linux-kernel - Re: [PATCH v2 5/8] coresight: etm: Add an attribute for updating buffer

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Message-ID: <20250310180318.GH9682@e132581.arm.com>
Date: Mon, 10 Mar 2025 18:03:18 +0000
From: Leo Yan <leo.yan@....com>
To: Suzuki K Poulose <suzuki.poulose@....com>
Cc: Mike Leach <mike.leach@...aro.org>,
	James Clark <james.clark@...aro.org>,
	Jonathan Corbet <corbet@....net>,
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Namhyung Kim <namhyung@...nel.org>, coresight@...ts.linaro.org,
	linux-arm-kernel@...ts.infradead.org, linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 5/8] coresight: etm: Add an attribute for updating
 buffer

On Mon, Mar 10, 2025 at 04:37:44PM +0000, Suzuki Kuruppassery Poulose wrote:

[...]

> > How about we dynamically set the default flag in the Perf tool?
> > 
> > - If users set explictly the 'update_buf_on_pause' flag, then the
> >    setting will be respected.
> > - If users don't set the flag, perf tool detects it is TRBE sinks,
> >    then it can set 'update_buf_on_pause' flag as false.
> 
> Not really possible. There could be systems with mixed sinks. e.g.  TRBE
> for some CPU and ETR for others (say due to a non-functioning TRBE).
> 
> > - If users don't set the flag, perf tool detects it is ETF/ETB/ETR
> >    sinks, it sets the flag as true.
> 
> And in the cases above, perf event cannot run on all the CPUs, because
> some sinks don't support it.
> 
> Why do we need a flag, when the effect is not user (read, perf decoder)
>  visible and at the same time improves some scenarios (read non-TRBE cases)
> ?

Indeed in this case the flag is redundant.

> I would say, let the driver always update on pause, depending on the
> sink.

It is fine for me.  I will move towards this direction.

Thanks,
Leo