Re: [PATCH v2] perf: arm_spe: Relax period restriction

[James Clark <james.clark@linaro.org>]

On 28/05/2025 2:25 pm, Leo Yan wrote:
> The minimum interval specified the PMSIDR_EL1.Interval field is a
> hardware recommendation. However, this value is set by hardware designer
> before the production. It may not accurately reflects actual hardware
> limitations, and tools currently have no way to test shorter periods.
> 
> This change relaxes the limitation by allowing any non-zero periods.
> This gives chance for experimenting smaller periods.
> 
> The downside is that small periods may increase the risk of AUX ring
> buffer overruns. When an overrun occurs, the perf core layer will
> trigger an irq work to disable the event and wake up the tool in user
> space to read the trace data. After the tool finishes reading, it will
> re-enable the AUX event.
> 
> Signed-off-by: Leo Yan <leo.yan@arm.com>
> ---
> 
> Changes from v1:
> - Shifted bits with FIELD_PREP().
> - Removed warning log which is not quite useful. (James Clark)
> 
>   drivers/perf/arm_spe_pmu.c | 10 +++++++---
>   1 file changed, 7 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/perf/arm_spe_pmu.c b/drivers/perf/arm_spe_pmu.c
> index 3efed8839a4e..e40e5daa838d 100644
> --- a/drivers/perf/arm_spe_pmu.c
> +++ b/drivers/perf/arm_spe_pmu.c
> @@ -308,12 +308,16 @@ static u64 arm_spe_event_to_pmscr(struct perf_event *event)
>   
>   static void arm_spe_event_sanitise_period(struct perf_event *event)
>   {
> -	struct arm_spe_pmu *spe_pmu = to_spe_pmu(event->pmu);
>   	u64 period = event->hw.sample_period;
>   	u64 max_period = PMSIRR_EL1_INTERVAL_MASK;
>   
> -	if (period < spe_pmu->min_period)
> -		period = spe_pmu->min_period;
> +	/*
> +	 * As per the Arm ARM (DDI 0487 L.a), section D24.7.12 PMSIRR_EL1,
> +	 * Sampling Interval Reload Register, the INTERVAL field (bits [31:8])
> +	 * states: "Software must set this to a nonzero value."
> +	 */
> +	if (period < FIELD_PREP(PMSIRR_EL1_INTERVAL_MASK, 1))
> +		period = FIELD_PREP(PMSIRR_EL1_INTERVAL_MASK, 1);
>   	else if (period > max_period)
>   		period = max_period;
>   	else

Reviewed-by: James Clark <james.clark@linaro.org>