Re: [PATCH 07/19] coresight: trbe: Refactor AUX flag setting

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

On 12/12/2025 17:27, Leo Yan wrote:
> On Fri, Dec 12, 2025 at 04:50:52PM +0200, James Clark wrote:
> 
> [...]
> 
>>> The trace can be stopped on Fill mode (stop on wrap) or Stop on trigger, so
>>> it is not (only) about buffer error.  In these cases, even though the trace
>>> unit is stopped, the buffer may still have space available, tracing can be
>>> directly re-enabled in interrupt handler, thus the COLLISION/PARTIAL flags
>>> are better choices.
>>
>> I suppose this is the bit that I don't understand. If there is space left
>> then why is the stop hit at all?
> 
> For instance, the buffer becomes empty in the middle:
> 
>     +----------------------------+
>     |$$$$$$$|            |$$$$$$$|
>     +----------------------------+
>     ` base  `wptr        `limit
> 
> In this case, the trace unit stops when it reaches the limit.
> Meanwhile, the tool might have already read out the old trace data so
> we will get some free space, thus no need to set TRUNCATED flag.

Hmmm I see, I still think that means that Perf is being too slow and 
disabling the event wouldn't be that bad.

Also isn't there a very small chance to hit this? I'm wondering if you 
ever saw PARTIAL flags in practice?

I think the following all have to be true:

  * 3 out of 4 sections were full on the last interrupt
  * Perf hadn't managed to finish reading even 1 section in that time
  * The last section fills up
  * Perf had managed to finish reading 1 section between the time of
    the previous interrupt and the hard stop

There would have to be quite a high variability in read speed to make 
that possible. Maybe it happens and it's worthwhile optimizing for. The 
comments could explain it better if we do decide to keep it.