linux-kernel - Re: [PATCH 1/3] coresight: Add config flag to enable branch broadcast

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Date:   Thu, 9 Dec 2021 09:21:20 +0000
From:   Suzuki K Poulose <suzuki.poulose@....com>
To:     James Clark <james.clark@....com>, mathieu.poirier@...aro.org,
        coresight@...ts.linaro.org
Cc:     Mike Leach <mike.leach@...aro.org>, Leo Yan <leo.yan@...aro.org>,
        John Garry <john.garry@...wei.com>,
        Will Deacon <will@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-perf-users@...r.kernel.org
Subject: Re: [PATCH 1/3] coresight: Add config flag to enable branch broadcast

On 08/12/2021 16:09, James Clark wrote:
> When enabled, all taken branch addresses are output, even if the branch
> was because of a direct branch instruction. This enables reconstruction
> of the program flow without having access to the memory image of the
> code being executed.
> 
> Use bit 8 for the config option which would be the correct bit for
> programming ETMv3. Although branch broadcast can't be enabled on ETMv3
> because it's not in the define ETM3X_SUPPORTED_OPTIONS, using the
> correct bit might help prevent future collisions or allow it to be
> enabled if needed.
> 
> Signed-off-by: James Clark <james.clark@....com>
> ---
>   drivers/hwtracing/coresight/coresight-etm-perf.c   | 2 ++
>   drivers/hwtracing/coresight/coresight-etm4x-core.c | 9 +++++++++
>   include/linux/coresight-pmu.h                      | 2 ++
>   3 files changed, 13 insertions(+)
> 
> diff --git a/drivers/hwtracing/coresight/coresight-etm-perf.c b/drivers/hwtracing/coresight/coresight-etm-perf.c
> index c039b6ae206f..43bbd5dc3d3b 100644
> --- a/drivers/hwtracing/coresight/coresight-etm-perf.c
> +++ b/drivers/hwtracing/coresight/coresight-etm-perf.c
> @@ -52,6 +52,7 @@ static DEFINE_PER_CPU(struct coresight_device *, csdev_src);
>    * The PMU formats were orignally for ETMv3.5/PTM's ETMCR 'config';
>    * now take them as general formats and apply on all ETMs.
>    */
> +PMU_FORMAT_ATTR(branch_broadcast, "config:"__stringify(ETM_OPT_BRANCH_BROADCAST));
>   PMU_FORMAT_ATTR(cycacc,		"config:" __stringify(ETM_OPT_CYCACC));
>   /* contextid1 enables tracing CONTEXTIDR_EL1 for ETMv4 */
>   PMU_FORMAT_ATTR(contextid1,	"config:" __stringify(ETM_OPT_CTXTID));
> @@ -97,6 +98,7 @@ static struct attribute *etm_config_formats_attr[] = {
>   	&format_attr_sinkid.attr,
>   	&format_attr_preset.attr,
>   	&format_attr_configid.attr,
> +	&format_attr_branch_broadcast.attr,
>   	NULL,
>   };
>   
> diff --git a/drivers/hwtracing/coresight/coresight-etm4x-core.c b/drivers/hwtracing/coresight/coresight-etm4x-core.c
> index bf18128cf5de..d2bafb50c66a 100644
> --- a/drivers/hwtracing/coresight/coresight-etm4x-core.c
> +++ b/drivers/hwtracing/coresight/coresight-etm4x-core.c
> @@ -692,6 +692,15 @@ static int etm4_parse_event_config(struct coresight_device *csdev,
>   		ret = cscfg_csdev_enable_active_config(csdev, cfg_hash, preset);
>   	}
>   
> +	/* branch broadcast - enable if selected and supported */
> +	if (attr->config & BIT(ETM_OPT_BRANCH_BROADCAST)) {
> +		if (!drvdata->trcbb) {
> +			ret = -EINVAL;
> +			goto out;
> +		} else
> +			config->cfg |= BIT(ETM4_CFG_BIT_BB);

nit: For consistent styling, I would recommend to wrap the else case
also in { }.

Otherwise looks good to me.

Suzuki