com>
> ---
> Changes since v2:
>   - The perf tool patch for reporting collisions is queued.
> ---
>  drivers/hwtracing/coresight/coresight-trbe.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/hwtracing/coresight/coresight-trbe.c b/drivers/hwtracing/coresight/coresight-trbe.c
> index de99dd0aecd3..a1a15fa6c4ae 100644
> --- a/drivers/hwtracing/coresight/coresight-trbe.c
> +++ b/drivers/hwtracing/coresight/coresight-trbe.c
> @@ -612,7 +612,7 @@ static unsigned long arm_trbe_update_buffer(struct coresight_device *csdev,
>  		 * for correct size. Also, mark the buffer truncated.
>  		 */
>  		write = get_trbe_limit_pointer();
> -		perf_aux_output_flag(handle, PERF_AUX_FLAG_TRUNCATED);
> +		perf_aux_output_flag(handle, PERF_AUX_FLAG_COLLISION);

The rational in the changelog should be added here as a comment.

>  	}
>  
>  	offset = write - base;
> @@ -705,7 +705,7 @@ static void trbe_handle_overflow(struct perf_output_handle *handle)
>  	 * Mark the buffer as truncated, as we have stopped the trace
>  	 * collection upon the WRAP event, without stopping the source.
>  	 */
> -	perf_aux_output_flag(handle, PERF_AUX_FLAG_TRUNCATED);
> +	perf_aux_output_flag(handle, PERF_AUX_FLAG_COLLISION);

Same.

>  	perf_aux_output_end(handle, size);
>  	event_data = perf_aux_output_begin(handle, event);
>  	if (!event_data) {
> -- 
> 2.24.1
>