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

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Message-ID: <20250310155015.GG9682@e132581.arm.com>
Date: Mon, 10 Mar 2025 15:50:15 +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 01:29:26PM +0000, Suzuki Kuruppassery Poulose wrote:
> Hi Leo
> 
> On 10/03/2025 10:49, Leo Yan wrote:
> > Add an attribute for updating buffer when the AUX trace is paused.  And
> > populate the value to the 'update_buf_on_pause' flag during the AUX
> > setting up.
> 
> Do we need this attribute in the uAPI ?

This uAPI allows users to perform AUX pause and resume without the long
latency caused by copying hardware trace data.

E.g., a user can specify a large AUX buffer size using option "-m,128M".
If the buffer is considered large enough to accommodate hardware trace
data for a small program, the 'update_buf_on_pause' flag can be set to
false, the copying will be deferred until the end of the perf session.

I am bias to keep this uAPI.  If you prefer to remove it, I am also
fine with that.

> Could we do this by default for
> sinks without interrupt ? This definitely improves the quality of trace
> collected for such sinks and the driver can transparently do this.

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.
- If users don't set the flag, perf tool detects it is ETF/ETB/ETR
  sinks, it sets the flag as true.

Thanks,
Leo