linux-kernel - Re: [PATCH 13/17] coresight etr: Do not clean ETR trace buffer

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Date:   Thu, 2 Nov 2017 14:36:23 -0600
From:   Mathieu Poirier <mathieu.poirier@...aro.org>
To:     Suzuki K Poulose <suzuki.poulose@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        rob.walker@....com, mike.leach@...aro.org,
        coresight@...ts.linaro.org
Subject: Re: [PATCH 13/17] coresight etr: Do not clean ETR trace buffer

On Thu, Oct 19, 2017 at 06:15:49PM +0100, Suzuki K Poulose wrote:
> We zero out the entire trace buffer used for ETR before it
> is enabled, for helping with debugging. Since we could be
> restoring a session in perf mode, this could destroy the data.

I'm not sure to follow you with "... restoring a session in perf mode ...". 
When operating from the perf interface all the memory allocated for a session is
cleanup after, there is no re-using of memory as in sysFS.

> Get rid of this step, if someone wants to debug, they can always
> add it as and when needed.
> 
> Cc: Mathieu Poirier <mathieu.poirier@...aro.org>
> Signed-off-by: Suzuki K Poulose <suzuki.poulose@....com>
> ---
>  drivers/hwtracing/coresight/coresight-tmc-etr.c | 7 ++-----
>  1 file changed, 2 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/hwtracing/coresight/coresight-tmc-etr.c b/drivers/hwtracing/coresight/coresight-tmc-etr.c
> index 31353fc34b53..849684f85443 100644
> --- a/drivers/hwtracing/coresight/coresight-tmc-etr.c
> +++ b/drivers/hwtracing/coresight/coresight-tmc-etr.c
> @@ -971,8 +971,6 @@ static void tmc_etr_enable_hw(struct tmc_drvdata *drvdata,
>  		return;
>  
>  	drvdata->etr_buf = etr_buf;
> -	/* Zero out the memory to help with debug */
> -	memset(etr_buf->vaddr, 0, etr_buf->size);

I agree, this can be costly when dealing with large areas of memory.

>  
>  	CS_UNLOCK(drvdata->base);
>  
> @@ -1267,9 +1265,8 @@ int tmc_read_unprepare_etr(struct tmc_drvdata *drvdata)
>  	if (drvdata->mode == CS_MODE_SYSFS) {
>  		/*
>  		 * The trace run will continue with the same allocated trace
> -		 * buffer. The trace buffer is cleared in tmc_etr_enable_hw(),
> -		 * so we don't have to explicitly clear it. Also, since the
> -		 * tracer is still enabled drvdata::buf can't be NULL.
> +		 * buffer. Since the tracer is still enabled drvdata::buf can't
> +		 * be NULL.
>  		 */
>  		tmc_etr_enable_hw(drvdata, drvdata->sysfs_buf);
>  	} else {
> -- 
> 2.13.6
>