linux-kernel - Re: [PATCH v2 06/28] coresight: etm4x: Add context synchronization before enabling trace

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Message-ID: <aGUSZpR1QHNgLu6f@e129823.arm.com>
Date: Wed, 2 Jul 2025 12:05:10 +0100
From: Yeoreum Yun <yeoreum.yun@....com>
To: Leo Yan <leo.yan@....com>
Cc: Suzuki K Poulose <suzuki.poulose@....com>,
	Mike Leach <mike.leach@...aro.org>,
	James Clark <james.clark@...aro.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	Yabin Cui <yabinc@...gle.com>, Keita Morisaki <keyz@...gle.com>,
	Yuanfang Zhang <quic_yuanfang@...cinc.com>,
	coresight@...ts.linaro.org, linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 06/28] coresight: etm4x: Add context synchronization
 before enabling trace

> According to the software usage PKLXF in Arm ARM (ARM DDI 0487 L.a), a
> Context synchronization event is required before enabling the trace
> unit. An ISB is added to meet this requirement.
>
> Improved the barrier comments to provide more accurate information by
> specifying which section of the document the requirement comes from and
> clarifying its intended purpose.
>
> Fixes: 1ab3bb9df5e3 ("coresight: etm4x: Add necessary synchronization for sysreg access")
> Signed-off-by: Leo Yan <leo.yan@....com>
> ---
>  drivers/hwtracing/coresight/coresight-etm4x-core.c | 37 +++++++++++++++++++---
>  1 file changed, 33 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/hwtracing/coresight/coresight-etm4x-core.c b/drivers/hwtracing/coresight/coresight-etm4x-core.c
> index af9d3b2319c5f49ccd40dfa0ccf0f694ce9e2f4f..e3a73718d0c903ee9c72b97028b56565f5ee1053 100644
> --- a/drivers/hwtracing/coresight/coresight-etm4x-core.c
> +++ b/drivers/hwtracing/coresight/coresight-etm4x-core.c
> @@ -445,13 +445,37 @@ static int etm4_enable_trace_unit(struct etmv4_drvdata *drvdata)
>  		etm4x_relaxed_write32(csa, TRCRSR_TA, TRCRSR);
>
>  	etm4x_allow_trace(drvdata);
> +
> +	/*
> +	 * According to software usage PKLXF in Arm ARM (ARM DDI 0487 L.a),
> +	 * execute a Context synchronization event to guarantee the trace unit
> +	 * will observe the new values of the System registers.
> +	 */
> +	if (!csa->io_mem)
> +		isb();
> +

But, when write to SYS_TRFCR_EL1 in etm4x_allow_trace(), it already does
isb(). Is it redundant?

> --
> 2.34.1
>

--
Sincerely,
Yeoreum Yun