Re: [PATCH v4 02/11] coresight: tmc-etr: Add barrier after updating AUX ring buffer

[Suzuki K Poulose]

On 11/07/2021 11:40, Leo Yan wrote:
> Since a memory barrier is required between AUX trace data store and
> aux_head store, and the AUX trace data is filled with memcpy(), it's
> sufficient to use smp_wmb() so can ensure the trace data is visible
> prior to updating aux_head.
>
> Signed-off-by: Leo Yan <leo.yan@xxxxxxxxxx>
> ---
>   drivers/hwtracing/coresight/coresight-tmc-etr.c | 8 ++++++++
>   1 file changed, 8 insertions(+)
>
> diff --git a/drivers/hwtracing/coresight/coresight-tmc-etr.c b/drivers/hwtracing/coresight/coresight-tmc-etr.c
> index acdb59e0e661..713205db15a1 100644
> --- a/drivers/hwtracing/coresight/coresight-tmc-etr.c
> +++ b/drivers/hwtracing/coresight/coresight-tmc-etr.c
> @@ -1563,6 +1563,14 @@ tmc_update_etr_buffer(struct coresight_device *csdev,
>   	 */
>   	if (etr_perf->snapshot)
>   		handle->head += size;
> +
> +	/*
> +	 * It requires the ordering between the AUX trace data and aux_head
> +	 * store, below smp_wmb() ensures the AUX trace data is visible prior
> +	 * to updating aux_head.
> +	 */

Please could we reword this a bit, something like :

	/*
	 * Ensure that the AUX trace data is visible before the aux_head
	 * is updated via perf_aux_output_end(), as expected by the
	 * perf ring buffer.
	 */
> +	smp_wmb();
> +

Reviewed-by: Suzuki K Poulose <suzuki.poulose@xxxxxxx>