Re: [PATCH v2 15/23] coresight: trbe: Report truncated trace on buffer update errors

[Leo Yan]

On Wed, Nov 11, 2026 at 10:47:10AM +0000, Suzuki K Poulose wrote:

> > @@ -903,6 +903,7 @@ static unsigned long arm_trbe_update_buffer(struct coresight_device *csdev,
> >   		 * errors and as such buffer is empty.
> >   		 */
> >   		if (act != TRBE_FAULT_ACT_WRAP) {
> > +			perf_aux_output_flag(handle, PERF_AUX_FLAG_TRUNCATED);
> 
> Do we need this though ? If the buffer has space left, we shouldn't use
> TRUNCATED. TRUNCATED causes the event to be stopped and the userspace
> needs to re-enable the event.

Here setting TRUNCATED flag is intentional for fatal faults. The following
patch restricts this handling to TRBE_FAULT_ACT_FATAL only.

We can consider this from two perspectives:

 1. We distinguish TRUNCATED from PARTIAL (see patch 23). PARTIAL
    indicates a recoverable trace gap without disabling the perf event,
    requiring tools to reset the decoder explicitly.

    In contrast, TRUNCATED streams contain ASYNC packets for decoder
    synchronization, so reset decoder is not needed.
 
 2. The AUX sampling calls update buffer callback [1], any fatal faults
    will set TRUNCATED to disable hardware and waiting for userspace to
    re-enable.

Thanks,
Leo

[1] https://lore.kernel.org/linux-arm-kernel/20260915-arm_cs_support_aux_sample-v1-16-35cf6f297d20@xxxxxxx/