Re: [PATCH v2 2/9] perf/cxl: Clear stale event fields before reprogramming a counter

On Wed, 29 Jul 2026 07:55:48 -0700
Dave Jiang <[email protected]> wrote:

> cxl_pmu_event_start() ORs the event group id, event mask, edge and invert
> selections into a configurable counter's config register without clearing
> those fields first, and cxl_pmu_event_stop() leaves them set. Reusing a
> counter for another event therefore ORs the new selection onto the old,
> counting the wrong events with stale edge/invert behaviour.
> 
> Clear EVENT_GRP_ID_IDX, EVENTS, EDGE and INVERT before applying the new
> values.

Maybe use FIELD_MODIFY() instead of mask then fill?

Otherwise LGTM.

> 
> Fixes: 5d7107c72796 ("perf: CXL Performance Monitoring Unit driver")
> Reported-by: [email protected]
> Closes: https://sashiko.dev/#/patchset/[email protected]?part=1
> Assisted-by: Claude:claude-opus-4-8
> Signed-off-by: Dave Jiang <[email protected]>
> ---
>  drivers/perf/cxl_pmu.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/drivers/perf/cxl_pmu.c b/drivers/perf/cxl_pmu.c
> index e77c0650a7eb..3138514157cd 100644
> --- a/drivers/perf/cxl_pmu.c
> +++ b/drivers/perf/cxl_pmu.c
> @@ -641,6 +641,7 @@ static void cxl_pmu_event_start(struct perf_event *event, int flags)
>  	}
>  
>  	cfg = readq(base + CXL_PMU_COUNTER_CFG_REG(hwc->idx));
> +	cfg &= ~(CXL_PMU_COUNTER_CFG_EDGE | CXL_PMU_COUNTER_CFG_INVERT);
>  	cfg |= FIELD_PREP(CXL_PMU_COUNTER_CFG_INT_ON_OVRFLW, 1);
>  	cfg |= FIELD_PREP(CXL_PMU_COUNTER_CFG_FREEZE_ON_OVRFLW, 1);
>  	cfg |= FIELD_PREP(CXL_PMU_COUNTER_CFG_ENABLE, 1);
> @@ -651,6 +652,8 @@ static void cxl_pmu_event_start(struct perf_event *event, int flags)
>  
>  	/* Fixed purpose counters have next two fields RO */
>  	if (test_bit(hwc->idx, info->conf_counter_bm)) {
> +		cfg &= ~(CXL_PMU_COUNTER_CFG_EVENT_GRP_ID_IDX_MSK |
> +			 CXL_PMU_COUNTER_CFG_EVENTS_MSK);
>  		cfg |= FIELD_PREP(CXL_PMU_COUNTER_CFG_EVENT_GRP_ID_IDX_MSK,
>  				  hwc->event_base);
>  		cfg |= FIELD_PREP(CXL_PMU_COUNTER_CFG_EVENTS_MSK,