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Subject: Re: [PATCH] coresight: tmc-etr: Remove perf_data check.
To:     yabinc@google.com, mathieu.poirier@linaro.org,
        alexander.shishkin@linux.intel.com
Cc:     linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org
References: <20190808193122.76679-1-yabinc@google.com>
From:   Suzuki K Poulose <suzuki.poulose@arm.com>
Message-ID: <84df6071-ef7e-c3d6-6ffa-fcfcbab0c8e6@arm.com>
Date:   Fri, 9 Aug 2019 10:22:07 +0100
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Hi Yabin,


Thank you for the analysis and the patch.

On 08/08/2019 20:31, Yabin Cui wrote:
> When tracing etm data of multiple threads on multiple cpus through
> perf interface, each cpu has a unique etr_perf_buffer while sharing
> the same etr device. There is no guarantee that the last cpu starts
> etm tracing also stops last. So the perf_data check is no longer valid.
> 
> Signed-off-by: Yabin Cui <yabinc@google.com>
> ---
>   drivers/hwtracing/coresight/coresight-tmc-etr.c | 9 ---------
>   drivers/hwtracing/coresight/coresight-tmc.h     | 2 --
>   2 files changed, 11 deletions(-)
> 
> diff --git a/drivers/hwtracing/coresight/coresight-tmc-etr.c b/drivers/hwtracing/coresight/coresight-tmc-etr.c
> index 17006705287a..0418440e0141 100644
> --- a/drivers/hwtracing/coresight/coresight-tmc-etr.c
> +++ b/drivers/hwtracing/coresight/coresight-tmc-etr.c
> @@ -1484,20 +1484,12 @@ tmc_update_etr_buffer(struct coresight_device *csdev,
>   		goto out;
>   	}
>   
> -	if (WARN_ON(drvdata->perf_data != etr_perf)) {
> -		lost = true;
> -		spin_unlock_irqrestore(&drvdata->spinlock, flags);
> -		goto out;
> -	}
> -

I think some sort of sanity check is a good idea to make sure we don't loose the
context. Even when different CPUs have different etr_perf buffer, the underlying
etr_buf should be the same. So, we should be able to simply convert the check
to, something like :

	struct etr_perf_buffer *perf_buf = drvdata->perf_data;

	...

	if (WARN_ON(perf_buf->etr_buf != etr_perf->buf)) {
		....
	}

Does that solve the problem for you ?

Suzuki