linux-kernel - Re: [PATCH 4/7] perf cs-etm: Don't use hard coded config bits when setting up ETMCR

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Message-ID: <20251202114300.GV724103@e132581.arm.com>
Date: Tue, 2 Dec 2025 11:43:00 +0000
From: Leo Yan <leo.yan@....com>
To: James Clark <james.clark@...aro.org>
Cc: Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Namhyung Kim <namhyung@...nel.org>,
	Mark Rutland <mark.rutland@....com>,
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	Jiri Olsa <jolsa@...nel.org>, Ian Rogers <irogers@...gle.com>,
	Adrian Hunter <adrian.hunter@...el.com>,
	Suzuki K Poulose <suzuki.poulose@....com>,
	Mike Leach <mike.leach@...aro.org>,
	John Garry <john.g.garry@...cle.com>, Will Deacon <will@...nel.org>,
	Leo Yan <leo.yan@...ux.dev>, linux-perf-users@...r.kernel.org,
	linux-kernel@...r.kernel.org, coresight@...ts.linaro.org,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 4/7] perf cs-etm: Don't use hard coded config bits when
 setting up ETMCR

On Mon, Dec 01, 2025 at 04:41:07PM +0000, Coresight ML wrote:

[...]

> @@ -746,7 +779,7 @@ static void cs_etm_get_metadata(struct perf_cpu cpu, u32 *offset,
>  	case CS_ETMV3:
>  		magic = __perf_cs_etmv3_magic;
>  		/* Get configuration register */
> -		info->priv[*offset + CS_ETM_ETMCR] = cs_etm_get_config(itr);
> +		info->priv[*offset + CS_ETM_ETMCR] = cs_etm_guess_etmcr(itr);

I still think cs_etm_get_config() is better than cs_etm_guess_etmcr().

For ETMv3, we directly pass CONFIG to the kernel, and after validation
in the dirver, then the value will be set to ETMCR.  If we already know
the config value is consistent between user space and kernel, why
introduce a redundant "guess" operation here?

Thanks,
Leo