linux-kernel - Re: [PATCH v2 14/17] coresight: trbe: Make sure we have enough space

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Message-ID: <60e75e7b-4a04-03d4-c861-88dd5fadef99@arm.com>
Date:   Wed, 22 Sep 2021 15:28:21 +0530
From:   Anshuman Khandual <anshuman.khandual@....com>
To:     Suzuki K Poulose <suzuki.poulose@....com>,
        linux-arm-kernel@...ts.infradead.org
Cc:     linux-kernel@...r.kernel.org, maz@...nel.org,
        catalin.marinas@....com, mark.rutland@....com, james.morse@....com,
        leo.yan@...aro.org, mike.leach@...aro.org,
        mathieu.poirier@...aro.org, will@...nel.org, lcherian@...vell.com,
        coresight@...ts.linaro.org
Subject: Re: [PATCH v2 14/17] coresight: trbe: Make sure we have enough space



On 9/21/21 7:11 PM, Suzuki K Poulose wrote:
> The TRBE driver makes sure that there is enough space for a meaningful
> run, otherwise pads the given space and restarts the offset calculation
> once. But there is no guarantee that we may find space or hit "no space".

So what happens currently when it neither finds the required minimum buffer
space for a meaningful run nor does it hit the "no space" scenario ?

> Make sure that we repeat the step until, either :
>   - We have the minimum space
>    OR
>   - There is NO space at all.
> 
> Cc: Anshuman Khandual <anshuman.khandual@....com>
> Cc: Mathieu Poirier <mathieu.poirier@...aro.org>
> Cc: Mike Leach <mike.leach@...aro.org>
> Cc: Leo Yan <leo.yan@...aro.org>
> Signed-off-by: Suzuki K Poulose <suzuki.poulose@....com>
> ---
>  drivers/hwtracing/coresight/coresight-trbe.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/hwtracing/coresight/coresight-trbe.c b/drivers/hwtracing/coresight/coresight-trbe.c
> index 3373f4e2183b..02f9e00e2091 100644
> --- a/drivers/hwtracing/coresight/coresight-trbe.c
> +++ b/drivers/hwtracing/coresight/coresight-trbe.c
> @@ -451,10 +451,14 @@ static unsigned long trbe_normal_offset(struct perf_output_handle *handle)
>  	 * If the head is too close to the limit and we don't
>  	 * have space for a meaningful run, we rather pad it
>  	 * and start fresh.
> +	 *
> +	 * We might have to do this more than once to make sure
> +	 * we have enough required space.

OR no space at all, as explained in the commit message.
Hence this comment needs an update.

>  	 */
> -	if (limit && ((limit - head) < trbe_min_trace_buf_size(handle))) {
> +	while (limit && ((limit - head) < trbe_min_trace_buf_size(handle))) {
>  		trbe_pad_buf(handle, limit - head);
>  		limit = __trbe_normal_offset(handle);
> +		head = PERF_IDX2OFF(handle->head, buf);

Should the loop be bound with a retry limit as well ?

>  	}
>  	return limit;
>  }
>