[PATCH v2] perf: arm_spe: Relax period restriction

[PATCH v2] perf: arm_spe: Relax period restriction

[Leo Yan]

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
-- 
2.34.1

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

[Leo Yan]

On Wed, May 28, 2025 at 02:25:01PM +0100, 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>

Gentle ping. thanks!

> ---
> 
> 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
> -- 
> 2.34.1
> 

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

[James Clark]

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>