linux-kernel - Re: [PATCH v7 13/13] coresight: docs: Document etm4x timestamp interval option

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Message-ID: <20251126151448.GM724103@e132581.arm.com>
Date: Wed, 26 Nov 2025 15:14:48 +0000
From: Leo Yan <leo.yan@....com>
To: Mike Leach <mike.leach@...aro.org>,
	James Clark <james.clark@...aro.org>,
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	Jonathan Corbet <corbet@....net>,
	Randy Dunlap <rdunlap@...radead.org>, coresight@...ts.linaro.org,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	linux-doc@...r.kernel.org
Subject: Re: [PATCH v7 13/13] coresight: docs: Document etm4x timestamp
 interval option

On Wed, Nov 26, 2025 at 02:44:37PM +0000, Coresight ML wrote:

[...]

> > As far as I recall when this command line parameter was a bool then:
> > perf -e cs_etm/timestamp/ <program>
> > is sufficient to turn on timestamping.
> 
> Hmm... with the latest perf, we must assign value to `timestamp`,
> otherwise perf will report error:
> 
>   # /mnt/build/perf record -e cs_etm/timestamp/ -C 0 -- taskset -c 0 ls
>   event syntax error: 'cs_etm/timestamp/'
>                        \___ Bad event or PMU
>   
>   Unable to find PMU or event on a PMU of 'cs_etm'
>   
>   event syntax error: 'cs_etm/timestamp/'
>                        \___ no value assigned for term
>   
>   event syntax error: 'cs_etm/timestamp/'
>                        \___ no value assigned for term
>   Run 'perf list' for a list of valid events

Apologize for this misinformation.  When `timestamp` is bool, it does
support the `-e cs_etm/timestamp/` format.

> > This is worth mentioning so users can correctly assess what happens
> > for any existing scripts they might have.
> > 
> > Based on this then the same command must set the timestamp to 1 -
> > which will have the same effect as before as we do not want to break
> > existing behaviour.

Not sure if we need to record such info.  Seems to me, it is weird that
record a common behaviour for perf formats in this doc.

The perf error log would be sufficient for users to setup a proper
format?

Thanks,
Leo